Semiconductor memory cell having information storage transistor and switching transistor

FIELD OF THE INVENTION
The present invention relates to a semiconductor memory cell consisting of two transistors or one transistor formed by combining two transistors.
DESCRIPTION OF THE PRIOR ART
FIG. 17 shows a conventional dynamic memory cell consisting of a single transistor and a capacitor; the memory cell of this structure is generally known as a one-transistor memory cell and used as a semiconductor memory cell suitable for high density devices. In such a memory cell, the capacitance of the capacitor needs to be high enough to cause a voltage change on the associated bit line. However, the trend to smaller plan area of the semiconductor memory cell necessitates a reduction in the size of the capacitor formed in a horizontal plate-like shape, giving rise to the problem that when reading information stored as a charge on the memory cell capacitor, the information is masked by noise, or that, because of increasing stray capacitance on the bit line with each generation of memory cells, only a small voltage change can be caused to the bit line. In one approach to solving this problem, there is proposed a dynamic memory cell having a trench capacitor cell structure (see FIG. 18) or a stacked capacitor cell structure. However, because of processing limitations on how deep the trench can be formed and how high the stack (stacked layers) can be made, there is a limit to increasing the capacitance of the capacitor. Accordingly, it is said that dynamic memory cells of these structures encounter the limit of miniaturization in the dimensional region beyond the low submicron rule.
Furthermore, with transistors forming the semiconductor memory cells, reducing the transistor's plan area beyond the low submicron rule would introduce such problems as dielectric strength degradation, punch-through, etc., which would increase the possibility of leakage even under the rated voltage condition. With conventional transistor structures, therefore, it becomes difficult to ensure proper operation of the memory cell when the memory cell size is reduced.
To overcome the above limitations on the capacitance, there is proposed a memory cell structure wherein the memory cell is constructed with two transistors and the transistor channel current is sensed.
In the memory cell disclosed in Japanese Patent Unexamined Publication No. 63-19847, for example, a capacitor C.sub.2 coupled to the gate and drain of a MOS transistor Q.sub.1 is connected to a word line, as shown in FIG. 3 accompanying the same Patent Publication. Furthermore, the drain of the transistor Q.sub.1 is connected to the gate of an SOI transistor Q.sub.2 of the complementary type to the transistor Q.sub.1. The drain of the transistor Q.sub.2 is in turn connected to a fixed potential V.sub.D, while the source of each of the transistors, Q.sub.1 and Q.sub.2, is connected to a bit line.
In the memory cell disclosed in the above Patent Publication, since the gate electrode of the SOI (silicon-on-insulator) transistor Q.sub.2 is formed only on one principal surface side of the channel region thereof having two principal surfaces, an extra capacitor C.sub.2 is needed to complete the structure of the memory cell. A further problem is that because of the charge or electric field applied (through an insulating film) to the other principal surface of the channel region of the SOI transistor Q.sub.2, the operation of the SOI transistor Q.sub.2 becomes unstable and it is difficult to reduce the channel length.
On the other hand, the memory cell disclosed in Japanese Patent Unexamined Publication No. 1-145850 is constructed with a write transistor 18, a read transistor 19 (an SOI o transistor), and a protective capacitor 20, as shown in FIG. 1 accompanying the same Patent Publication. The source of the write transistor 18 is connected to the gate of the read transistor 19. The cathode of the protective diode 20 is connected to the drain of the read transistor 19. Further, the anode of the protective diode 20 and the drain of the write transistor 18 are connected to a bit line, while the source of the read transistor 19 and the gate of the write transistor 18 are connected to a word line.
In the memory cell disclosed in the above Patent Publication, since the read transistor 19 is an SOI transistor, when the source potential is caused to change to the same polarity as the gate potential (when writing a "1") the potential of the channel region changes with the change of the gate potential, resulting in an incomplete writing condition. It is therefore difficult to read the "1" with the expected certainty. Furthermore, in the read transistor 19, the gate electrode is formed only on one principal surface side of the channel region having two principal surfaces. Therefore, this structure also has the problem that because of the charge or electric field applied (through an insulating film) to the other principal surface side of the channel region of the read transistor 19, the operation of the read transistor 19 becomes unstable and it is difficult to reduce the channel length.
The memory cells disclosed in Japanese Patent Unexamined Publication Nos. 62-141693 and 62-254462 each comprise a write transistor T1, a read transistor T2, and a storage capacitor C, as shown in FIG. 1 accompanying the former Patent Publication. The drains of the transistors T1 and T2 are connected to a bit line, and the source of the transistor T1 is connected to the storage capacitor and also to a first gate of the transistor T2. Further, the gate of the transistor T1 is connected to a write selection line, while a second gate (or a channel forming region such as a well) of the transistor T2 is connected to a read selection line.
When the read transistor T2 is formed from an SOI transistor, the first and second gates are respectively formed on the upper and lower surface sides of the channel region of the transistor T2 (refer to Japanese Patent Application No. 55-93521 and Japanese Patent Unexamined Publication No. 57-18364). This structure eliminates the problem that has placed a limitation on the reduction of the channel length. For the write transistor T1, on the other hand, only bulk-type transistors are disclosed as preferred embodiments, and therefore, there is a limit to the miniaturization of the memory cell as a whole. Furthermore, the structure requiring each word line to be divided into a read line and a write line has the problem of increased chip area or increased number of stacked layers.
The present invention is concerned with the structure of a memory cell constructed with two transistors and yet capable of solving the above enumerated problems. An object of the invention is to provide a semiconductor memory cell, a semiconductor memory cell for ASICs (Application Specific Integrated Circuits), and even a semiconductor memory cell with a single transistor formed by combining two transistors, of the structure that ensures stable transistor operation, that does not require the provision of a large-capacitance capacitor as required in prior art DRAMs, and that allows the channel length to be reduced and achieves miniaturization of the cell.
SUMMARY OF THE INVENTION
To achieve the above object, in accordance with a first aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 1, comprising:
an information storage transistor TR.sub.1 comprising a semiconductor channel layer Ch.sub.1 having first and second opposing principal surfaces; first and second conductive gates, G.sub.1 and G.sub.2, respectively disposed opposite the two principal surfaces of the semiconductor channel layer Ch.sub.1 with first and second barrier layer respectively interposed therebetween; and first and second conductive regions, L.sub.1 and L.sub.2, respectively connected to either end of the semiconductor channel layer Ch.sub.1, and
a switching transistor TR.sub.2 comprising a semiconductor channel forming region Ch.sub.2 having a third principal surface; a third conductive gate G.sub.3 disposed opposite the third principal surface of the semiconductor channel forming region Ch.sub.2 with a third barrier layer interposed therebetween; and third and fourth conductive layers, L.sub.2 and L.sub.4, each formed in a surface region of the semiconductor channel forming region Ch.sub.2 and near either end of the third conductive gate G.sub.3, wherein
the fourth conductive layer L.sub.4 is connected to the second conductive gate G.sub.2,
the first conductive gate G.sub.1 and the third conductive gate G.sub.3 are connected to a first memory-cell-selection line,
the first conductive layer L.sub.1 and the third conductive layer L.sub.3 are connected to a second memory-cell-selection line,
the second conductive layer L.sub.2 is connected to a fixed potential including zero potential, and
the semiconductor channel forming region Ch.sub.2 is connected to a read/write selection line.
In the above structure, the conductive layers may be formed from a low-resistivity semiconductor, a silicide, a two-layered structure of silicide and semiconductor, a metal, or the like. The barrier layers serve as barriers to channel carriers, and may be formed from an insulating material or a wide-gap semiconductor material.
In one preferred mode of the semiconductor memory cell according to the first aspect of the invention, as shown in the schematic diagram of FIG. 4,
the information storage transistor TR.sub.1 is formed from a transistor of a first conductivity type,
the switching transistor TR.sub.2 is formed from a transistor of a conductivity type opposite to the first conductivity type,
the first conductive layer L.sub.1 is connected to the second line via a fifth conductive layer L.sub.5 which forms a rectifier junction with the first conductive layer L.sub.1, and
the semiconductor channel forming region Ch.sub.2 is connected to a second fixed potential including zero potential.
To achieve the above object, in accordance with a second aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 7, comprising:
an information storage transistor TR.sub.1 comprising a first semiconductor channel layer Ch.sub.1 having first and second opposing principal surfaces; first and second conductive gates, G.sub.1 and G.sub.2, respectively disposed opposite the two principal surfaces of the first semiconductor channel layer Ch.sub.1 with first and second barrier layers respectively interposed therebetween; and first and second conductive regions, L.sub.1 and L.sub.2, each connected to either end of the first semiconductor channel layer Ch.sub.1, and
a switching transistor TR.sub.2 comprising a second semiconductor channel layer Ch.sub.2 having third and fourth opposing principal surfaces; third and fourth conductive gates, G.sub.3 and G.sub.4, respectively-disposed opposite the two principal surfaces of the second semiconductor channel layer Ch.sub.2 with third and fourth barrier layers respectively interposed therebetween; and third and fourth conductive layers, L.sub.3 and L.sub.4, each connected to either end of the second semiconductor channel region Ch.sub.2, wherein
the fourth conductive layer L.sub.4 is connected to the second conductive gate G.sub.2,
the first conductive gate G.sub.1 and the third conductive gate G.sub.3 are connected to a first memory-cell-selection line,
the first conductive layer L.sub.1 and the third conductive layer L.sub.3 are connected to a second memory-cell-selection line,
the second conductive layer L.sub.2 is connected to a fixed potential including zero potential, and
the fourth conductive gate G.sub.4 is connected to a read/write selection line.
In one preferred mode of the semiconductor memory cell according to the second aspect of the invention, as shown in the schematic diagram of FIG. 10,
the information storage transistor TR.sub.1 is formed from a transistor of a first conductivity type,
the switching transistor TR.sub.2 is formed from a transistor of a conductivity type opposite to the first conductivity type,
the first conductive layer L.sub.1 is connected to the second line via a fifth conductive layer L.sub.5 which forms a rectifier junction with the first conductive layer L.sub.1, and
the fourth conductive gate G.sub.4 is connected to a second fixed potential including zero potential.
To achieve the above object, in accordance with a third aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 12(A), comprising:
an information storage transistor TR.sub.1 of a first conductivity type, comprising a first semiconductor channel forming region Ch.sub.1 ; a first conductive gate G.sub.1 formed above the surface thereof with a first barrier layer interposed therebetween; a first conductive region SC.sub.1 ; and a second conductive region SC.sub.2, and
a switching transistor TR.sub.2 of a second conductivity type opposite to the first conductivity type, comprising a second semiconductor channel forming region Ch.sub.2 ; a second conductive gate G.sub.2 formed above the surface thereof with a second barrier layer interposed therebetween; a third conductive region SC.sub.3 ; and a fourth conductive region SC.sub.4, wherein
the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 and the second conductive gate G.sub.2 of the switching transistor TR.sub.2 are connected to a first memory-cell-selection line;
the fourth conductive region SC.sub.4 of the switching transistor TR.sub.2 is connected to the first semiconductor channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 ;
the third conductive region SC.sub.3 of the switching transistor TR.sub.2 is connected to a second memory-cell-selection line, and
the first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected to a read line.
The second conductive region SC.sub.2 of the information storage transistor TR.sub.1 is supplied with a fixed potential including zero potential. Preferably, the first conductive region SC.sub.1 is connected to the second line or the third conductive region SC.sub.3 via a rectifier junction. Preferably, the first conductive region SC.sub.1 is formed from a semiconductor as a common region with the second semiconductor channel forming region Ch.sub.2, the rectifier junction being formed between the common region and the third conductive region SC.sub.3, and the first semiconductor channel forming region Ch.sub.1 and the fourth conductive region SC.sub.4 are formed from a common region. Furthermore, the second conductive gate G.sub.2 may be formed common with the first conductive gate G.sub.1, as in a fifth aspect of the invention hereinafter described. In this case, it is desirable that the third conductive region SC.sub.3 further include a metal layer of Mo, Al, or the like, or a silicide layer that forms a Schottky junction with the first conductive region SC.sub.1.
In the above structure, the conductive regions may be formed from a low-resistivity semiconductor, a silicide, a two-layered structure of silicide and semiconductor, a metal, or the like.
To achieve the above object, in accordance with a fourth aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 14(A), comprising:
an information storage transistor TR.sub.1 of a first conductivity type, comprising a first semiconductor channel forming region Ch.sub.1 ; a first conductive gate G.sub.1 formed above the surface thereof with a first barrier layer interposed therebetween; a first conductive region SC.sub.1 ; and a second conductive region SC.sub.2, and
a switching transistor TR.sub.2 of a second conductivity type opposite to the first conductivity type, comprising a second semiconductor channel forming region Ch.sub.2 ; a second conductive gate G.sub.2 formed above the surface thereof with a second barrier layer interposed therebetween; a third conductive region SC.sub.3 ; and a fourth conductive region SC.sub.4, wherein
the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 and the second conductive gate G.sub.2 of the switching transistor TR.sub.2 are connected to a first memory-cell-selection line;
the fourth conductive region SC.sub.4 of the switching transistor TR.sub.2 is connected to the first semiconductor channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 ;
the third conductive region SC.sub.3 of the switching transistor TR.sub.2 is connected to a second memory-cell-selection line,
the second conductive region SC.sub.2 of the information storage transistor TR.sub.1 is connected to a fixed potential, and
the first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected to the third conductive region SC.sub.3 of the switching transistor TR.sub.2, forming a rectifier junction therebetween.
A third conductive gate G.sub.3 may be further provided opposite a third principal surface of the second semiconductor channel forming region Ch.sub.2 with a third barrier layer interposed therebetween.
To achieve the above object, in accordance with a fifth aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 15(A), comprising:
a first semiconductor region SC.sub.1 of a first conductivity type formed in a surface region of a semiconductor substrate or on an insulating substrate,
a first conductive region SC.sub.2 formed in a surface region of the first semiconductor region SC.sub.1 in contacting relationship forming a rectifier junction therebetween,
a second semiconductor region SC.sub.3 of a second conductivity type formed in a surface region of the first semiconductor region SC.sub.1 but spaced apart from the first conductive region SC.sub.2,
a second conductive region SC.sub.4 formed in a surface region of the second semiconductor region SC.sub.3 in contacting relationship forming a rectifier junction therebetween, and
a conductive gate G disposed in such a manner as to form a bridge over a barrier layer between the first semiconductor region SC.sub.1 and the second conductive region SC.sub.4 and between the first conductive region SC.sub.2 and the second semiconductor region SC.sub.3, wherein
the conductive gate G is connected to a first memory-cell-selection line,
the first conductive region SC.sub.2 is connected to a write information selection line, and
the second conductive region SC.sub.4 is connected to a second memory-cell-selection line.
To achieve the above object, in accordance with a sixth aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 16(A), comprising:
a first semiconductor region SC.sub.1 of a first conductivity type formed in a surface region of a semiconductor substrate or on an insulating substrate,
a first conductive region SC.sub.2 formed in a surface region of the first semiconductor region SC.sub.1 in contacting relationship forming a rectifier junction therebetween,
a second semiconductor region SC.sub.3, of a second conductivity type opposite to the first conductivity type, formed in a surface region of the first semiconductor region SC.sub.1 but spaced apart from the first conductive region SC.sub.2,
a second conductive region SC.sub.4 formed in a surface region of the second semiconductor region SC.sub.3 in contacting relationship forming a rectifier junction therebetween, and
a conductive gate G disposed in such a manner as to form a bridge over a barrier layer between the first semiconductor region SC.sub.1 and the second conductive region SC.sub.4 and between the first conductive region SC.sub.2 and the second semiconductor region SC.sub.3, wherein
the conductive gate G is connected to a first memory-cell-selection line, and
the first semiconductor region SC.sub.1 is connected to a second memory-cell-selection line.
In the semiconductor memory cell according to the fifth and sixth aspects of the invention, the first semiconductor region SC.sub.1 (corresponding to the channel forming region Ch.sub.2), the first conductive region SC.sub.2 (corresponding to a source/drain region), the second semiconductor region SC.sub.3 (corresponding to a source/drain region), and the conductive gate G constitute the switching transistor TR.sub.2. Likewise, the second semiconductor region SC.sub.3 (corresponding to the channel forming region Ch.sub.1), the first semiconductor region SC.sub.1 (corresponding to a source/drain region), the second conductive region SC.sub.4 (corresponding to a source/drain region), and the conductive gate G constitute the information storage transistor TR.sub.1.
The semiconductor channel layer or semiconductor channel forming region can be formed from a silicon, GaAs, or the like, using a known method. Each conductive gate can be formed from a metal, an impurity-added or doped silicon or polysilicon, a silicide, highly doped GaAs, or the like, using a known method. Each barrier layer can be formed from SiO.sub.n, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, GaAlAs, or the like, using a known method. The conductive layers, conductive regions, or semiconductor regions can be formed from a doped silicon or polysilicon, a silicide, a highly doped GaAs, or the like, using a known method.
In the semiconductor memory cell of the present invention, one conductive gate of the information storage transistor and one conductive gate of the switching transistor are connected to the first memory-cell-selection line. The first memory-cell-selection line, therefore, need not be provided more than one, and the chip area can be reduced.
In the semiconductor memory cell of the first and second aspects of the present invention, the fourth conductive layer is connected to the second conductive gate. When writing information, the switching transistor conducts, and the information is stored in the form of a potential or charge on the second conductive gate of the information storage transistor. The threshold voltage of the information storage transistor required at the first conductive gate, when reading information, varies depending on the potential or charge (information) stored on the second conductive gate. This is because the space-charge regions overlap across the channel layer near the first and second gates. Therefore, when reading information, the operation of the information storage transistor can be controlled by applying an appropriately selected potential to the first conductive gate. Information reading is accomplished by sensing the operating condition (for example, magnitude of the channel current) of the information storage transistor.
In the information storage transistor, a conductive gate is provided opposite each of the two principal surfaces of the semiconductor channel layer. This structure stabilizes the operation of the information storage transistor and facilitates short-channel transistor design. This also eliminates the need for a large capacitor as required in prior art DRAMs.
The gate threshold voltage of the switching transistor is controlled by the potential applied to the read/write selection line. When writing information, the switching transistor is turned on in order to store the potential or charge on the second conductive gate, and when the write operation is completed, it is turned off. The stored information is retained as a potential or charge on the second conductive gate until the information is read out.
In the semiconductor memory cell according to the second aspect of the present invention, the switching transistor has the fourth conductive gate. This serves to further stabilize the operation of the switching transistor. This is achieved by applying to the read/write selection line a signal for putting the switching transistor in an off condition without fail during information read operation.
In the preferred modes of the semiconductor memory cell according to the first and second aspects of the present invention, the fifth conductive layer is provided which forms a rectifier junction with the first conductive layer. The provision of this fifth conductive layer serves to prevent without fail a current from flowing into the information storage transistor during the writing of information.
In the semiconductor memory cell according to the third and fourth aspects of the present invention, the fourth conductive region of the switching transistor is connected to the channel forming region of the information storage transistor. In the semiconductor memory cell according to the fifth aspect of the invention, the second semiconductor region, which forms a source/drain region of the switching transistor, corresponds to the channel forming region of the information storage transistor TR.sub.1.
when writing information, the switching transistor conducts, and the information is stored as a potential or charge in the channel forming region of the information storage transistor. The threshold voltage of the information storage transistor required at the first conductive gate (in the third and fourth aspects of the invention) or the conductive gate (in the fifth aspect of the invention) for read operations varies depending on the potential or charge (information) stored in the channel forming region. Therefore, when reading information, by applying an appropriately selected potential to the first conductive gate or the conductive gate, the stored information state of the information storage transistor can be identified by the magnitude of the channel current (including zero current). Information reading is accomplished by sensing the operating condition of the information storage transistor.
In the semiconductor memory cell according to the sixth aspect of the present invention, the second semiconductor region SC.sub.3, which forms a source/drain region of the switching transistor, corresponds to the channel forming region Ch.sub.1 of the information storage transistor. Further, the first semiconductor region SC.sub.1 that corresponds to the channel forming region of the switching transistor and also to a source/drain region of the information selection transistor is connected to the second memory-cell-selection line.
The threshold voltage of the information storage transistor required at each conductive gate for read operations can be varied by appropriately selecting the potential applied to the second memory-cell-selection line. Therefore, the on/off conditions of the information storage transistor and the switching transistor can be controlled by appropriately selecting the potential applied to the first memory-cell-selection line.
When writing information, the first line is set at a potential high enough to turn on the switching transistor, and the capacitor between the regions SC.sub.1 and SC.sub.3 of the switching transistor is charged depending on the potential on the second line. As a result, the information is stored in the channel forming region (the second semiconductor region SC.sub.3) of the information storage transistor in the form of a charge or a potential difference from the region SC.sub.1.
When reading information, the region SC.sub.1 is supplied with a read potential, and the potential or charge (information) stored in the channel forming region of the information storage transistor is converted to a charge or a potential difference between the second semiconductor region SC.sub.3, which corresponds to the channel forming region Ch.sub.1, and the second conductive region SC.sub.4, which corresponds to a source/drain region. The threshold voltage of the information storage transistor required at the conductive gate varies depending on the charge (information). Therefore, the on/off operations of the information storage transistor can be controlled, when reading information, by applying an appropriately selected potential to the conductive gate. Information reading is accomplished by sensing the operating condition of the information storage transistor.
In the semiconductor memory cell of the invention, the stored information is retained in the form of a potential, potential difference, or charge, but since the stored information decays with time because of leakage currents due to junction leaks, etc., refreshing is necessary, and the memory cell operates in the same manner as other DRAM cells.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the principle of operation of a semiconductor memory cell according to a first aspect of the present invention.
FIG. 2 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 1 of the present invention.
FIG. 3 is a diagram showing a modified example of Embodiment 1.
FIG. 4 is a diagram showing the principle of operation of a semiconductor memory cell according to a preferred mode of the first aspect of the invention.
FIG. 5 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 2 of the present invention.
FIG. 6 is a diagram showing a modified example of Embodiment 2.
FIG. 7 is a diagram showing the principle of operation of a semiconductor memory cell according to a second aspect of the present invention.
FIG. 8 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 3 of the present invention.
FIG. 9 is a diagram showing a modified example of Embodiment 3.
FIG. 10 is a diagram showing the principle of operation of a semiconductor memory cell according to a preferred mode of the second aspect of the present invention.
FIG. 11 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 4 of the present invention.
FIGS. 12A to 12C shows the principle of operation of a semiconductor memory cell according to a third aspect of the present invention along with schematic cross-sectional views showing a portion thereof.
FIG. 13 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to a modified example of the third aspect of the present invention.
FIGS. 14A to 14C shows the principle of operation of a semiconductor memory cell according to a fourth aspect of the present invention along with schematic cross-sectional views showing a portion thereof.
FIGS. 15A to 15B shows the principle of operation of a semiconductor memory cell according to a fifth aspect of the present invention along with a schematic cross-sectional view showing a portion thereof.
FIGS. 16A to 16C shows the principle of operation of a semiconductor memory cell according to a sixth aspect of the present invention along with schematic cross-sectional views showing a portion thereof.
FIG. 17 is a conceptual diagram showing a prior art one-transistor memory cell.
FIG. 18 is a cross-sectional view of a memory cell having a prior art trench capacitor cell structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
The semiconductor memory cell of the invention will be described below in accordance with preferred embodiments thereof.
EMBODIMENT 1
Embodiment 1 is concerned with a semiconductor memory cell according to a first aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 1 and cross sections of a portion of which are shown schematically in FIGS. 2.and 3, comprises an information storage transistor TR.sub.1 and a switching transistor TR.sub.2.
The information storage transistor TR.sub.1 comprises a semiconductor channel layer Ch.sub.1, a first conductive gate G.sub.1, a second conductive gate G.sub.2, and first and second conductive layers, L, and L.sub.2, each connected to either end of the semiconductor channel layer Ch.sub.1. The switching transistor TR.sub.2 comprises a semiconductor channel forming region Ch.sub.2, a third conductive gate G.sub.3, and third and fourth conductive layers, L.sub.3 and L.sub.4, each formed in the surface area of the semiconductor channel forming region Ch.sub.2 in contacting relationship forming a rectifier junction therewith. The third conductive gate G.sub.3 is formed in such a manner as to bridge the third conductive layer L.sub.3 and the fourth conductive layer L.sub.4.
The semiconductor channel layer Ch.sub.1 has two opposing principal surfaces, the first principal surface MS.sub.1 and the second principal surface MS.sub.2. The first conductive gate G.sub.1 is formed opposite the principal surface MS.sub.1 of the semiconductor channel layer with a first barrier layer BL.sub.1 interposed therebetween. Likewise, the second conductive gate G.sub.2 is formed opposite the principal surface MS.sub.2 of the semiconductor channel layer with a second barrier layer BL.sub.2 interposed therebetween. The semiconductor channel forming region Ch.sub.2 has a third principal surface MS.sub.3. The third conductive gate G.sub.3 is formed opposite the third principal surface MS.sub.3 of the semiconductor channel forming region Ch.sub.2 with a third barrier layer BL.sub.3 interposed therebetween.
The fourth conductive layer L.sub.4 is connected to the second conductive gate G.sub.2. The first conductive gate G.sub.1 and the third conductive gate G.sub.3 are connected to a first memory-cell-selection line (for example, a word line). In the structural example shown in FIG. 2, the first conductive gate G.sub.1 and the third conductive gate G.sub.3 are common. The first conductive layer L.sub.1 and the third conductive layer L5 are connected to a second memory-cell-selection line (for example, a bit line). The second conductive layer L.sub.2 is connected to a fixed potential including zero potential. The semiconductor channel forming region Ch.sub.2 is connected to a read/write selection line. The read/write selection line may be a common well or a substrate. In Embodiment 1, a common well is used. The second memory-cell-selection line (for example, a bit line) need not be provided more than one, and the chip area can be reduced.
The conductive layers may be formed from a low-resistivity semiconductor, a silicide, a two-layered structure of silicide and semiconductor, a metal or the like. The barrier layers serve as barriers to channel carriers, and may be formed from an insulating material or a wide-gap semiconductor material.
In Embodiment 1, the information storage transistor TR.sub.1 has an SOI structure. That is, with the multilayered structure shown in FIG. 2, the total area that the information storage transistor and the switching transistor take up can be made approximately equal to the area that one transistor takes up, thus allowing the chip area to be reduced.
The operation of the semiconductor memory cell will be described below, taking for example a case in which the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are both n-type transistors.
Potentials applied to the various lines for a memory write are designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.w
Second memory-cell-selection line (e.g., bit line) "0" write: V.sub.0
"1" write: V.sub.1
Read/write selection line: V.sub.B.sbsb.--.sub.W
Potentials applied to the various lines for a memory read are designated as follows. Note that during a read cycle, the read/write selection line is reverse biased.
First memory-cell-selection line (e.g., word line): V.sub.R
Read/write selection line: V.sub.B.sbsb.--.sub.R
For read/write, the fixed potential to which the second conductive layer L.sub.2 is connected is designated as follows:
Fixed potential to which the second conductive layer L.sub.2 is connected: V.sub.2
The threshold voltages of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 for memory read/write operations are designated as follows:
"0" read/write: V.sub.TH1.sbsb.--.sub.0
"1" read/write: V.sub.TH1.sbsb.--.sub.1
The potential of the second conductive gate G.sub.2 is different between a "0" read/write and a "1" read/write. As a result, the threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 differs between a "0" read/write and a "1" read/write. The information storage transistor TR.sub.1 is provided with two conductive gates, G.sub.1 and G.sub.2, on opposite sides of the semiconductor channel layer Ch.sub.1 ; this structure serves to stabilize the operation of the information storage transistor TR.sub.1, and facilitates short-channel design. Furthermore, a capacitor having as large capacitance as required in prior art DRAMs is not required.
The threshold voltage of the switching transistor TR.sub.2 required at the third conductive gate G.sub.3 for a write operation is designated as V.sub.TH2.sbsb.--.sub.W. Further, the threshold voltage of the switching transistor TR.sub.2 required at the third conductive gate G.sub.3 for a read operation is designated as V.sub.TH2.sbsb.--.sub.R. The threshold voltage of the switching transistor TR.sub.2 required at the third conductive gate G.sub.3 is different between memory write and memory read because the potential applied to the read/write selection line differs between write and read operations.
The relationships between the various potentials are set as follows:
.linevert split.V.sub.W .linevert split.>.linevert split.V.sub.TH2.sbsb.--.sub.W .linevert split.
.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.
.linevert split.V.sub.TH2.sbsb.--.sub.R .linevert split.>.linevert split.V.sub.R .linevert split.
The operation of the semiconductor memory cell of Embodiment 1 will be described below.
Information write
When writing information "0" (second line potential: V.sub.0) or "1" (second line potential: V.sub.1), the potential of the first line is V.sub.W. Therefore, the potential at the third conductive gate G.sub.3 of the switching transistor TR.sub.2 is also V.sub.W. Since the potential of the read/write selection line is V.sub.B.sbsb.--.sub.W, the threshold voltage, V.sub.TH2.sbsb.--.sub.W, of the switching transistor TR.sub.2 required at the third conductive gate has the following relationship with respect to V.sub.W.
.linevert split.V.sub.W .linevert split.>.linevert split.V.sub.TH2.sbsb.--.sub.W .linevert split.
As a result, the switching transistor TR.sub.2 is ON. Therefore, the potential at the second conductive gate G.sub.2 of the information transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 (when writing information "1").
When writing information, the potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.W. The threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 is V.sub.TH1.sbsb.--.sub.0 or V.sub.TH1.sbsb.--.sub.1. If the relationship
.linevert split.V.sub.W .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.>V.sub.TH1.sbsb.--.sub.1 .linevert split.
exists between these potentials, the information storage transistor TR.sub.1 is also ON, but writing is performed whether the information storage transistor TR.sub.1 is ON or OFF.
As described above, when writing information "0" or "1", the potential at the second conductive gate G.sub.2 of the information storage transistor TR.sub.1 is V.sub.0 or V.sub.1. That is, the second conductive gate G.sub.2 is held at a potential corresponding to the information "0" or "1", and this condition is substantially maintained within a prescribed time until the information is read out. During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are set at such potentials that neither transistor will conduct.
Information read
When reading information "0" or "1", the potential of the first line is V.sub.R. Therefore, the potential at the third conductive gate G.sub.3 of the switching transistor TR.sub.2 is V.sub.R. Since the potential of the read/write selection line is V.sub.B.sbsb.--.sub.R, a reverse bias is applied between the source and the channel forming region of the switching transistor TR.sub.2. V.sub.B.sbsb.--.sub.R is set so that the threshold voltage, V.sub.TH2.sbsb.--.sub.R, of the switching transistor TR.sub.2 required at the third conductive gate has the following relationship with respect to V.sub.R.
.linevert split.V.sub.TH2.sbsb.--.sub.R .linevert split.>.linevert split.V.sub.R .linevert split.
The switching transistor TR.sub.2 is thus held OFF.
When reading information, the potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.R. The threshold voltage of the information storage transistor TR.sub.1 at the first conductive gate G.sub.1 is V.sub.TH1.sbsb.--.sub.0 or V.sub.TH1.sbsb.--.sub.1. The threshold voltage of the information storage transistor TR.sub.1 is dependent on the potential of the second conductive gate G.sub.2. The following relationship exists between these potentials.
.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.
Therefore, when the stored information is "0", the information storage transistor TR.sub.1 is OFF. On the other hand, when the stored information is "1", the information storage transistor TR.sub.1 is ON.
Thus, the information storage transistor TR.sub.1 is set to an ON or OFF condition, depending on the stored information. Therefore, when the stored information is "1", current flows to the second line; when the stored information is "0", no current but the leakage current flows to the second line. The stored information can be read in this manner by the information storage transistor TR.sub.1.
The above-described operating conditions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are summarized in Table 1. The potential values given in Table 1 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.
TABLE 1______________________________________ Unit: voltsMemory write "0" write "1" write______________________________________1st line potential V.sub.W 2.0 V.sub.W 2.02nd line potential V.sub.0 0 V.sub.1 1.0Read/write line V.sub.B-W V.sub.B-WThreshold of TR.sub.2 V.sub.TH2-W 0.5 V.sub.TH2-W 0.5required at 3rd gate3rd gate potential V.sub.W 2.0 V.sub.W 2.0Condition of TR.sub.2 ON ON2nd gate potential V.sub.0 0 V.sub.1 1.0Threshold of TR.sub.1 V.sub.TH1-0 1.1 V.sub.TH1-1 0.5required at 1st gate1st gate potential V.sub.W 2.0 V.sub.W 2.0Condition of TR.sub.1 ON ON______________________________________Memory read "0" read "1" read______________________________________1st line potential V.sub.R 1.0 V.sub.R 1.0Read/write line V.sub.B-R V.sub.B-RThreshold of TR.sub.2 V.sub.TH2-R 1.5 V.sub.TH2-R 1.5required at 3rd gate3rd gate potential V.sub.R 1.0 V.sub.R 1.0Condition of TR.sub.2 OFF OFF2nd gate potential V.sub.0 0 V.sub.1 1.0Threshold of TR.sub.1 V.sub.TH1-0 1.1 V.sub.TH1-1 0.5required at 1st gate1st gate potential 1.0 1.0Condition of TR.sub.2 OFF ONFixed potential V.sub.2 0 V.sub.2 02nd line current OFF ON______________________________________
A modified example of the semiconductor memory cell of Embodiment 1 illustrated in FIG. 2 is shown schematically in cross section in FIG. 3. In the semiconductor memory cell shown in FIG. 2, the first conductive gate G.sub.1 and the third conductive gate G.sub.3 are common. By contrast, in the semiconductor memory cell shown in FIG. 3, the second conductive gate G.sub.2 and the fourth conductive layer L.sub.4 are common. Also, the read/write selection line is formed from a well. While a slightly larger plan area is required as compared with the memory cell shown in FIG. 2, the memory cell shown in FIG. 3 have the advantages of relatively smooth surface topography, which is advantageous in lithography, and a less number of contacts.
EMBODIMENT 2
Embodiment 2 is concerned with a preferred mode of the semiconductor memory according to the first aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 4 and cross sections of a portion of which are shown schematically in FIGS. 5 and 6, comprises an information storage transistor TR.sub.1 and a switching transistor TR.sub.2. The structures of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are fundamentally the same as those of Embodiment 1, except for the points hereinafter described.
The information transistor TR.sub.1 is formed from a transistor of a first conductivity type, for example, a p-type transistor, and the switching transistor TR.sub.2 is formed from a transistor of the opposite conductivity type to that of the information storage transistor TR.sub.1, for example, an n-type transistor. In this case, if the first conductive layer L.sub.1 and the second conductive layer L.sub.2 are formed from semiconductor material, then their conductivity type should be p type. Likewise, if the first conductive gate G.sub.1, the second conductive gate G.sub.2, the third conductive gate G.sub.3, the third conductive layer L.sub.3, and the fourth conductive layer L.sub.4 are formed from semiconductor material, then their conductivity type should be n.sup.+ type. Alternatively, these regions may be formed from a silicide, a two-layer structure of silicide and semiconductor, or a metal. The semiconductor channel forming region Ch.sub.2 is connected to a second fixed potential including zero potential.
Further, the first conductive layer L.sub.1 is connected to the second line via a fifth conductive layer L.sub.5 which forms a rectifier junction with the first conductive layer. The provision of the fifth conductive layer serves to prevent without fail a current from flowing into the information storage transistor TR.sub.1 during the writing of information. Furthermore, there is no possibility of the information write voltage being applied to the information storage transistor TR.sub.1 and thereby interfering with the write operation as was the case with the prior art.
The operation of the semiconductor memory cell of Embodiment 2 will be described below.
The potentials of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are set to satisfy the following relationships.
.linevert split.V.sub.W .linevert split.>.linevert split.V.sub.TH2.sbsb.--.sub.W .linevert split.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
Information write
When writing information "0" (second line potential: V.sub.0) or "1" (second line potential: V.sub.1), the potential of the first line is V.sub.W (>0). Therefore, the potential at the third conductive gate G.sub.3 of the switching transistor TR.sub.2 is also V.sub.W (>0). Since the second fixed potential is V.sub.B.sbsb.--.sub.W, if V.sub.W is set as
.linevert split.V.sub.W .linevert split.>.linevert split.V.sub.TH2.sbsb.--.sub.W +V.sub.0 or V.sub.1 .linevert split.
where V.sub.TH2.sbsb.--.sub.W is the threshold voltage of the switching transistor TR.sub.2 required at the third conductive gate G.sub.3, then the switching transistor TR.sub.2 is ON. Therefore, the potential at the second conductive gate G.sub.2 of the information transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 (when writing information "1").
when writing information, the potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.W (>0). Therefore, when V.sub.0 or V.sub.1 <V.sub.W -V.sub.TH1.sbsb.--.sub.1, the information storage transistor TR.sub.1 is OFF. Even if it is ON, current flow is blocked by the presence of the rectifier junction between the first conductive layer L.sub.1 and the fifth conductive layer L.sub.5.
As described above, when writing information "0" or "1", the potential at the second conductive gate G.sub.2 of the information storage transistor TR.sub.1 is V.sub.0 or V.sub.1. That is, the second conductive gate G.sub.2 is held at a potential corresponding to the information "0" or "1", and this condition is substantially maintained until the information is read out. During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are set at such potentials that neither transistor will conduct.
Information read
When reading information "0" or "1", the potential of the first line is V.sub.R (<0). Therefore, the potential at the third conductive gate G.sub.3 of the switching transistor TR.sub.2 is V.sub.R (<0), and the switching transistor TR.sub.2 remains OFF unless the potential of the second line is made more negative than V.sub.R -V.sub.TH2.sbsb.--.sub.R. Normally, for a read operation, the second line is set at a small potential and information is read by sensing the current.
The potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.R (<0). The threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 is V.sub.TH1.sbsb.--.sub.0 or V.sub.TH1.sbsb.--.sub.1. The threshold voltage of the information storage transistor TR.sub.1 is dependent on the potential of the second conductive gate G.sub.2. The following relationship exists between these potentials.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
Therefore, when the stored information is "0", the information storage transistor TR.sub.1 is ON. On the other hand, when the stored information is "1", the information storage transistor TR.sub.1 is OFF.
Thus, the information storage transistor TR.sub.1 is set to an ON or OFF condition, depending on the stored information. The second conductive layer L.sub.2 of the information storage transistor TR.sub.1 is connected to the fixed potential (V.sub.2); therefore, when the stored information is "0", current flows to the second line, and when the stored information is "1", current does not flow to the second line. The stored information can be read in this manner by the information storage transistor TR.sub.1.
A cross section of a portion of the semiconductor memory cell of Embodiment 2 is shown schematically in FIG. 5. The semiconductor memory cell of Embodiment 2 is fundamentally the same as the semiconductor memory cell shown in FIG. 2, except that the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are of different conductivity types and that the first conductive layer L.sub.1 is connected to the second line via the n-type fifth conductive layer L.sub.5 which forms a rectifier junction with the p-type first conductive layer L.sub.1.
Table 2 summarizes the operating conditions of the semiconductor memory cell in which the information storage transistor TR.sub.1 is formed from a p-type transistor and the switching transistor TR.sub.2 is formed from an n-type transistor.
TABLE 2______________________________________ Unit: voltsMemory write "0" write "1" write______________________________________1st line potential V.sub.W 1.5 V.sub.W 1.52nd line potential V.sub.0 0 V.sub.1 1.0Read/write line V.sub.B-W V.sub.B-WThreshold of TR.sub.2 V.sub.TH2-W 0.5 V.sub.TH2-W 0.5required at 3rd gate3rd gate potential V.sub.W 1.5 V.sub.W 1.5Condition of TR.sub.2 ON ON2nd gate potential V.sub.0 0 V.sub.1 1.01st gate potential V.sub.W 1.5 V.sub.W 1.5Condition of TR.sub.1 OFF OFF______________________________________Memory read "0" read "1" read______________________________________1st line potential V.sub.R -1.0 V.sub.R -1.03rd gate potential V.sub.R -1.0 V.sub.R -1.0Condition of TR.sub.2 OFF OFF2nd gate potential V.sub.0 0 V.sub.1 1.0Threshold of TR.sub.1 V.sub.TH1-0 -0.5 V.sub.TH1-1 -1.1required at 1st gate1st gate potential -1.0 -1.0Condition of TR.sub.1 ON OFFFixed potential V.sub.2 V.sub.22nd gate current ON OFF______________________________________
The potential values given in Table 2 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.
A modified example of the semiconductor memory cell illustrated in FIG. 5 is shown schematically in cross section in FIG. 6. The semiconductor memory cell shown is fundamentally the same as the semiconductor memory cell shown in FIG. 3, except that the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are of different conductivity types and that the first conductive layer L.sub.1 is connected to the second line via the n-type fifth conductive layer L.sub.5 which forms a rectifier junction with the p-type first conductive layer L.sub.1.
Embodiment 3
Embodiment 3 is concerned with a semiconductor memory cell according to a second aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 7 and cross sections of a portion of which are shown schematically in FIGS. 8 and 9, comprises an information storage transistor TR.sub.1 and a switching transistor TR.sub.2. The information storage transistor TR.sub.1 comprises a first semiconductor channel layer Ch.sub.1, a first conductive gate G.sub.1, a second conductive gate G.sub.2, and first and second conductive layers, L.sub.1 and L.sub.2, each connected to either end of the first semiconductor channel layer Ch.sub.1. The switching transistor TR.sub.2 comprises a second semiconductor channel layer Ch.sub.2, a third conductive gate G.sub.3, a fourth conductive gate G.sub.4, and third and fourth conductive layers, L.sub.3 and L.sub.4, each connected to either end of the second semiconductor channel layer Ch.sub.2. In Embodiment 3, the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 both have an SOI structure (including a structure in which the barrier layer is formed from a wide-gap semiconductor, for example, GaAlAs as compared with GaAs).
The first semiconductor channel layer Ch.sub.1 has two opposing principal surfaces, the first principal surface MS.sub.1 and the second principal surface MS.sub.2. The first conductive gate G.sub.1 is formed opposite the principal surface MS.sub.1 of the first semiconductor channel layer with a first barrier layer BL.sub.1 interposed therebetween. Likewise, the second conductive gate G.sub.2 is formed opposite the principal surface MS.sub.2 of the first semiconductor channel layer with a second barrier layer BL.sub.2 interposed therebetween.
The second semiconductor channel layer Ch.sub.2 has two opposing principal surfaces, the third principal surface MS.sub.3 and the fourth principal surface MS.sub.4. The third conductive gate G.sub.3 is formed opposite the third principal surface MS.sub.3 of the second semiconductor channel layer Ch.sub.2 with a third barrier layer BL.sub.3 interposed therebetween. Likewise, the fourth conductive gate G.sub.4 is formed opposite the fourth principal surface MS.sub.4 of the second semiconductor channel layer Ch.sub.2 with a third barrier layer BL.sub.4 interposed therebetween.
The fourth conductive layer L.sub.4 is connected to the second conductive gate G.sub.2. In Embodiment 3, the fourth conductive layer L.sub.4 and the second conductive gate G.sub.2 are common. The first conductive gate G.sub.1 and the third conductive gate G.sub.3 are connected to a first memory-cell-selection line (for example, a word line). The first conductive layer L.sub.1 and the third conductive layer L.sub.3 are connected to a second memory-cell-selection line (for example, a bit line). The second conductive layer L.sub.2 is connected to a fixed potential including zero potential. The fourth conductive gate G.sub.4 is connected to a read/write selection line.
The semiconductor memory cell of Embodiment 3 is different from the semiconductor memory cell of Embodiment 1 in that the switching transistor TR.sub.2 has the fourth conductive gate G.sub.4. This serves to further stabilize the operation of the switching transistor TR.sub.2 and yet allows further miniaturization.
When the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are both n-type transistors, the operation of the semiconductor memory cell is the same as that described in Embodiment 1 (see Table 1), and therefore, detailed explanation thereof is not repeated here.
A modified example of the semiconductor memory cell of Embodiment 3 illustrated in FIG. 8 is shown schematically in cross section in FIG. 9. In the semiconductor memory cell shown in FIG. 8, the second conductive gate G.sub.2 and the fourth conductive layer L.sub.4 are common. By contrast, in the semiconductor memory cell shown in FIG. 9, the first conductive gate G.sub.1 and the third conductive gate G.sub.3 are formed in the same substrate surface. The first conductive gate G.sub.1 and the third conductive gate G.sub.3 may be connected to each other. While a slightly larger plan area is required than that of the memory cell shown in FIG. 8, the memory cell shown in FIG. 9 has the advantage that the first semiconductor channel layer Ch.sub.1 and the second semiconductor channel layer Ch.sub.2 can be formed using a single high-quality semiconductor layer whereas two such layers are needed in the structure of the semiconductor memory cell shown in FIG. 8. There is a further advantage that the first conductive gate G.sub.1 and the third conductive gate G.sub.3 can be made common.
Embodiment 4
Embodiment 4 is concerned with a preferred mode of the semiconductor memory cell according to the second aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 10 and a cross section of a portion of which is shown schematically in FIG. 11, comprises an information storage transistor TR.sub.1 and a switching transistor TR.sub.2. The structures of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are fundamentally the same as those of Embodiment 3, except for the points hereinafter described.
The information transistor TR.sub.1 is formed from a transistor of a first conductivity type, for example, a p-type transistor, and the switching transistor TR.sub.2 is formed from a transistor of the opposite conductivity type to that of the information storage transistor TR.sub.1, for example, an n-type transistor. In this case, if the first conductive layer L.sub.1 and the second conductive layer L.sub.2 are formed from semiconductor material, then their conductivity type should be p type. Likewise, if the first conductive gate G.sub.1, the second conductive gate G.sub.2, the third conductive gate G.sub.3, the fourth conductive gate G.sub.4, the third conductive layer L.sub.3, and the fourth conductive layer L.sub.4 are formed from semiconductor material, then their conductivity type should be n.sup.+ type. The fourth conductive gate G.sub.4 is connected to a second fixed potential including zero potential.
Further, the first conductive layer L.sub.1 is connected to the second line via an n-type fifth conductive layer L.sub.5 which forms a rectifier junction with the p-type first conductive layer L.sub.1.
A cross section of a portion of the semiconductor memory cell of Embodiment 4 is shown schematically in FIG. 11. The semiconductor memory cell of Embodiment 3 is fundamentally the same as the semiconductor memory cell shown in FIG. 9, except that the first and second conductive layers, L.sub.1 and L.sub.2, are of different conductivity type and that the first conductive layer L.sub.1 is connected to the second line via the n.sup.+ fifth conductive layer L.sub.5 which forms a rectifier junction with the p-type first conductive layer L.sub.1. In FIG. 11, the fifth conductive layer L.sub.5 and the rectifier junction between the first conductive layer L.sub.1 and the fifth conductive layer L.sub.5 are shown in schematic form for simplicity. It will also be appreciated that the first conductive gate G.sub.1 and the third conductive gate G.sub.3 may be connected to each other.
When the information storage transistor TR.sub.1 is formed from a p-type transistor and the switching transistor TR.sub.2 from an n-type transistor, the operation of the semiconductor memory cell is the same as that described in Embodiment 2 (see Table 2), and therefore, detailed explanation thereof is not repeated here.
Embodiment 5
Embodiment 5 is concerned with a semiconductor memory cell according to a third aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 12(A) and a cross section of a portion of which is shown schematically in FIG. 12(B), comprises an information storage transistor TR.sub.1 formed from a field-effect transistor of a first conductivity type (for example, n type) and a switching transistor TR.sub.2 formed from a field-effect transistor of a second conductivity type (for example, p type).
The information storage transistor TR.sub.1 comprises a first channel forming region Ch.sub.1 of the second conductivity type, a first conductive gate G.sub.1 formed above the first semiconductor channel forming region Ch.sub.1 with a first barrier layer interposed therebetween, and first and second conductive regions, SC.sub.1 and SC.sub.2, separated by the first conductive gate G.sub.1. The first conductive gate G.sub.1 is formed in such a manner as to bridge the first conductive region SC.sub.1 and the second conductive region SC.sub.2.
The switching transistor TR.sub.2 comprises a second semiconductor channel forming region Ch.sub.2 of the first conductivity type, a second conductive gate G.sub.2 formed above the second semiconductor channel forming region Ch.sub.2 with a second barrier layer interposed therebetween, a third conductive region SC.sub.3, and a fourth conductive region SC.sub.4. The second conductive gate G.sub.2 is formed in such a manner as to bridge the third conductive region SC.sub.3 and the fourth conductive region SC.sub.4.
More specifically, the conductive regions are the source and drain regions. The first and second conductive regions, SC.sub.1 and SC.sub.2, are each formed from a low-resistivity semiconductor of the opposite conductivity type to that of the first semiconductor channel forming region Ch.sub.1, or from a metal or a silicide that forms a rectifier junction with the first semiconductor channel forming region Ch.sub.1. Likewise, the third and fourth conductive regions, SC.sub.3 and SC.sub.4, are each formed from a low-resistivity semiconductor of the opposite conductivity type to that of the second semiconductor channel forming region Ch.sub.2, or from a metal or a silicide that forms a rectifier junction with the second semiconductor channel forming region Ch.sub.2.
The first conductive gate G.sub.1 of the storage transistor TR.sub.1 and the second conductive gate G.sub.2 of the switching transistor TR.sub.2 are connected to a first memory-cell-selection line (for example, a word line). The fourth conductive region SC.sub.4 of the switching transistor TR.sub.2 is connected to, or formed common with, the channel forming region Ch.sub.1 of the storage transistor TR.sub.1. The third conductive region SC.sub.3 of the switching transistor TR.sub.2 is connected to a second memory-cell-selection line (for example, a bit line. The first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected to a read line. The second conductive region SC.sub.2 is connected to a fixed potential including zero potential.
Potentials applied to the various lines for a memory write are designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.w
Second memory-cell-selection line (e.g., bit line)
"0" write: V.sub.0
"1" write: V.sub.1
Read potential is designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.R
Also, for a memory read, the potential of the read line to which the first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected is designated as follows:
Read line potential: V.sub.2
The threshold voltages of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 for read operations are designated as follows:
"0" read: V.sub.TH1.sbsb.--.sub.0
"1" read: V.sub.TH1.sbsb.--.sub.1 The potential of the channel forming region Ch.sub.1 is different between a "0" read and a "1" read. As a result, the threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 differs between a "0" read and a "1" read. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM shown in FIG. 17.
The potential of the information storage transistor TR.sub.1 is set to satisfy the following relationship.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
The operation of the semiconductor memory cell of Embodiment 5 will be described below.
Information write
When writing information "0" (second line potential: V.sub.0) or "1" (second line potential: V.sub.1), the potential of the first line is set at V.sub.W (<0). As a result, the potential at the second conductive gate G.sub.2 of the switching transistor TR.sub.2 is also V.sub.W (<0). Therefore, the switching transistor TR.sub.2 is ON. Consequently, the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 (when writing information "1". Or V.sub.W -V.sub.TH2 when .linevert split.V.sub.W .linevert split.<.linevert split.V.sub.1 +v.sub.TH2 .linevert split.).
When writing information, the potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.W (<0). Therefore, the information storage transistor TR.sub.1 is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 or V.sub.W -V.sub.TH2 (when writing information "1"). This condition changes with time because of leakage currents (between the substrate and the semiconductor channel forming region Ch.sub.1 of the information storage transistor TR.sub.1, off currents of the switching transistor TR.sub.2, etc.), but is maintained within an allowable range until the information is read out. More specifically, during the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are set at such potentials that neither transistor will conduct. Furthermore, the so-call refresh operation is performed before the change in the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 becomes large enough to cause errors in read operations.
Information read
When reading information "0" or "1", the potential of the first line is V.sub.R (>0). As a result, the potential at the second conductive gate G.sub.2 of the switching transistor TR.sub.2 is V.sub.R (>0), and the switching transistor TR.sub.2 is OFF.
The potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.R (>0). Also, the threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 is V.sub.TH1.sbsb.--.sub.0 or V.sub.TH1.sbsb.--.sub.1. The threshold voltage of the information storage transistor TR.sub.1 is dependent on the potential of the channel forming region Ch.sub.1. The following relationship exists between these threshold voltages.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
Therefore, when the stored information is "0", the information storage transistor TR.sub.1 is ON. On the other hand, when the stored information is "1", the information storage transistor TR.sub.1 is OFF.
Thus, the information storage transistor TR.sub.1 is set to an ON or OFF condition, depending on the stored information. Since the first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected to the read line or the second line, current flows or does not flow to the information storage transistor TR.sub.1, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR.sub.1. A mode in which the first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected to the second line will be described hereinafter.
The above-described operating conditions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are summarized in Table 3. The potential values given in Table 3 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.
TABLE 3______________________________________ Unit: voltsMemory write "0" write "1" write______________________________________1st line potential V.sub.W -3.0 V.sub.W -3.02nd line potential V.sub.0 0 V.sub.1 -2.02nd gate potential V.sub.W -3.0 V.sub.W -3.0Condition of TR.sub.2 ON ONPotential of channel V.sub.0 0 V.sub.1 -2.0forming region1st gate potential V.sub.W -3.0 V.sub.W -3.0Condition of TR.sub.1 OFF OFF______________________________________Memory read "0" read "1" read______________________________________1st line potential V.sub.R 1.0 V.sub.R 1.02nd gate potential V.sub.R 1.0 V.sub.R 1.0Condition of TR.sub.2 OFF OFFPotential of channel V.sub.0 0 V.sub.1 -2.0forming regionThreshold of TR.sub.1 V.sub.TH1-0 0.5 V.sub.TH1-1 1.1required at 1st gate1st gate potential 1.0 1.0Condition of TR.sub.1 ON OFFRead line potential 0.5 V* 0.5 V*______________________________________ *2nd line potential: 1 V for FIG. 12(c)
In Embodiment 5, it is possible to omit the read line. More specifically, the read line may be formed common with the second line. In this case, the first conductive region SC.sub.1 is connected to the second line via a p-n junction or a Schottky junction type rectifying diode D.sub.2, as indicated by a dotted line in FIG. 12(B). This semiconductor memory cell structure can be realized, for example, in the structure shown in FIG. 12(C). In the structure shown in FIG. 12(C), the diode D.sub.2 is formed by forming the first conductive region SC.sub.1 from a semiconductor and forming the third conductive region SC.sub.3 in the surface area thereof, the conductivity type of the third conductive region SC.sub.3 being opposite to that of the first conductive region SC.sub.1. Furthermore, in the semiconductor memory cell shown in FIG. 12(C), the first semiconductor channel forming region Ch.sub.1 and the fourth conductive region SC.sub.4 are formed of a common region. Likewise, the first conductive region SC.sub.1 and the second semiconductor channel forming region Ch.sub.2 are formed of a common region.
In the thus structured semiconductor memory cell, the readout voltage applied to the second line must be small enough that a large forward current will not flow across the junction between the first conductive region SC.sub.1 and the third. conductive region SC.sub.3 (0.4 V or less in the case of a p-n junction); otherwise, latch-up may occur. One way to prevent the latch-up condition is to form the third conductive region SC.sub.3 from a silicide or a metal so that the third conductive region SC.sub.3 and the first conductive region SC.sub.1 form a junction such as a Schottky junction where majority carriers essentially form the forward current.
As a structure that can utilize the conventional MOS technology, the third conductive region SC.sub.3 may be formed, as shown in FIG. 13, for example, from a p-type semiconductor region SC.sub.3P and a metal layer of Mo, Al, etc. or a silicide layer, SC.sub.3S, that can form a Schottky junction with the first conductive region SC.sub.1. In the semiconductor memory cell shown in FIG. 13, the first conductive gate G.sub.1 and the second conductive gate G.sub.2 are formed common. Also, the reference sign E designates an electrode Al or Al/TiN/Ti triple layer formed on the third conductive region SC.sub.3, and the first conductive region SC.sub.1 is formed, for example, from an n-type semiconductor.
Thus, this embodiment provides the structure that can also be employed in CMOS fabrication, and therefore, the structure, among others, the structure shown in FIG. 12(B), is particularly suitable for DRAMs integrated in CMOS ASICs.
Embodiment 6
Embodiment 6 is concerned with a semiconductor memory cell according to a fourth aspect of the invention. The semiconductor memory cell of Embodiment 6, the principle of operation of which is shown in FIG. 14(A) and a cross section of a portion of which is shown schematically in FIG. 13(B), comprises an information storage transistor TR.sub.1 of a first conductivity type (for example, n type) and a switching transistor TR.sub.2 of a second conductivity type (for example, p type) opposite to the first conductivity type.
The information storage transistor TR.sub.1 comprises a first conductive gate G.sub.1, first and second conductive regions, SC.sub.1 and SC.sub.2, formed, for example, from an n.sup.+ semiconductor, and a first semiconductor channel forming region Ch.sub.1 (common with a p-well in the example shown in FIG. 14(B)). The first conductive region SC.sub.1 and the second conductive region SC.sub.2 each form a rectifier junction with the surface area of the first semiconductor channel forming region Ch.sub.1. The first conductive gate G.sub.1 is formed opposite a first principal surface of the first semiconductor channel forming region Ch.sub.1, with a first barrier layer interposed therebetween, in such a manner as to bridge the first and second conductive regions SC.sub.1 and SC.sub.2.
The switching transistor TR.sub.2 comprises a second conductive gate G.sub.2, third and fourth conductive regions, SC.sub.3 and SC.sub.4, formed, for example, from a p.sup.+ semiconductor, and a second semiconductor channel forming region Ch.sub.2. The third conductive region SC.sub.3 and the fourth conductive region SC.sub.4 are connected to the second semiconductor channel forming region Ch.sub.2. The second conductive gate G.sub.2 is formed opposite a second principal surface of the second semiconductor channel forming region Ch.sub.2 with a second barrier layer interposed therebetween.
The first conductive gate G.sub.1 of the information storage transistor TR.sub.1 and the second conductive gate G.sub.2 of the switching transistor TR.sub.2 are connected to a first memory-cell-selection line (for example, a word line). The fourth conductive region SC.sub.4 of the switching transistor TR.sub.2 is connected to the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1. The third conductive region SC.sub.3 of the switching transistor TR.sub.2 is connected to a second memory-cell-selection line (for example, a bit line). The second conductive region SC.sub.2 of the information storage transistor TR.sub.1 is connected to a fixed potential. The first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected to the third conductive region SC.sub.3 of the switching transistor TR.sub.2, forming at their boundary a rectifier junction of p-n junction or Schottky junction type. With the impurity concentration in the first conductive region SC.sub.1 at the junction adjusted to 2.times.10.sup.19 cm.sup.-3 or less, the dielectric strength or leakage current characteristic at the rectifier junction can be improved.
Potentials applied to the various lines for a memory write are designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.W
Second memory-cell-selection line (e.g., bit line)
"0" write: V.sub.0
"1" write: V.sub.1
Read potential is designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.R
Also, for a memory read, the fixed potential to which the second conductive region SC.sub.2 of the information storage transistor TR.sub.1 is connected is designated as follows:
Fixed potential: V.sub.2
The threshold voltages of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 for read operations are designated as follows:
"0" read: V.sub.TH1.sbsb.--.sub.0
"1" read: V.sub.TH1.sbsb.--.sub.1
The potential of the channel forming region Ch.sub.1 is different between a "0" read and a "1" read. As a result, the threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 differs between a "0" read and a "1" read. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM.
The potential of the information storage transistor TR.sub.1 is set to satisfy the following relationship.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
The operation of the semiconductor memory cell of Embodiment 6 will be described below.
Information write
When writing information "0" (second line potential: V.sub.0) or "1" (second line potential: V.sub.1), the potential of the first line is set at V.sub.W (<0). As a result, the potential at the second conductive gate G.sub.2 of the switching transistor TR.sub.2 is also V.sub.W (<0). Therefore, the switching transistor TR.sub.2 is ON. Consequently, the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 (when writing information "1". Or V.sub.W -V.sub.TH2 when .linevert split.V.sub.W .linevert split.<.linevert split.V.sub.1 +V.sub.TH2 .linevert split.).
When writing information, the potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.W (<0). Therefore, the information storage transistor TR.sub.1 is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 or V.sub.W -V.sub.TH2 (when writing information "1"). This condition changes with time because of leakage currents (between the substrate and the semiconductor channel forming region Ch.sub.1 of the information storage transistor TR.sub.1, off currents of the switching transistor TR.sub.2, etc.), but is maintained within an allowable range until the information is read out. More specifically, during the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are set at such potentials that neither transistor will conduct. Furthermore, the so-call refresh operation is performed before the change in the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 becomes large enough to cause errors in read operations.
The first conductive region SC.sub.1 of the information storage transistor TR.sub.1 is connected to the third conductive region SC.sub.3 of the switching transistor TR.sub.2, forming at their boundary a rectifier junction of p-n junction or Schottky junction type. This perfectly prevents the current flow to the first conductive region SC.sub.1 during the writing of information.
Information read
When reading information "0" or "1", the potential of the first line is V.sub.R (>0). As a result, the potential at the second conductive gate G.sub.2 of the switching transistor TR.sub.2 is V.sub.R (>0), and the switching transistor TR.sub.2 is OFF.
The potential at the first conductive gate G.sub.1 of the information storage transistor TR.sub.1 is V.sub.R (>0). Also, the threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 is V.sub.TH1.sbsb.--.sub.0 or V.sub.TH1.sbsb.--.sub.1. The threshold voltage of the information storage transistor TR.sub.1 is dependent on the potential of the channel forming region Ch.sub.1. The following relationship exists between these threshold voltages.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
Therefore, when the stored information is "0", the information storage transistor TR.sub.1 is ON. On the other hand, when the stored information is "1", the information storage transistor TR.sub.1 is OFF.
Thus, the information storage transistor TR.sub.1 is set to an ON or OFF condition, depending on the stored information. Since the second conductive region SC.sub.2 of the information storage transistor TR.sub.1 is connected to the fixed potential, current flows or does not flow to the information storage transistor TR.sub.1, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR.sub.1. The above-described operating conditions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are summarized in Table 4. The potential values given in Table 4 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.
TABLE 4______________________________________ Unit: voltsMemory write "0" write "1" write______________________________________1st line potential V.sub.W -2.5 V.sub.W -2.52nd line potential V.sub.0 0 V.sub.1 -2.02nd gate potential V.sub.W -2.5 V.sub.W -2.5Condition of TR.sub.2 ON ONPotential of TR.sub.1 V.sub.0 0 V.sub.1 -1.0channel formingregion1st gate potential V.sub.W -2.5 V.sub.W -2.5Condition of TR.sub.1 OFF OFF______________________________________Memory read "0" read "1" read______________________________________1st line potential V.sub.R 0.9 V.sub.R 0.92nd gate potential V.sub.R 0.9 V.sub.R 0.9Condition of TR.sub.2 OFF OFFPotential of TR.sub.1 V.sub.0 0 V.sub.1 -2.0channel formingregionThreshold of TR.sub.1 V.sub.TH1-0 0.5 V.sub.TH1-1 1.1required at 1st gate1st gate potential 0.9 0.9Condition of TR.sub.1 ON OFF2nd line potential V.sub.2 (.apprxeq.1.0) V.sub.2 (.apprxeq.1.0)______________________________________
FIG. 14(C) shows a modified example of the semiconductor memory cell of Embodiment 6. In the semiconductor memory cell shown in FIG. 14(C), an auxiliary gate (a third conductive gate) G.sub.3 is formed opposite the second conductive gate G.sub.2 and facing a third principal surface of the second semiconductor channel forming region Ch.sub.2 with a third barrier layer interposed therebetween. The auxiliary gate G.sub.3 has the function of shielding the semiconductor forming region Ch.sub.2 from potentials induced at and ions adhering to the surface of the semiconductor memory cell, thereby ensuring stable operation of the semiconductor memory cell. Furthermore, by connecting the auxiliary gate G.sub.3 to the read/write selection line, read or write operating margins can be improved. The semiconductor memory cell thus constructed has further improved characteristics as compared with the semiconductor memory cell whose principle of operation is shown in FIG. 4.
Embodiment 7
Embodiment 7 is concerned with a semiconductor memory cell according to a fifth aspect of the invention. The semiconductor memory cell of Embodiment 7, the principle of operation of which is shown in FIG. 15(A) and a cross section of a portion of which is shown schematically in FIG. 15(B), comprises: a first semiconductor region SC.sub.1 of a first conductivity type (for example, n type); a first conductive region SC.sub.2 formed from a material of a second conductivity type opposite to the first conductivity type (for example, p.sup.+) or from other material such as a metal or a silicide that forms a rectifier junction with the first semiconductor region SC.sub.1 ; a second semiconductor region SC.sub.3 of the second conductivity type (for example, p.sup.+ type); a second conductive region SC.sub.4 formed from a material of the first conductivity type (for example, n.sup.+) or from other material such as a metal or a silicide that forms a rectifier junction with the second semiconductor region SC.sub.3 ; and a conductive gate G disposed in such a manner as to form a bridge over a barrier layer between the first semiconductor region SC.sub.1 and the second conductive region SC.sub.4 and between the first conductive region SC.sub.2 and the second semiconductor region SC.sub.3.
The first semiconductor region SC.sub.1 is formed in the surface area of a semiconductor substrate or on an insulating substrate. The first conductive region SC.sub.2 is formed in the surface area of the first semiconductor region SC.sub.1. The second semiconductor region SC.sub.3 is formed in the surface area of the first semiconductor region SC.sub.1 but spaced apart from the first conductive region SC.sub.2. The second conductive region SC.sub.4 is formed in the surface area of the second semiconductor region SC.sub.3.
The conductive gate G is connected to a first memory-cell-selection line (for example, a word line). The first conductive region SC.sub.2 is connected to a write information setting line. The second conductive region SC.sub.4 is connected to a second memory-cell-selection line (for example, a bit line).
The first semiconductor region SC.sub.1 (corresponding to the channel forming region Ch.sub.2), the first conductive region SC.sub.2 (corresponding to a source/drain region), the second semiconductor region SC.sub.3 (corresponding to a source/drain region), and the conductive gate G constitute the switching transistor TR.sub.2. Likewise, the second semiconductor region SC.sub.3 (the surface area below the conductive gate G corresponds to the channel forming region Ch.sub.1 (p)), the first semiconductor region SC.sub.1 (corresponding to a source/drain region), the second conductive region SC.sub.4 (corresponding to a source/drain region), and the conductive gate G constitute the information storage transistor TR.sub.1.
Potentials applied to the various lines for a memory write are designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.W
Write information setting line
"0" write: V.sub.0
"1" write: V.sub.1
Read potential is designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.R
Also, for a memory read, the potential of the second memory-cell-selection line (for example, bit line) to which the second conductive region SC.sub.4 is connected is designated as follows:
Second memory-cell-selection line: V.sub.2
A fixed potential, including zero potential, is applied to the first semiconductor region SC.sub.1.
The threshold voltages of the information storage transistor TR.sub.1 required at the conductive gate G for read operations are designated as follows:
"0" read: V.sub.TH1.sbsb.--.sub.0
"1" read: V.sub.TH1.sbsb.--.sub.1
The potential of the channel forming region Ch.sub.1 is different between a "0" read and a "1" read. As a result, the threshold voltage of the information storage transistor TR.sub.1 at the conductive gate G differs between a "0" read and a "1" read. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM.
The potential of the information storage transistor TR.sub.1 is set to satisfy the following relationship.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
The operation of the semiconductor memory cell of Embodiment 7 will be described below.
Information write
When writing information "0" (write information setting line potential: V.sub.0) or "1" (write information setting line potential: V.sub.1), the potential of the first line is set at V.sub.W (<0). As a result, the potential at the conductive gate G of the switching transistor TR.sub.2 is also V.sub.W (<0). Therefore, the switching transistor TR.sub.2 is ON. Consequently, the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 (when writing information "1". Or V.sub.W -V.sub.TH2 when .linevert split.V.sub.W .linevert split.<.linevert split.V.sub.1 +V.sub.TH2 .linevert split.).
During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are set at such potentials that neither transistor will conduct. To achieve this, the potential of the first line should be set at 0 (V) and the potential of the second line at V.sub.1, for example.
When writing information, the potential at the conductive gate G of the information storage transistor TR.sub.1 is V.sub.W (<0). Therefore, the information storage transistor TR.sub.1 is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is V.sub.0 (when writing information "0") or V.sub.1 or V.sub.W -V.sub.TH2 (when writing information "1"). This condition changes with time because of leakage currents (between the first semiconductor region SC.sub.1 and the semiconductor channel forming region Ch.sub.1 of the information storage transistor TR.sub.1, off currents of the switching transistor TR.sub.2, etc.), but is maintained within an allowable range until the information is read out. The so-call refresh operation is performed before the change in the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 becomes large enough to cause errors in read operations.
Information read
When reading information "0" or "1", the potential of the first line is V.sub.R (>0). As a result, the potential at the conductive gate G of the switching transistor TR.sub.2 is V.sub.R (>0), and the switching transistor TR.sub.2 is OFF.
The potential at the conductive gate G of the information storage transistor TR.sub.1 is V.sub.R (>0). Also, the threshold voltage of the information storage transistor TR.sub.1 required at the conductive gate G is V.sub.TH1.sbsb.--.sub.0 or V.sub.TH1.sbsb.--.sub.1. The threshold voltage of the information storage transistor TR.sub.1 is dependent on the potential of the channel forming region Ch.sub.1. The following relationship exists between these potentials.
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
Therefore, when the stored information is "0", the information storage transistor TR.sub.1 is ON. On the other hand, when the stored information is "1", the information storage transistor TR.sub.1 is OFF.
Thus, the information storage transistor TR.sub.1 is set to an ON or OFF condition, depending on the stored information. Since the second conductive region SC.sub.4 is connected to the second line, current flows or does not flow to the information storage transistor TR.sub.1, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR.sub.1.
The above-described operating conditions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are summarized in Table 5. The potential values given in Table 5 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.
TABLE 5______________________________________ Unit: voltsMemory write "0" write "1" write______________________________________1st line potential V.sub.W -3.0 V.sub.W -3.0Potential of write V.sub.0 0 V.sub.1 -2.0information settinglineGate potential V.sub.W -3.0 V.sub.W -3.0Condition of TR.sub.2 ON ONPotential of channel V.sub.0 0 V.sub.1 -2.0forming regionCondition of TR.sub.1 OFF OFF______________________________________Memory read "0" read "1" read______________________________________1st line potential V.sub.R 1.0 V.sub.R 1.0Gate potential V.sub.R 1.0 V.sub.R 1.0Condition of TR.sub.2 OFF OFFPotential of channel V.sub.0 0 V.sub.1 -2.0forming regionThreshold of TR.sub.1 V.sub.TH1-0 0.5 V.sub.TH1-1 1.1required at 1st gateCondition of TR.sub.1 ON OFF2nd line potential 0.5 0.5______________________________________
Embodiment 8
Embodiment 8 is concerned with a semiconductor memory cell according to a sixth aspect of the invention. The semiconductor memory cell of Embodiment 8, the principle of operation of which is shown in FIG. 16(A) and a cross section of a portion of which is shown schematically in FIG. 16(B), comprises: a first semiconductor region SC.sub.1 of a first conductivity type (for example, n type); a first conductive region SC.sub.2 formed from a material of a second conductivity type opposite to the first conductivity type (for example, p.sup.+) or from other material such as a metal or a silicide that forms a rectifier junction with the first semiconductor region SC.sub.1); a second semiconductor region SC.sub.3 of the second conductivity type (for example, p.sup.+ type); a second conductive region SC.sub.4 formed from a material of the first conductivity type (for example, n.sup.+) or from other material such as a metal or a silicide that forms a rectifier junction with the second semiconductor region SC.sub.3); and a conductive gate G formed in such a manner as to bridge the second conductive region SC.sub.4 and a portion of the first semiconductor region SC.sub.1 flanked by the second semiconductor region SC.sub.3 and the first conductive region SC.sub.2 and also bridge the first conductive region SC.sub.2 and the second semiconductor region SC.sub.3. The semiconductor memory cell shown in FIG. 16(B) is depicted in more detail in FIG. 16(C). For further embodiment of the semiconductor memory cell structure shown in FIG. 15(B), part of the structure of the semiconductor memory cell structure shown in FIG. 16(C) can be applied to the structure shown in FIG. 15(B).
The first semiconductor region SC.sub.1 is formed on a semiconductor substrate as an isolated region in the surface area thereof, or on an insulating substrate as a region attached to a support substrate with an insulating layer such as an SiO.sub.2 layer interposed therebetween. The first conductive region SC.sub.2 is formed in the surface area of the first semiconductor region SC.sub.1. The second semiconductor region SC.sub.3 is formed in the surface area of the first semiconductor region SC.sub.1 but spaced apart from the first conductive region SC.sub.2. The second conductive region SC.sub.4 is formed in the surface area of the second semiconductor region SC.sub.3.
The conductive gate G is connected to a first-memory-cell selection line (for example, a word line). The first semiconductor region SC.sub.1 is connected to a second memory-cell-selection line (for example, a bit line). The first conductive region SC.sub.2 is connected to a first fixed potential. The second conductive region SC.sub.4 is connected to a second fixed potential.
The first semiconductor region SC.sub.1 (corresponding to the channel forming region Ch.sub.2), the first conductive region SC.sub.2 (corresponding to a source/drain region), the second semiconductor region SC.sub.3 (corresponding to a source/drain region), and the conductive gate G constitute the switching transistor TR.sub.2. Likewise, the second semiconductor region SC.sub.3 (the surface area below the conductive gate G corresponds to the channel forming region Ch.sub.1 (p)), the first semiconductor region SC.sub.1 (corresponding to a source/drain region), the second conductive region SC.sub.4 (corresponding to a source/drain region), and the conductive gate G constitute the information storage transistor TR.sub.1.
The first fixed potential to which the first conductive region SC.sub.2 is connected is designated as V.sub.3 (.ltoreq.0). Likewise, the second fixed potential to which the second conductive region SC.sub.4 is connected is designated as V.sub.4 (.gtoreq.0).
Potentials applied to the various lines for a memory write are designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.W
Second memory-cell-selection line (e.g., bit line)
"0" write: V.sub.0
"1" write: V.sub.1
Likewise, the threshold voltages of the switching transistor TR.sub.2 required at the conductive gate for write operations are designated as follows:
"0" write: V.sub.TH2W.sbsb.--.sub.0
"1" write: V.sub.TH2W.sbsb.--.sub.1
Read potential is designated as follows:
First memory-cell-selection line (e.g., word line): V.sub.R
Also, the threshold voltages of the information storage transistor TR.sub.1 required at the conductive gate G for read operations are designated as follows:
"0" read: V.sub.TH1R.sbsb.--.sub.0
"1" read: V.sub.TH1E.sbsb.--.sub.1
Generally, the potentials of the channel forming regions Ch.sub.1 and Ch.sub.2 are different between a "0" read/write and a "1" read/write. As a result, the threshold voltages of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 required at the conductive gate G differ between a "0" read/write and a "1" read/write. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM. The potential at the conductive gate of the switching transistor TR.sub.2, relative to V.sub.W, is set as shown below, for example, for simplicity of explanation.
.linevert split.V.sub.W .linevert split.>.linevert split.V.sub.TH2W.sbsb.--.sub.1 .linevert split. or .linevert split.V.sub.TH2W.sbsb.--.sub.0 .linevert split. whichever larger
The potential at the conductive gate of the information storage transistor TR.sub.1, relative to V.sub.R, is set as follows:
.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.
The operation of the semiconductor memory cell of Embodiment 8 will be described below.
Information write
When writing information "0" (second line potential: V.sub.0) or "1" (second line potential: V.sub.1), the potential of the first line is set at V.sub.W (<0). As a result, the potential at the conductive gate G of the switching transistor TR.sub.2 is also V.sub.W (<0). V.sub.W is
.linevert split.V.sub.W .linevert split.>.linevert split.V.sub.TH2W.sbsb.--.sub.1 .linevert split. or .linevert split.V.sub.TH2W.sbsb.--.sub.0 .linevert split. whichever larger
Therefore, for a write operation, the switching transistor TR.sub.2 is ON. Consequently, the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is
"0" information write: V.sub.3
"1" information write: V.sub.3
When writing information, the potential at the conductive gate G of the information storage transistor TR.sub.1 is V.sub.W (<0). Therefore, the information storage transistor TR.sub.1 is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 is V.sub.3 for both "0" and "1" information write. During the retention period, since TR.sub.2 is also OFF, when the second line potential at this time is denoted by V.sub.5 (.linevert split.V.sub.5 .linevert split..ltoreq..linevert split.V.sub.0 .linevert split. or .linevert split.V.sub.1 .linevert split.) the potential of the first semiconductor channel Ch.sub.1 is .gamma.{V.sub.3 -(V.sub.0 -V.sub.5)} or .gamma.{V.sub.3 -(V.sub.1 -V.sub.5)}, where .gamma. is the ratio of the capacitance between the first semiconductor region SC.sub.1 and the second semiconductor region SC.sub.3 to the total capacitance between the second semiconductor region SC.sub.3 and the other areas (including the first semiconductor region SC.sub.1) than the second semiconductor region SC.sub.3. This condition changes with time because of leakage currents (between the first semiconductor region SC.sub.1 and the semiconductor channel forming region Ch.sub.1 of the information storage transistor TR.sub.1, off currents of the switching transistor TR.sub.2, etc.), but is maintained within an allowable range until the information is read out.
During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are set at such potentials that neither transistor will conduct. To achieve this the potential of the first line should be set at 0 (V) and the potential of the second line at V.sub.5, for example. The so-called refresh operation is performed before the change in the potential of the channel forming region Ch.sub.1 of the information storage transistor TR.sub.1 becomes large enough to cause errors in read operations.
Information read
When reading information "0" or "1", the potential of the first line is V.sub.R (>0), and the potential of the second line is V.sub.6 (.linevert split.V.sub.6 .linevert split..ltoreq..linevert split.V.sub.0 .linevert split. or .linevert split.V.sub.1 .linevert split.). As a result, the potential at the conductive gate G of the switching transistor TR.sub.2 is V.sub.R (>0), and the switching transistor TR.sub.2 is OFF.
The potential at the conductive gate G of the information storage transistor TR.sub.1 is V.sub.R (>0). Also, the threshold voltage of the information storage transistor TR.sub.1 required at the first conductive gate G.sub.1 is V.sub.TH1R.sbsb.--.sub.0 or V.sub.TH1R.sbsb.--.sub.1 which is given by the potential between the second semiconductor region SC.sub.3 and the second conductive region SC.sub.4 when .gamma.{V.sub.3 -(V.sub.0 -V.sub.6)} or .gamma.{V.sub.3 -(V.sub.1 -V.sub.6)} is met. The threshold voltage of the information storage transistor TR.sub.1 is dependent on the potential of the channel forming region Ch.sub.1. The following relationship exists between these potentials.
.linevert split.V.sub.TH1.sbsb.--.sub.0 .linevert split.>.linevert split.V.sub.R .linevert split.>.linevert split.V.sub.TH1.sbsb.--.sub.1 .linevert split.
Therefore, when the stored information is "0", the information storage transistor TR.sub.1 is OFF. On the other hand, when the stored information is "1", the information storage transistor TR.sub.1 is ON.
Thus, the information storage transistor TR.sub.1 is set to an ON or OFF condition, depending on the stored information. Since the second conductive region SC.sub.4 is connected to the second fixed potential, current flows or does not flow to the information storage transistor TR.sub.1, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR.sub.1.
The above-described operating conditions of the information storage transistor TR.sub.1 and the switching transistor TR.sub.2 are summarized in Table 6. The potential values given in Table 6 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.
TABLE 6______________________________________ Unit: voltsMemory write "0" write "1" write______________________________________1st line potential V.sub.W -2.0 V.sub.W -2.02nd line potential V.sub.0 2.0 V.sub.1 01st fixed potential V.sub.2 V.sub.2Gate potential V.sub.W -2.0 V.sub.W -2.0Thereshold of TR.sub.2 V.sub.TH2W-0 -1.2 V.sub.TH2W-1 -0.5required at gateCondition of TR.sub.2 ON ONPotential of TR.sub.1 1st fixed 1st fixedchannel forming potential potentialregion e.g. 0 e.g. 0Condition of TR.sub.1 OFF OFF______________________________________Memory read "0" read "1" read______________________________________1st line potential V.sub.R 1.0 V.sub.R 1.0Gate potential V.sub.R 1.0 V.sub.R 1.0Condition of TR.sub.2 OFF OFFPotential of TR.sub.1 -0.75 +0.25channel formingregionThreshold of TR.sub.1 V.sub.TH1R-0 0.85 V.sub.TH1R-1 0.5required at 1st gateCondition of TR.sub.1 OFF ON2nd line potential 0.5 0.52nd fixed potential 0.25 0.25______________________________________
The semiconductor memory cell of the invention has been described above in accordance with preferred embodiments, but it will be appreciated that the invention is not limited to those described preferred embodiments. For example, in the first and second aspects of the invention described in Embodiments 1 and 3, the information storage transistor TR.sub.1 and the switching transistor TR.sub.1 may be both formed from p-type transistors. Furthermore, in the preferred modes of the first and second aspects of the invention described in Embodiments 2 and 4, the information storage transistor TR.sub.1 may be formed from an n-type transistor and the switching transistor TR.sub.2 from a p-type transistor. Moreover, in the third to fifth aspects described in Embodiments 5 to 8, the information storage transistor TR.sub.1 may be formed from a p-type transistor and the switching transistor TR.sub.2 from an n-type transistor. It will also be appreciated that the arrangement of elements in each transistor shown is only illustrative and may be modified as needed.
The present invention is applicable not only to memory cells formed from silicon semiconductors but also to memory cells formed from compound semiconductors such as GaAs, for example.
In the semiconductor memory cell of the present invention, the operation of the information storage transistor is determined in depending relationship on the potential or charge (information) stored on the second conductive gate or in the channel forming region of the information storage transistor, and the information read out as the transistor current during refresh intervals is independent of the magnitude of the capacitance of a capacitor (for example, the capacitance of the second conductive gate plus added capacitance, etc.) if such a capacitor is added. This therefore serves to solve the problem of capacitor capacitance encountered in prior art semiconductor memory cells; if a capacitor is added for refresh interval adjustment, a very large capacitor such as those required in the prior art DRAMs is not required. Furthermore, the maximum area of the semiconductor memory cell is equal to or smaller than the area of two transistors.
In the semiconductor memory cell according to the first and second aspects of the invention, a conductive gate is provided opposite each of two principal surfaces of the semiconductor channel layer of the information storage transistor. This structure eliminates the problem of unstable transistor operation inherent in prior art transistor structures; that is, the structure of the invention is effective in stabilizing the operation of the information storage transistor, and facilitates short-channel transistor design. The first conductive gate and the third conductive gate are both connected to the first memory-cell-selection line. Accordingly, the first memory-cell-selection line need not be provided more than one, and the chip area can be reduced. Furthermore, depending on the structure, it is possible to form a memory cell within an area equal to one transistor area.
In the semiconductor memory cell according to the second aspect of the invention, a fourth gate is provided as an extra gate of the switching transistor. The provision of this extra gate serves to further stabilize the operation of the switching transistor.
In the preferred modes of the semiconductor memory cell according to the first and second aspects of the invention, a fifth conductive layer is provided. The provision of this fifth conductive layer prevents without fail a current from flowing to the information storage transistor during the information write operation.
The semiconductor memory cell fabrication process according to the third aspect of the invention is compatible with the fabrication process for CMOS ASICs, as shown, for example, in FIG. 12(B). The process therefore allows DRAM functions to be incorporated in an ASIC without requiring extra processing steps although the semiconductor memory cell area increases slightly. On the other hand, in the semiconductor memory cell structure shown in FIG. 12(C), for example, one extra processing step may be required as compared to the CMOS ASIC structure, but the semiconductor memory cell area can be reduced nearly by half.
According to the fourth aspect of the invention, the semiconductor memory cell can be implemented on an area approximately equal to one transistor area.
According to the fifth and sixth aspects of the invention, the semiconductor memory cell can be implemented on an area approximately equal to one transistor area by using conventional semiconductor memory cell fabrication techniques without relying on SOI technology.

Number	Date	Country
4-352200	Dec 1992	JPX
5-130130	May 1993	JPX
5-246264	Sep 1993	JPX

Number	Name	Date
4330849	Tagei et al.	May 1982
4384300	Iizuka	May 1983
5128731	Lien et al.	Jul 1992
5283457	Matloubian	Feb 1994

	Number	Date	Country
Parent	420068	Apr 1995
Parent	164812	Dec 1993

Semiconductor memory cell having information storage transistor and switching transistor

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