TECHNICAL FIELD
The present invention relates to a static induction transistor, a method of manufacture thereof, and an electric power conversion apparatus using said static induction transistor.
BACKGROUND OF THE INVENTION
As electric power converters are required to be of large power and high frequency, and a semiconductor switching element used therein is required not only to have a large controllable current, but also to be of low loss and to be capable of operation at high speeds.
In order to respond such requirements, switching elements with a constituent element made of SiC (silicon carbide) have been proposed. For example, a power MOSFET has been studied, as disclosed in IEEE Electron Devices Letters, Vol. 18, No. 3, p. 93-95(1997), “High-Voltage Double-Implanted Power MOSFET's in 6H—SiC”. However, since an inversion layer with low carrier mobility is used for a channel layer forming a current passage, a problem exists in that the ON voltage of this power MOSFET becomes high.
In order to avoid this problem, there is a static induction transistor in which an inversion layer is not used as a channel layer (disclosed in, for example, IEEE Trans. on Electron Devices, Vol. ED-22, p. 185-197, 1975, “Field-effect Transistor versus Analog Transistor (Static Induction Transistor)).
FIG. 2 is a sectional view of a static induction transistor of the type known in the art. The semiconductor substrate comprises an n+-type region 1, an n−-type region 2 and a p-type region 5, as well as a source electrode 11, a drain electrode 12 and a gate electrode 13. The potential of the gate is made lower than that of the source, whereby a depletion layer is extended to a region provided by the p-type region 5, a so-called channel region, whereby a current flowing through the drain electrode 12 and the source electrode 11 can be turned off. Since a substrate made of SiC is used for the channel region, quite a low ON-resistance can be realized. This is reported, for example, in International Conference on Silicon Carbide, III-nitrides and Related Materials-1997, Abstract p. 443 (1997), “Electrical Characteristics of A Novel Gate Structure 4H—SiC Power Static Induction Transistor”.
In the basic structure in FIG. 2, however, a problem exists in that the OFF characteristics are very bad. That is, in order to turn a current off, a layer gate voltage must be applied. This is caused by the fact that the impurity diffusion coefficient of SiC is small. In silicon, when a deep p-type region is to be formed, thermal diffusion is used, but in SiC, this process can not be applied. As a result, a local p-type region, such as the p-type region 5, is formed by ion implantation, but even if boron with a relatively small atomic weight is implanted at an energy of about 2 Mev, which represents high energy implantation, the depth is about 2 pm at most. The implantation at a higher energy can make the junction deep, but a defect may remain which can not be removed by heat treatment afterward. As a result, a leakage current increases and the OFF characteristics become bad.
Also, when ion implantation with high energy is partially performed, it is difficult to form an implantation mask withstanding this state.
In FIG. 2, Xj is called the channel length, and Wch is called the channel width. In place of increasing Xj, means for decreasing the channel width Wch may be taken into consideration. In this case, however, Wch must be atomized significantly. Therefore, a problem exists in that the ON characteristics are significantly deteriorated.
In order to solve the above-mentioned problems, a structure is proposed based on the idea that a gate is constituted by a surface p-type region and an embedded p-type region and a channel is laid in the lateral direction. For example, Japanese Patent Laid-open 59-150474 discloses a specific example of this proposal as applied to a static induction thyristor, and FIG. 3 is a sectional view of a static induction transistor made of SiC based on this proposal. The semiconductor substrate comprises an n−-type region 1, an n−type region 2 and a p-type region 5, as well as a source electrode 11, a drain electrode 12 and a gate electrode 13. In this example, an n+-type source region 4 and a first gate region 5 of p type are provided on one main surface of the semiconductor substrate, and a second gate region 3 comprising a p-type embedded layer including projection parts of the n+-type source region 4 and the n+-type source region 5 is formed at a position deeper than both regions 4, 5. The second gate region 3 has a vacant longitudinal channel portion Wvch within a surface in parallel to the main surface. When the second gate region 3 has the same potential as the first gate region 5 and a negative potential with respect to the source electrode 11 is applied to the gate electrode 13, a current between the source and the drain can be turned off.
A difference in operation of this example from the previous example shown in FIG. 2 will be explained as follows. FIG. 4 is a sectional view of the example shown in FIG. 3 in the conduction state. In FIG. 4, numeral 21 designates a flow of electrons. In this case, electrons injected from the source electrode 11 flow through the channel in a lateral direction. And then, while turning in direction to the drain side, the electrons flow into the drain electrode 12. That is, the channel becomes aligned in the lateral direction. In this example, since the channel is in the lateral direction, the channel length is not limited by the ion implantation depth or the like, but can be adjusted freely by microfabrication technology, such as photo etching. Further, since the channel width can be adjusted by the width of the epitaxial growth and the ion implantation energy in the case of forming the p-type gate region, a high control property can be obtained. According to this example, as above-described, a SiC static induction transistor which has excellent OFF characteristics can be obtained without deteriorating the ON characteristics significantly.
In the example shown in FIG. 3, however, a problem exists in that pattern matching work with considerably high accuracy is required for the exact control of the channel length, which strongly influences the pinch-off characteristics of the channel. That is, in the element using the conventional constituent material of Si, in order for the voltage of the junction between the gate and the source to be made high, a part of the n−-type region 2 is interposed between the n+-type source region 4 and the first gate region 5. When the idea in the conventional case of Si is applied to an element using the constituent material of SiC as it is, the necessary width for the n−-type region to be interposed becomes about 1 μm. Thus, a significantly high accuracy is required for the pattern matching when the n+-type region 4 and the first gate region 5 are formed. As a result, the manufacturing of the element having constant pinch-off characteristics becomes very difficult.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a structure of a static induction transistor having excellent OFF characteristics without deteriorating the ON characteristics.
Another object of the present invention is to provide a structure which will enable the manufacture of the above-mentioned static induction transistor at a high yield factor and a method of manufacturing the same.
Another object of the present invention is to provide an electric power conversion apparatus of high performance using the above-mentioned static induction transistor.
In the static induction transistor according to the invention, a semiconductor substrate having an energy band gap greater than that of silicon includes a first gate region of a second conduction type and a second gate region of a second conduction type positioned respectively at the surface and the inside of a first semiconductor region of a first conduction type serving as a drain region. The first gate region is positioned at the surface of the first semiconductor region and is in contact with a second semiconductor region of a first conduction type serving as a source region. According to the present invention, since the second semiconductor region and the first gate region are in contact with each other, high accuracy is not required for the alignment of a pattern in the second semiconductor region and a pattern in the first gate region. Further, since the energy band gap of the semiconductor material for the semiconductor substrate is greater than that of silicon, a high withstanding voltage can be obtained even if the second semiconductor region and the first gate region are in contact with each other. Consequently, the OFF characteristics of the static induction transistor are improved.
In the static induction transistor according to the invention, in a semiconductor substrate having an energy band gap greater than that of silicon, a first semiconductor layer of a first conduction type serving as a drain region and a gate electrode form a Schottky junction. Thus, a high gate withstanding voltage can be obtained. Further, according to the present invention, since the pn junction is not used, but the Schottky junction is used in the gate electrode part, there is no problem of alignment between semiconductor layer patterns when the high gate withstanding voltage is to be obtained.
The first conduction type and the second conduction type, as above-described, are p type or n type, respectively, and are in an opposite conduction type to each other.
In a method of manufacturing a static induction transistor according to the present invention, a gate region is formed by the epitaxial method. Consequently, a static induction transistor having a high gate withstanding voltage can be produced at a high yield factor.
In an electric power conversion apparatus according to the present invention, the static induction transistor according to the present invention as above-described is turned on or off and thereby electric power conversion is performed. Consequently, an the electric power conversion apparatus of high performance is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing a first embodiment of a static induction transistor made of SiC to which the invention is applied.
FIG. 2 is a sectional view showing an example of a static induction transistor in the prior art.
FIG. 3 is a sectional view showing another example of a static induction transistor in the prior art.
FIG. 4 is a sectional view showing flow of electrons in the conduction state of the static induction transistor in FIG. 3.
FIG. 5 is a sectional view showing a second embodiment of a static induction transistor made of SiC to which the invention is applied.
FIG. 6 is a sectional view showing a third embodiment of a static induction transistor made of SiC to which the invention is applied.
FIG. 7 is a sectional view showing a fourth embodiment of a static induction transistor made of SiC to which the invention is applied.
FIG. 8A is a top plan view and FIG. 8B is a sectional view taken along line AA′ in FIG. 8A, showing a more concrete embodiment of a static induction transistor made of SiC to which the invention is applied.
FIG. 9A is a top plan view, FIG. 9B is a sectional view taken along line AA′ in FIG. 9A and FIG. 9C is a sectional view taken along line ab in FIG. 9A, representing another embodiment of a two-dimensional layout of unit cells.
FIG. 10A is a top plan view, FIG. 10B is a sectional view taken along line AA; in FIG. 10A and FIG. 10C is a sectional view taken along line ab in FIG. 10A, representing still another embodiment of a two-dimensional layout of unit cells.
FIG. 11A is a top plan view, FIG. 11B is a sectional view taken along line AA′ in FIG. 11A and FIG. 11C is a sectional view taken along line ab in FIG. 11A, showing other coupling means of a gate region of unit cells.
FIGS. 12A to 12D are sectional views showing a part of a characteristic fabricating process of the embodiment in FIG. 1.
FIGS. 13A to 13D are sectional views showing a part of a characteristic fabricating process of the embodiment in FIG. 5.
FIG. 14 is a schematic circuit diagram showing a main circuit of an embodiment of an inverter device using a static induction transistor of SiC to which the invention is applied.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will be described in detail with reference to various embodiments as follows.
FIG. 1 is a sectional view showing a static induction transistor made of SiC (silicon carbide) representing a first embodiment of the present invention. A 20 semiconductor substrate comprises an n+-type region 1, an n−-type region 2 being in contact with the n+-type region 1 and having an impurity concentration lower than that of the n+-type region 1, an n+-type region 4 being in contact with the surface of the n−-type region 2 and having an impurity concentration higher than that of the n−-type region 2, and p-type region 5 serving as a first gate region. A source electrode 11, a drain electrode 12 and a gate electrode 13 are electrically connected to the n+-type region 4, the n+-type region 1 and the p-type region 5, respectively. The n−-type region is connected to the drain electrode 12 through the n+-type region 4, but the drain electrode may be in ohmic contact therewith directly. Further, the n+-type region 4 and the p-type region 5 are provided at one main surface of the semiconductor substrate, and a second gate region 3 comprising a p-type embedded layer including projection parts of the n+-type region 4 and the p-type region 5 are formed at a deep position of both regions. The n−-type region 2 has a longitudinal channel part Wvch by which the second gate region 3 is lost within the plane in parallel to the main surface including the second gate region. The length Lch of an overlapping portion of respective projections of the first gate region 5 and the second gate region 3 with each other is made larger than the width WIch of the n−-type region located between the first gate region 5 and the second gate region 3, so that the depletion layer is easy to be pinched off between the first gate region and the second gate region when the negative potential is applied to the gate electrode 13.
The p-type region 3 is set at the floating state or the same potential as that of the n+-type region 4 or the same potential as that of the p-type region 5 operating as the gate region, and a negative potential with respect to the source electrode 11 is applied to the gate electrode 13. Thus, a current flowing between the source electrode and the drain electrode can be turned off. Although not shown in this embodiment, a second gate electrode may be provided in the second gate region and a control signal may be given to the second gate electrode.
In this embodiment, since the semiconductor substrate is of SiC having a maximum breakdown electric field intensity which is about ten times as large as that of Si, a high gate withstanding voltage of several tens V to several hundreds V can be obtained even if the n+-type region 4 and the p-type region 5 having a high impurity concentration are in contact with each other. Also, when the n+-type region 4 and the p-type region 5 are formed, since the patterns of both regions can be overlapped to be formed, the alignment accuracy can be reduced. That is, even with a low alignment accuracy, the gate withstanding voltage can be set without fail. In the static induction transistor, a reverse bias is applied to the junction between the gate and the source and the gate regions are mutually pinched off so that the voltage between the drain and the source can be blocked. Consequently, for the element of a high blocking voltage, the gate-source junction having as high a withstanding voltage as possible is required. As a result, according to this embodiment, the static induction transistor of high withstanding voltage can be obtained at a high manufacturing yield factor.
FIG. 5 shows a second embodiment of the present invention, which is a modification of the first embodiment. A first gate region is divided into a p-type part 51 having an impurity concentration higher than that of an n−-type region 2 which is in contact with an n+-type region 4, but relatively low, and a p+-type region 52 which is in contact with a gate electrode and has an impurity concentration higher than that of the p-type part 51. According to this embodiment, the gate source junction in a high withstanding voltage and a low leak current can be formed, while holding the gate electrode connection in a low resistance.
FIG. 6 is a sectional view of a static induction transistor made of SiC, which represents a third embodiment of the present invention. In this embodiment, on one main surface of the semiconductor substrate, a Schottky electrode 14 is provided to form a Schottky junction in an n−-type region 2. This constitution performs the same action as that of the first gate region 5 and the gate electrode 13 in the first embodiment as above-described. In this embodiment, a high gate withstanding voltage can be obtained by the Schottky junction on the semiconductor surface made of SiC. That is, a pn junction need not be formed on the surface of the semiconductor substrate in order to obtain the desired gate withstanding voltage. Consequently, there is no problem of alignment to form the pn junction, and the static induction transistor having a high withstanding voltage can be manufactured at a high yield factor.
Also in this embodiment, the p-type region 3 is set at the floating state or the same potential as that of the n+-type source region 4 or the same potential as that of the gate electrode 14, and a negative potential with respect to the source electrode 11 is applied to the gate electrode 14. Thus, a current flowing between the source and the drain can be turned off. Also in this embodiment, la second gate electrode may be provided in the second gate region 3 and a control signal may be given to the second gate electrode.
FIG. 7 shows a fourth embodiment of the present invention, which represents another modification of the first embodiment. On a plane where a second gate region 3 is provided, there is a p+-type embedded region 31 at a position spaced apart from the second gate region. The region 31 is in the floating state electrically. When the voltage is blocked between the drain electrode and the source electrode, the region 31 has a function of facilitating the pinch-off between the first gate region and the second gate region, and provides the OFF characteristics of a high withstanding voltage. This embodiment shows an example in which the p+-type embedded region 31 is added to the first embodiment, but it may be added also to the second embodiment and the third embodiment. Although one p+-type embedded region is shown in this embodiment, two or more p+-type embedded regions may be provided.
The individual embodiments have been described based on the sectional structure of a unit cell of the semiconductor element. In a more concrete structure, however, a plurality of cells are arranged within one semiconductor substrate. FIGS. 8A and 8B show such an embodiment. FIG. 8A shows an arrangement in which basic cells described in the first embodiment are arranged within the same basic body, and FIG. 8B is a sectional view in the position of the line AA′ in FIG. 8A. Constituent parts in FIGS. 8A and 8B designated by the same reference numerals as those in FIG. 1 indicate parts which are equivalent in structure, conduction type and function. Although not shown in FIGS. 8A and 8B, source electrodes 11 of individual cells are electrically connected to each other and the respective cells are connected so that they are operated in parallel within the semiconductor substrate. In FIGS. 8A and 8B, only four cells are shown, but the number of cells can be increased in response to the current capacity of the semiconductor substrate. This embodiment shows an example in which a unit cell is of square shape. However, the two-dimensional shape of the cell is not limited to a square shape, but may be a rectangular shape, square shape with rounded corners, polygonal shape or circular shape. In the case of a circular cell, however, in a portion where the cells are arranged in a cross (in FIG. 8A, a portion where lines ab and cd intersect orthogonally), the longitudinal channel part Wvch becomes wide. As a result, the pinch-off becomes insufficient and the voltage OFF characteristics of the high voltage may be deteriorated. Consequently, the shape of the cell is preferably a square shape or polygonal shape having sides and corners with small roundness (radius of curvature). Further, as the shape of a cell arranged usually within a chip is nearly a square shape, a similar shape to the chip is excellent. The concrete cell arrangement structure of the present invention will be described based on a square cell as follows.
FIGS. 9A, 9B and 9C show another embodiment of a two-dimensional layout of unit cells. FIG. 9A is a front surface view of a semiconductor substrate, FIG. 9B is a sectional view at a position of line AA′, and FIG. 9C is a sectional view at a position of line ab. Constituent parts in FIGS. 9A, 9B and 9C designated by the same reference numerals as those in FIGS. 8A and 8B indicate parts which are equivalent in structure, conduction type and function. This embodiment is different from the embodiment in FIGS. 8A and 8B in that extended parts 33 of second gate regions 3 are provided to couple the second gate regions 3 of individual cells. Although not shown in FIGS. 9A, 9B and 9C, the second gate regions are coupled electrically and a second gate electrode is connected to the second gate region by a low resistance. As above-described, the potential of the second gate electrode is made the same potential as that of the n+-type source region 4 or the same potential as that of the gate region 5 operating as the first gate region, and can be controlled in ON/OFF control. In order that a decrease in the area of the longitudinal channel part forming a current passage at the ON state is made minimum and the second gate regions are coupled, in this embodiment, coupling parts are provided in the four sides of the square cell. Of course, the coupling parts need not be provided on all four sides, but may be provided only on one to three sides.
FIGS. 10A, 10B and 10C show still another embodiment of a two-dimensional layout of unit cells. FIG. 10A is a front surface view of a semiconductor substrate, FIG. 10B is a sectional view at a position of line AA′, and FIG. 10C is a sectional view at a position of line ab. Constituent parts in FIGS. 10A, 10B and 10C designated by the same reference numerals as those in FIGS. 9A, 9B and 9C indicate parts which are equivalent in structure, conduction type and function. This embodiment is characterized in that the coupling parts 33 of the second gate regions 3 are provided at four corners of the square cell. In a part where the cells are arranged in cross (in FIG. 10A, a part where lines ab and cd intersect orthogonally), the width Wvch of the longitudinal channel part becomes wide. Thus, the pinch-off becomes insufficient there, and the voltage OFF characteristics at the high voltage may be deteriorated. In this embodiment, the part with the cells arranged in a cross becomes the coupling part. Thus, this embodiment is more desirable than the embodiment shown in FIGS. 9A, 9B and 9C where the coupling is performed at the side parts of the square cell in that the deterioration of the OFF characteristics can be prevented. Of course, even in this embodiment, if the width Wvch of the longitudinal channel part is set to be narrow in a range in which the OFF characteristics are not deteriorated, the coupling parts need not be provided in all four corners, but may be provided only in one to three corners.
FIGS. 11A, 11B and 11C show an embodiment having other coupling means of the second gate region of unit cells. FIG. 11A is a front surface view of a semiconductor substrate, FIG. 11B is a sectional view at a position of line AA′, and FIG. 11C is a sectional view at a position of line ab. Constituent parts in FIGS. 11A, 11B and 11C designated by the same reference numerals as those in FIGS. 8A and 8B indicate parts which are equivalent in structure, conduction type and function. At the corners of the square cell, p-type layers 34 are provided in a portion where it extends through, from one main surface of the semiconductor substrate, the first p-type gate region 5 and reaches the second gate region 3. The second gate regions 3 of unit cells are coupled by the p-type layers 34, and further the second gate region 3 is electrically coupled with the first p-type gate region 5. Thus, there is an advantage in that the gate control to make the two gate regions the same potential becomes possible without providing a second gate electrode newly. This embodiment shows an example in which the p-type layers 34 are provided in all corners of the square cell, although the intended function can be obtained even when the p-type layers 34 are provided in a part of the corners of the square cell. Also, when the coupling parts 33 of the second gate regions 3 are provided at the sides and the corners of the square cell as shown in FIGS. 9 and 10, the p-type layers 34 can be applied as the connecting means of the first gate region and the second gate region.
The embodiment relating to the cell arrangement has been described regarding a cell of square shape, but, of course, it can be applied also in a cell structure of rectangular shape or polygonal shape.
The embodiment relating to the cell arrangement has been described in the embodiment of FIG. 1 regarding the basic cell of the present invention, but, of course, an arrangement of a plurality of cells can be applied also to the cell structure shown in other embodiments of the present invention as above described.
According to the individual embodiments as above described, a static induction transistor made of SiC can be realized which has excellent OFF characteristics and is capable of being manufactured easily.
FIGS. 12A to 12D show a part of the process of fabricating a static induction transistor made of SiC in the first embodiment. Ion implantation of aluminum or boron is performed from a surface of an n−-type region 2 of a semiconductor substrate made of SiC using a resist (not shown, and so forth) as a mask. Thus, (FIG. 12A) a p-type region 3 is formed, and (FIG. 12B) the n−-type region 2 is laminated and grown by the epitaxial method. Next, (FIG. 13C) using a resist as a mask, ion implantation of nitrogen is performed to form an n+-type region 4, and ion implantation of aluminum or boron is performed, whereby a p-type region 5 is formed so that both regions come in contact with each other. Thereafter, a source electrode 11, a drain electrode 12 and a gate electrode 13 are formed. Thus, an element is completed.
FIGS. 13A to 13D show a part of the process of fabricating a static induction transistor made of SiC in the second embodiment. Ion implantation of aluminum or boron is performed from a surface of an n−-type region 2 of a semiconductor substrate made of SiC using a resist as a mask. Thus, (FIG. 13A) a p-type region 3 serving as a second gate region is formed, and (FIG. 13B) the n−-type region 2 is grown by the epitaxial method and further a p-type region 51 serving as a first gate region is laminated on the n−-type region 2. Next, using a resist as a mask, ion implantation of nitrogen is performed to form an n+-type region 4. Further, (FIG. 13C) using a resist as a mask, ion implantation of boron preferably aluminum is performed, whereby a p+-type region 52 is formed. Thereafter, a source electrode 11, a drain electrode 12 and a gate electrode 13 are formed. Thus, an element is completed.
Since the p-type region 51 is formed by epitaxial growth, in comparison with the manufacturing method shown in FIGS. 9A, 9B and 9C where any of the n+-type region 4 and the p-type region 5 is formed by ion implantation, a problem of an increase of the leakage current of the gate source junction due to a crystal defect remaining in the overlapped portion of the ion implantation layers can be avoided, and a junction which has excellent blocking characteristics can be obtained.
FIG. 14 shows an example of the constitution of an inverter device for driving a three-phase induction motor using static induction transistors made of SiC to which the present invention is applied and diodes connected to the transistors in inverse-parallel connection. The six static induction transistors SW11, SW12, SW21, SW22, SW31, SW32 are turned on or off such that the DC power is converted into AC power and the three-phase induction motor is driven. The static induction transistor made of SiC according to the present invention has little loss, and a cooling system therefor can be simplified. That is, a low cost and highly efficient system using the inverter device can be attained.
The embodiments of the present invention have been described, but the present invention covers for more application areas or derivative areas.
In the individual embodiments as above described, the semiconductor material used for the semiconductor substrate is SiC, but also another semiconductor material can be applied. Particularly, a wide gap semiconductor material, such as diamond or gallium nitride, with a larger energy band gap than that of Si is effective.
The present invention can be applied also to a static induction transistor made of SiC where the conduction type of each region is inverted in each embodiment as above-described.
According to the present invention as above-described, static induction transistors made of SiC and which have excellent ON characteristics can be realized without difficulty in the process of manufacture.
Patent Application
20050006649
