The present specification relates to a vertical floating body memory cell. Moreover, the present specification relates to transistors of such memory cells and a memory device comprising such memory cells and a special circuitry.
Floating body memory cells usually employ charge storage effects in an insulated floating semiconductor body. The floating body may be disposed between two regions. A first region may be connected to a bit line and a second region may be connected to a source line. A gate electrode is configured to switch a current between the two regions by a select voltage applied to the gate electrode. By applying a suitable write signal to at least one of the terminals of the memory cell, a charge may be injected in or removed from the floating body. By applying a suitable read signal to at least one of the terminals, an output signal may be caused in the bit line. The output signal depends on the amount and/or type of charge stored in the floating body region. Usually, floating body memory cells need less space and, accordingly, may be arranged at a very dense pitch. Nevertheless, due to leakage currents, the retention time of these floating body memory cells can be problematic. Accordingly, there is a need for providing floating body memory cells having an improved retention time.
The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles. Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers designate corresponding similar parts.
In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” “horizontal,” “vertical,” etc. is used with reference to the orientation of the figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims.
The transistor further comprises a third portion 14. The third portion 14 is disposed between the first and second doped portions 12, 13. The first doped portion 12 adjoins the third portion 14 and a junction 33b, and the second doped portion 13 adjoins the third portion 14 at a junction 33a. The first and second doped portions 12, 13 as well as the third portion 14 are disposed in the semiconductor substrate 1 comprising a main surface 10. The transistor further comprises a gate electrode 15. The gate electrode 15 is adjacent to the third doped portion 14 and is insulated from the third portion 14 by a gate dielectric 16. The gate electrode 15 does not overlap at least one of the first and second doped portions 12, 13. Moreover, a line connecting the first and the second doped portions extends in the direction that is approximately vertical with respect to the main surface 10 of the substrate. In the context of the present specification, the term main surface 10 refers to an approximately horizontal surface. In other words, a line extending between the first and second doped portions 12, 13 is substantially perpendicular to the substrate surface, with the second doped portion 13 being below the first doped portion.
Accordingly, there may be a distance d between a boundary of the gate electrode 15 and either of the junctions 33a, 33b. In the context of the present specification the terms junctions and boundary refer to the metallurgical junction of the first or the second doped region and the third doped region. The junction is close to a portion of a vertical surface of the doped region, the vertical surface being adjacent to the gate electrode of the transistor 34. Moreover, the boundary of the gate electrode 15 refers to the portion of the boundary that is close to the substrate surface. The gate electrode may be arranged in such a manner that it does not overlap the whole length L of the channel of the transistor. For example, the length L of the channel may be less than 100 nm. By way of further example, the length L of the channel may be less than 75 nm and may be approximately 50 nm.
According to an implementation of the present invention, the gate electrode 15 may be disposed in a symmetric manner with respect to the first and second doped portions 12, 13. According to an alternative implementation, the gate electrode 15 may be disposed in an asymmetric manner with respect to the first and second doped portions 12, 13. For example, one of the first and second doped portions 12, 13 may overlap the gate electrode. As is further shown in
The gate electrodes 15 may be disposed over the bottom fill 20. Moreover, a top fill 21 which may be made of an insulating material that may be different or the same as the bottom fill is disposed over the gate electrodes 15. For example, segments of a cap layer 18 may be disposed over the first doped portions 12. The third portion 14 may be doped with a dopant having a conductivity type that is opposite to the conductivity type of the first and second doped portions, respectively. For example, the first and second doped portions 12, 13 may be n-doped, the third portion being p-doped, and vice versa. In the arrangement shown in
The first doped region 12 may be connected with a bit line 41, and the second doped portion 13 may be connected with a source line 42. Moreover, the gate electrodes 15 may be connected with a corresponding word line 43a, 43b. Upon application of suitable potentials to the bit lines 41, optionally the source lines 42 as well as optionally the word lines 43a, 43b, the transistor may be operated in a bipolar mode with doped portion 12, 13 acting as collector and emitter respectively and portion 14 acting as base of the bipolar transistor. For example, holes may be generated by impact ionization at top junction 33b, acting as a base current and hence, giving rise to an electron current flow between emitter and collector. In such a condition, the transistor action is influenced by bipolar effects so that a control of the transistor action via the field effect is of minor importance. As a consequence, an overlap of the gate electrode over the whole channel length is not required. Due to the distance d between the gate electrode 15 and any of the first and second doped portions, a gate induced drain leakage is reduced. As a consequence, the retention time of the transistor may be improved.
The source lines 42 may comprise a doped semiconductor material such as doped polysilicon or they may be made of another conductive material such as a metal. An insulating material 24 may be filled over the source line 42. In an upper portion of the isolation trenches 23, the bit lines 41 are disposed. The bit lines 41 may be made of a doped semiconductor material such as doped poly silicon or another conductive material such as a metal. On top of the bit lines 41, an arbitrary insulating 24 material may be disposed. In the arrangement shown in
Nevertheless, as is clearly to be understood, any arbitrary arrangement of source lines, bit lines, and word lines may be chosen. As is usual, the word lines should extend in a direction that is perpendicular to the direction to the bit lines. Accordingly, a memory cell may be formed at a cross point between word line and bit line. The source lines may extend parallel to the word lines or parallel to the bit lines. As a further alternative, the source line may be implemented as a so-called buried plate, for example a doped portion that connects a plurality of second doped portions 13. As a further alternative, also the word line may be implemented as a plate. In the context of the present specification, the term “plate” or “buried plate” refers to any conductive structure or pattern by which a plurality of memory cells may be simultaneously addressed. Accordingly, this term encompasses any type of grid, plate including holes or array of lines that are held at one common potential. The plate or buried plate usually is disposed in the substrate.
According to the implementation shown in
The gate electrode 55 is formed so as not to overlap with at least one of the first and the second doped portions 52, 54. Moreover, an intrinsic portion 60 may be disposed between the first doped portion 52 and the third portion 53. For example, there may be an overlap between the intrinsic portion and a gate electrode 55 or not. As is shown in
As is clearly to be understood, the basic concept underlying the structure shown in
In the arrangement shown in
The memory cells 68 may be implemented in the manner as has been described above with reference to
The gate electrode 75 may extend so that it does not reach the junction 79 between the first and the third doped portions, or so that it does not reach the second junction 80 between the second and third doped portions. As is clearly to be understood, the orientation shown in
Usually, for reading the information that is stored in the memory cell 68, a voltage is applied between the first and second doped portions of the memory cell. Additionally, a suitable voltage pulse at the gate electrode is applied. Depending on the charging state of the floating body, accumulated holes are released from the gate-to-body capacitor and a current is generated. In this case, an ignition may be obtained. The memory cell 78 may be interpreted in terms of a bipolar transistor. Accordingly, in this case, reading and writing may be accomplished in a manner that is schematically depicted in
In a standby mode the bit line, the word line, and the source line may be held at a reference voltage level. For writing a “1,” a voltage V1 may be applied to the bit line while holding a source line at a reference level. Moreover, a voltage W1 that is higher than the threshold voltage is applied to the gate electrode, inducing an electron current flowing between the first and the second doped portion. Due to the high voltage that is applied between the first and the second doped portions, impact ionization takes place at the first junction 79, causing a hole current to flow from the junction 79 towards the second doped region 72, acting as a base current for the intrinsic bipolar transistor formed from the three doped regions. In the case that impact ionization rate and bipolar current gain are high enough, the ignited bipolar current becomes approximately independent of the voltage that is supplied to the gate electrode. The voltage that is supplied to the gate electrode is decreased to a hold voltage below the threshold voltage of the transistor while the body potential remains at a high level. For example, the hold voltage may be equal to the reference voltage. Thereafter, the voltage that is applied to the bit line is set to the reference value. For performing a read operation, a voltage V2 is applied to the bit line. The voltage V2 may be selected in such a manner that it does not cause a punch-through but it is high enough so as to cause an impact ionization to take place at the first junction 79. The voltage V2 may be equal to the voltage V1. Moreover, for initializing the transistor action, a voltage pulse is needed at the gate electrode. Accordingly, after applying a voltage V2 to the bit line, a voltage W2 is applied to the gate electrode that is lower than the threshold voltage and lower than W1 of the transistor. After applying this voltage pulse, an ignition of the bipolar current may be caused in case a “1” is stored in the transistor body, whereas no ignition is caused in case a “0” is stored in the transistor body. For writing a “0,” majority carriers have to be removed from the transistor body 73. Accordingly, a voltage V3 is applied between the bit line and the source line, V3 being not suitable to cause an impact ionization at the first junction 79. For example, V3 may be 0 V or at a low level or V3 may be negative with respect to the source line. Moreover, a gate voltage is applied to the gate electrode, the gate voltage being higher than the threshold voltage of the transistor. Accordingly, the body to source diode becomes forward biased in order to remove carriers from the transistor body 73. The signal that is applied to the wordline may be equal to the signal for writing a “1.” After setting the voltage of the bit line to the reference value, the gate voltage is set to the hold voltage, i.e., the voltage that is needed to hold the signal. As has been shown in this example, the circuitry 69 shown in
As a further example, the memory cell 68 may be implemented in the manner as has been described above with reference to
Examples of timing diagrams of voltages that are applied to the bit line, the word line and the source line are shown in
Nevertheless, as is clearly to be understood, these timing diagrams are given by way of example only and there may be many modifications and variations in which the memory device 67 shown in
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.
This application claims priority under 35 U.S.C. 119(e) from U.S. Provisional Patent Application Ser. No. 61/021,488, filed Jan. 16, 2008, and entitled “Floating Body Memory Cell with a Non-Overlapping Gate Electrode,” the entire contents of which are hereby incorporated by reference.
Number | Date | Country | |
---|---|---|---|
61021488 | Jan 2008 | US |