Patent Publication Number: US-10777675-B2

Title: Semiconductor device, method for manufacturing semiconductor device, inverter circuit, driving device, vehicle, and elevator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-121783, filed on Jun. 20, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a semiconductor device, a method for manufacturing a semiconductor device, an inverter circuit, a driving device, a vehicle, and an elevator. 
     BACKGROUND 
     Silicon carbide (SiC) is expected to be used as a material for a next-general semiconductor device. SiC has better physical properties than silicon (Si). For example, SiC has a bandgap that is three times wider than that of Si, a breakdown field strength that is about ten times higher than that of Si, and a thermal conductivity that is about three times higher than that of Si. These physical properties are used to achieve a semiconductor device which has low loss and can operate at a high temperature. 
     However, for example, when silicon carbide is used to form a metal insulator semiconductor (MIS) structure, there is a concern that the breakdown voltage of a gate insulating layer will become lower than the breakdown voltage of a semiconductor, since the breakdown voltage of silicon carbide is high. In particular, when the MIS structure is formed in a trench in order to increase an integrity of a device, the breakdown voltage of the gate insulating layer is reduced due to the concentration of the electric field at the bottom of the trench. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view schematically illustrating a semiconductor device according to a first embodiment. 
         FIG. 2  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in a semiconductor device manufacturing method according to the first embodiment. 
         FIG. 3  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 4  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 5  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 6  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 7  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 8  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 9  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 10  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIG. 11  is a cross-sectional view schematically illustrating the semiconductor device that is being manufactured in the semiconductor device manufacturing method according to the first embodiment. 
         FIGS. 12A and 12B  are diagrams illustrating the function and effect of the semiconductor device according to the first embodiment. 
         FIGS. 13A and 13B  are diagrams illustrating the function and effect of the semiconductor device according to the first embodiment. 
         FIGS. 14A and 14B  are diagrams illustrating the function and effect of the semiconductor device according to the first embodiment. 
         FIG. 15  is a diagram illustrating the function and effect of the semiconductor device according to the first embodiment. 
         FIG. 16  is a diagram illustrating the function and effect of the semiconductor device according to the first embodiment. 
         FIGS. 17A and 17B  are diagrams illustrating the function and effect of the semiconductor device according to the first embodiment. 
         FIG. 18  is a cross-sectional view schematically illustrating a modification example of the semiconductor device according to the first embodiment. 
         FIG. 19  is a cross-sectional view schematically illustrating a semiconductor device according to a second embodiment. 
         FIG. 20  is a diagram illustrating the function and effect of the semiconductor device according to the second embodiment. 
         FIG. 21  is a cross-sectional view schematically illustrating a semiconductor device according to a third embodiment. 
         FIG. 22  is a cross-sectional view schematically illustrating a semiconductor device according to a fourth embodiment. 
         FIG. 23  is a cross-sectional view schematically illustrating a semiconductor device according to a fifth embodiment. 
         FIG. 24  is a cross-sectional view schematically illustrating a semiconductor device according to a sixth embodiment. 
         FIG. 25  is a cross-sectional view schematically illustrating a semiconductor device according to a seventh embodiment. 
         FIG. 26  is a diagram schematically illustrating a driving device according to an eighth embodiment. 
         FIG. 27  is a diagram schematically illustrating a vehicle according to a ninth embodiment. 
         FIG. 28  is a diagram schematically illustrating a vehicle according to a tenth embodiment. 
         FIG. 29  is a diagram schematically illustrating an elevator according to an eleventh embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A semiconductor device according to an embodiment includes a silicon carbide layer having a first plane and a second plane, a first silicon carbide region of a first conductivity type provided in the silicon carbide layer, a second silicon carbide region of a second conductivity type provided between the first silicon carbide region and the first plane, a third silicon carbide region of the second conductivity type provided between the first silicon carbide region and the first plane, a fourth silicon carbide region of the first conductivity type provided between the second silicon carbide region and the first plane, a fifth silicon carbide region of the first conductivity type provided between the third silicon carbide region and the first plane, a gate electrode having a portion provided between the second silicon carbide region and the third silicon carbide region, a gate insulating layer having a portion provided between the second silicon carbide region and the gate electrode and a portion between the third silicon carbide region and the gate electrode, a sixth silicon carbide region of the second conductivity type provided between the first silicon carbide region and the second silicon carbide region, a part of the second silicon carbide region being interposed between the sixth silicon carbide region and the gate insulating layer, and the sixth silicon carbide region having a higher second-conductivity-type impurity concentration than the second silicon carbide region, and a seventh silicon carbide region of the second conductivity type provided between the first silicon carbide region and the third silicon carbide region, a part of the third silicon carbide region being interposed between the seventh silicon carbide region and the gate insulating layer, and the seventh silicon carbide region having a higher second-conductivity-type impurity concentration than the third silicon carbide region. 
     Hereinafter, embodiments of the invention will be described with reference to the drawings. In the following description, the same or similar members are denoted by the same reference numerals and the description thereof will not be repeated. 
     In the following description, n + , n, n − , p + , p, and p −  indicate the relative levels of impurity concentration in each conductivity type. That is, n +  indicates an n-type impurity concentration which is higher than that of n and n −  indicates an n-type impurity concentration which is lower than that of n. In addition, p +  indicates a p-type impurity concentration which is higher than that of p and p −  indicates a p-type impurity concentration which is lower than that of p. In some cases, an n +  type and an n −  type are simply referred to as an n type and a p +  type and a p −  type are simply referred to as a p type. 
     First Embodiment 
     A semiconductor device according to this embodiment includes a silicon carbide layer having a first plane and a second plane, a first silicon carbide region of a first conductivity type which is provided in the silicon carbide layer, a second silicon carbide region of a second conductivity type which is provided between the first silicon carbide region and the first plane, a third silicon carbide region of the second conductivity type which is provided between the first silicon carbide region and the first plane, a fourth silicon carbide region of the first conductivity type which is provided between the second silicon carbide region and the first plane, a fifth silicon carbide region of the first conductivity type which is provided between the third silicon carbide region and the first plane, a gate electrode having at least a portion provided between the second silicon carbide region and the third silicon carbide region, a gate insulating layer having at least a portion provided between the second silicon carbide region and the gate electrode and between the third silicon carbide region and the gate electrode, a sixth silicon carbide region of the second conductivity type which is provided between the first silicon carbide region and the second silicon carbide region, faces the gate insulating layer with the second silicon carbide region interposed therebetween, and has a higher second-conductivity-type impurity concentration than the second silicon carbide region, and a seventh silicon carbide region of the second conductivity type which is provided between the first silicon carbide region and the third silicon carbide region, faces the gate insulating layer with the third silicon carbide region interposed therebetween, and has a higher second-conductivity-type impurity concentration than the third silicon carbide region. 
       FIG. 1  is a cross-sectional view schematically illustrating the semiconductor device according to this embodiment. A metal oxide semiconductor field effect transistor (MOSFET)  100  is, for example, a double implantation MOSFET (DIMOSFET) in which a well region and a source region are formed by ion implantation. In addition, the MOSFET  100  is a trench gate MOSFET in which a gate insulating layer and a gate electrode are provided in a trench. 
     Hereinafter, an example in which a first conductivity type is an n type and a second conductivity type is a p type will be described. The MOSFET  100  is an n-type MOSFET having electrons as carriers. 
     The MOSFET  100  includes a silicon carbide layer (SiC layer)  10 , a source electrode  12 , a drain electrode  14 , a gate insulating layer  16 , a gate electrode  18 , an interlayer insulating film  20 , and a trench  50 . The SiC layer  10  includes a drain region (SiC substrate)  22 , a drift region (first silicon carbide region)  24 , a first p well region (second silicon carbide region)  26   a , a second p well region (third silicon carbide region)  26   b , a first source region (fourth silicon carbide region)  28   a , a second source region (fifth silicon carbide region)  28   b , a first high-concentration p region (sixth silicon carbide region)  32   a , a second high-concentration p region (seventh silicon carbide region)  32   b , a first contact region (eleventh silicon carbide region)  34   a , and a second contact region (twelfth silicon carbide region)  34   b.    
     The SiC layer  10  is, for example, a 4H—SiC single-crystal layer. 
     SiC can have a plurality of crystal forms. Examples of SiC include 4H—SiC which is a hexagonal crystal system, 6H—SiC which is a hexagonal crystal system, and 3C—SiC which is a cubic crystal system. The arrangement of atoms in SiC can be observed by, for example, a transmission electron microscope (TEM) to identify the crystal form of SiC. In addition, the arrangement of atoms in SiC can be observed by, for example, X-ray diffraction (XRD) to identify the crystal form of SiC. 
     The SiC layer  10  has a first plane and a second plane. In  FIG. 1 , the first plane is an upper plane and the second plane is a lower plane. Hereinafter, the first plane is referred to as a front surface and the second plane is referred to as a rear surface. 
     An example in which the first plane is inclined at an angle that is equal to or greater than 0 degrees and equal to or less than 8 degrees with respect to a (0001) face and the second plane is inclined at an angle that is equal to or greater than 0 degrees and equal to or less than 8 degrees with respect to a (000-1) face will be described. The (0001) face is referred to as a silicon face. The (000-1) face is referred to as a carbon face. 
     The drain region  22  is made of n-type SiC. The drain region  22  includes, for example, nitrogen (N) as n-type impurities. The n-type impurity concentration of the drain region  22  is, for example, equal to or greater than 1×10 18  cm −3  and equal to or less than 1×10 21  cm −3 . 
     The n-type impurity concentration of the drain region  22  in the second plane is preferably equal to or greater than 1×10 19  cm −3  and more preferably equal to or greater than 1×10 20  cm −3  in order to reduce the contact resistance between the drain electrode  14  and the drain region  22 . 
     The drift region  24  is provided on the drain region  22 . The drift region  24  is, for example, an n −  SiC region that is formed on the drain region  22  by epitaxial growth. The thickness of the drift region  24  is, for example, equal to or greater than 5 μm and equal to or less than 150 μm. 
     The drift region  24  includes, for example, nitrogen (N) as n-type impurities. The n-type impurity concentration of the drift region  24  is lower than the n-type impurity concentration of the drain region  22 . The n-type impurity concentration of the drift region  24  is, for example, equal to or greater than 1×10 14  cm −3  and equal to or less than 1×10 17  cm −3 . 
     The first p well region  26   a  and the second p well region  26   b  are provided between the drift region  24  and the first plane. The first p well region  26   a  and the second p well region  26   b  are made of p-type SiC. 
     The first p well region  26   a  is provided between the first source region  28   a  and the drift region  24 . The second p well region  26   b  is provided between the second source region  28   b  and the drift region  24 . The first p well region  26   a  and the second p well region  26   b  function as the channel regions of the MOSFET  100 . 
     The first p well region  26   a  and the second p well region  26   b  are made of p-type SiC. The first p well region  26   a  and the second p well region  26   b  include, for example, aluminum (Al) as p-type impurities. The p-type impurity concentration of the first p well region  26   a  and the second p well region  26   b  is, for example, equal to or greater than 5×10 15  cm −3  and equal to or less than 1×10 18  cm −3 . 
     The depth of the first p well region  26   a  and the second p well region  26   b  is, for example, equal to or greater than 0.4 μm and equal to or less than 0.9 μm. In the specification, the “depth” means the distance from the front surface of the SiC layer  10 . 
     The first source region  28   a  is provided between the first p well region  26   a  and the first plane. The second source region  28   b  is provided between the second p well region  26   b  and the first plane of the SiC layer  10 . 
     The first source region  28   a  and the second source region  28   b  are made of n +  SiC. The first source region  28   a  and the second source region  28   b  include, for example, nitrogen (N) as n-type impurities. 
     The n-type impurity concentration of the first source region  28   a  and the second source region  28   b  is higher than the n-type impurity concentration of the drift region  24 . The n-type impurity concentration of the first source region  28   a  and the second source region  28   b  is, for example, equal to or greater than 1×10 18  cm −3  and equal to or less than 1×10 21  cm −3 . 
     The n-type impurity concentration of the first source region  28   a  and the second source region  28   b  in the first plane is preferably equal to or greater than 1×10 19  cm −3  and more preferably equal to or greater than 1×10 20  cm −3  in order to reduce the contact resistance between the source electrode  12  and the first and second source regions  28   a  and  28   b.    
     The depth of the first source region  28   a  and the second source region  28   b  is less than the depth of the first p well region  26   a  and the second p well region  26   b  and is, for example, equal to or greater than 0.1 μm and equal to or less than 0.4 μm. 
     A trench  50  is formed in the SiC layer  10  so as to extend from the first plane to the second plane of the SiC layer  10 . The depth of the trench  50  is greater than the depth of the first p well region  26   a  and the second p well region  26   b.    
     The gate insulating layer  16  is provided in the trench  50 . The gate electrode  18  is provided on the gate insulating layer  16  in the trench  50 . 
     At least a portion of the gate electrode  18  is located between the first p well region  26   a  and the second p well region  26   b . At least a portion of the gate insulating layer  16  is provided between the first p well region  26   a  and the gate electrode  18 . At least a portion of the gate insulating layer  16  is provided between the second p well region  26   b  and the gate electrode  18 . 
     The gate insulating layer  16  includes, for example, silicon oxide or silicon oxynitride. The gate insulating layer  16  is, for example, a silicon oxide film. The thickness of the gate insulating layer  16  is, for example, equal to or greater than 40 nm and equal to or less than 100 nm. 
     The gate electrode  18  is made of, for example, polysilicon including n-type impurities or p-type impurities. 
     The first contact region  34   a  and the second contact region  34   b  are provided so as to come into contact with the first plane. The first source region  28   a  is interposed between the first contact region  34   a  and the gate insulating layer  16 . The second source region  28   b  is interposed between the second contact region  34   b  and the gate insulating layer  16 . 
     The first contact region  34   a  and the second contact region  34   b  are made of p +  SiC. The first contact region  34   a  and the second contact region  34   b  include, for example, aluminum (Al) as p-type impurities. 
     The p-type impurity concentration of the first contact region  34   a  and the second contact region  34   b  is higher than the p-type impurity concentration of the first p well region  26   a  and the second p well region  26   b . The p-type impurity concentration of the first contact region  34   a  and the second contact region  34   b  is, for example, equal to or greater than 1×10 18  cm −3  and equal to or less than 1×10 21  cm −3 . 
     The depth of the first contact region  34   a  and the second contact region  34   b  is less than the depth of the first p well region  26   a  and the second p well region  26   b  and is, for example, equal to or greater than 0.1 μm and equal to or less than 0.4 μm. 
     The first high-concentration p region  32   a  is located between the drift region  24  and the first p well region  26   a . The first p well region  26   a  is interposed between the first high-concentration p region  32   a  and the gate insulating layer  16 . 
     The second high-concentration p region  32   b  is located between the drift region  24  and the second p well region  26   b . The second p well region  26   b  is interposed between the second high-concentration p region  32   b  and the gate insulating layer  16 . 
     The first high-concentration p region  32   a  and the second high-concentration p region  32   b  are made of p-type SiC. The first high-concentration p region  32   a  and the second high-concentration p region  32   b  include, for example, aluminum (Al) as p-type impurities. 
     The p-type impurity concentration of the first high-concentration p region  32   a  and the second high-concentration p region  32   b  is higher than the p-type impurity concentration of the first p well region  26   a  and the second p well region  26   b . The p-type impurity concentration of the first high-concentration p region  32   a  and the second high-concentration p region  32   b  is higher than the p-type impurity concentration of the first p well region  26   a  and the second p well region  26   b  by, for example, one digit or more. The p-type impurity concentration of the first high-concentration p region  32   a  and the second high-concentration p region  32   b  is, for example, equal to or greater than 5×10 16  cm −3  and equal to or less than 110 20  cm −3 . 
     The thickness of the first high-concentration p region  32   a  and the second high-concentration p region  32   b  in a depth direction is, for example, equal to or greater than 0.05 μm and equal to or less than 0.2 μm. 
     The distance (“T” in  FIG. 1 ) between the first high-concentration p region  32   a  and the gate insulating layer  16  is, for example, equal to or greater than 0.05 μm and equal to or less than 0.7 μm. The distance between the second high-concentration p region  32   b  and the gate insulating layer  16  is, for example, equal to or greater than 0.05 μm and equal to or less than 0.7 μm. 
     The distance between the first and second high-concentration p regions  32   a  and  32   b  and the drift region  24  is, for example, equal to or less than 0.1 μm. 
     The boundary between the first high-concentration p region  32   a  and a region adjacent to the first high-concentration p region is defined as an isoconcentration line indicating concentration that is half of the maximum impurity concentration of the first high-concentration p region  32   a . In addition, the boundary between the second high-concentration p region  32   b  and a region adjacent to the second high-concentration p region  32   b  is defined as an isoconcentration line indicating concentration that is half of the maximum impurity concentration of the second high-concentration p region  32   b.    
     The interlayer insulating film  20  is provided on the gate electrode  18 . The interlayer insulating film  20  is, for example, a silicon oxide film. 
     The source electrode  12  is provided on the front surface of the SiC layer  10 . The source electrode  12  is electrically connected to the first source region  28   a , the second source region  28   b , the first contact region  34   a , and the second contact region  34   b . The source electrode  12  comes into contact with the first source region  28   a , the second source region  28   b , the first contact region  34   a , and the second contact region  34   b . The source electrode  12  has a function of applying potential to the first source region  28   a , the second source region  28   b , the first contact region  34   a , and the second contact region  34   b.    
     The source electrode  12  is made of metal. The metal forming the source electrode  12  has, for example, a stacked structure of titanium (Ti) and aluminum (Al). The metal forming the source electrode  12  may react with the SiC layer  10  to form metal silicide or metal carbide. 
     The drain electrode  14  is provided on the rear surface of the SiC layer  10 . The drain electrode  14  is electrically connected to the drain region  22 . 
     The drain electrode  14  is made of metal. The metal forming the drain electrode  14  is, for example, nickel silicide. 
     The concentration and distribution of impurities included in the SiC layer  10  can be measured by, for example, a secondary ion mass spectroscopy (SIMS). In addition, the relative level of impurity concentration can be determined from the level of carrier concentration which is calculated by, for example, scanning capacitance microscopy (SCM). For example, the depth of regions including impurities and the distance between the regions can be calculated by, for example, SIMS. For example, the distance between the region including impurities and the gate insulating layer can be calculated from a composite image of an SCM image and an atomic force microscope (AFM) image. 
     A method for manufacturing a semiconductor device according to this embodiment includes forming a first mask member on a first plane of a silicon carbide layer including a first region of a first conductivity type and having the first plane and a second plane, implanting second-conductivity-type impurity ions into the silicon carbide layer on both sides of the first mask member, using the first mask member as a mask, to form a second region of a second conductivity type, depositing a second mask member on the first plane and the first mask member, removing the second mask member on the first mask member, selectively removing the first mask member with respect to the second mask member, forming a third mask member on a side surface of the second mask member, forming a trench in the silicon carbide layer so as to be separated from the second region, using the second mask member and the third mask member as a mask, forming a gate insulating layer in the trench, and forming a gate electrode on the gate insulating layer in the trench. 
     Hereinafter, an example of the semiconductor device manufacturing method according to this embodiment will be described.  FIGS. 2 to 11  are cross-sectional views schematically illustrating the semiconductor device which is being manufactured in the semiconductor device manufacturing method according to this embodiment. 
     First, an n-type SiC substrate having a first plane which is a silicon face and a second plane which is a carbon face is prepared. The SiC substrate becomes the drain region  22 . The n-type SiC substrate is made of 4H—SiC. 
     Then, the n −  drift region (first region)  24  is formed on the first plane of the n-type SiC substrate by an epitaxial growth method ( FIG. 2 ). The SiC substrate and the n −  drift region  24  form the SiC layer  10 . 
     Then, aluminum (Al) ions which are p-type impurity ions are selectively implanted into the drift region  24  by photolithography and an ion implantation method. The p-type p well region (third region)  26  is formed by the ion implantation. 
     Then, aluminum (Al) ions which are p-type impurity ions are selectively implanted into the p well region  26  by photolithography and the ion implantation method. The first contact region  34   a  and the second contact region  34   b  are formed by the ion implantation. 
     Then, nitrogen (N) ions which are n-type impurity ions are selectively implanted into the p well region  26  by photolithography and the ion implantation method. The n +  source region (fourth region)  28  is formed by the ion implantation ( FIG. 3 ). 
     Then, a first mask member  60  is formed on the first plane. The first mask member  60  is formed by, for example, the deposition of a film by a vapor phase growth method, lithography, and dry etching. The first mask member  60  is, for example, a silicon oxide film. 
     Then, aluminum ions which are p-type impurity ions are selectively implanted into the SiC layer  10 , using the first mask member  60  as a mask. The p-type first high-concentration p region (second region)  32   a  and the p-type second high-concentration p region (second region)  32   b  are formed in the SiC layer  10  on both sides of the first mask member  60  ( FIG. 4 ). 
     Then, a second mask member  62  is deposited on the first plane and the first mask member  60  ( FIG. 5 ). The second mask member  62  is formed by, for example, the deposition of a film by the vapor phase growth method. The second mask member  62  is, for example, a silicon nitride film. 
     Then, the second mask member  62  on the first mask member  60  is removed ( FIG. 6 ). The second mask member  62  on the first mask member  60  is selectively removed by, for example, a chemical mechanical polishing (CMP) method, using the first mask member  60  as a stopper. 
     Then, the first mask member  60  is selectively removed with respect to the second mask member  62  ( FIG. 7 ). The first mask member  60  is removed by, for example, wet etching. 
     Then, a third mask member  64  is deposited on the first plane and the second mask member  62  ( FIG. 8 ). The third mask member  64  is formed by, for example, the deposition of a film by the vapor phase growth method. The third mask member  64  is, for example, a silicon nitride film. 
     Then, aportion of the third mask member  64  on the second mask member  62  is removed and a portion of the third mask member  64  on both sides of the second mask member  62  remains ( FIG. 9 ). The third mask member  64  is processed by, dry etching with high anisotropy such that only a portion of the third mask member  64  placed on the side surface of the second mask member  62  remains. 
     Then, the trench  50  is formed in the SiC layer  10 , using the second mask member  62  and the third mask member  64  as a mask ( FIG. 10 ). The trench  50  is formed such that the first high-concentration p region  32   a  and the second high-concentration p region  32   b  are separated from each other. The trench  50  is formed so as to be self-aligned with the first high-concentration p region  32   a  and the second high-concentration p region  32   b.    
     The p well region (third region)  26  is divided into the first p well region  26   a  and the second p well region  26   b  by the trench  50 . The source region (fourth region)  28  is divided into the first source region  28   a  and the second source region  28   b  by the trench  50 . 
     Then, a heat treatment is performed to activate the p-type impurities and the n-type impurities introduced into the SiC layer  10  by ion implantation. The heat treatment is performed, for example, in a non-oxidizing atmosphere. 
     Then, the second mask member  62  and the third mask member  64  are removed. Then, the gate insulating layer  16 , the gate electrode  18 , and the interlayer insulating film  20  are formed by a known processing technique ( FIG. 11 ). 
     Then, the source electrode  12  is formed on the front surface of the SiC layer  10  by a known processing technique. In addition, the drain electrode  14  is formed on the rear surface of the SiC layer  10 . 
     The MOSFET  100  illustrated in  FIG. 1  is formed by the above-mentioned manufacturing method. 
     Next, the function and effect of the semiconductor device according to this embodiment will be described.  FIGS. 12A to 17B  are diagrams illustrating the function and effect of the semiconductor device according to this embodiment. 
     The trench gate MOSFET has the problem that, in the off state of the MOSFET, the breakdown voltage of a gate insulating film is reduced due to the concentration of the electric field on the bottom of a trench. In particular, the breakdown voltage of the gate insulating film is reduced due to the concentration of the electric field on the corners of the trench, which results in a reduction in the breakdown voltage of the MOSFET. 
     The MOSFET  100  according to this embodiment includes the first high-concentration p region (sixth silicon carbide region)  32   a  and the second high-concentration p region (seventh silicon carbide region)  32   b . Therefore, the concentration of the electric field on the corners of the trench is reduced. As a result, the maximum electric field strength of the gate insulating layer  16  is reduced. 
     In the MOSFET  100 , the first p well region  26   a  and the second p well region  26   b , the first source region  28   a  and the second source region  28   b , the first high-concentration p region  32   a  and the second high-concentration p region  32   b , and the first contact region  34   a  and the second contact region  34   b  are symmetrically provided with the trench  50  interposed therebetween. Therefore, the pairs of regions have the same structure and function. In the following description, the first p well region  26   a  and the second p well region  26   b  are simply referred to as a “p well region”, the first source region  28   a  and the second source region  28   b  are simply referred to as a “source region”, the first high-concentration p region  32   a  and the second high-concentration p region  32   b  are simply referred to as a “high-concentration p region”, and the first contact region  34   a  and the second contact region  34   b  are simply referred to as a “contact region”. 
       FIGS. 12A and 12B  are diagrams illustrating the simulation results of the electric field strength distribution and the potential distribution of a MOSFET without a high-concentration p region when the MOSFET is turned off. In  FIG. 12A , the electric field strength distribution is represented by an equi-field-strength line. In  FIG. 12B , the potential distribution is represented by an equipotential line. In  FIGS. 12A and 12B , the corner of the trench is enlarged. 
     As illustrated in  FIG. 12A , when the MOSFET is turned off, the maximum electric field strength is generated in the gate insulating layer (a region surrounded by a dashed line in  FIG. 12A ) at the corner of the trench. As illustrated in  FIG. 12B , when the MOSFET is turned off, a large number of equipotential lines extend from the drift region to the p well region. In other words, the amount of shift of the position where potential is 0 V to the front surface is large in the p well region and the position where potential is 0 V is separated from the position of the gate electrode at the bottom of the trench where potential is 0 V in the vertical direction. 
     Therefore, as can be seen from  FIG. 12B , the interval between the equipotential lines is very small in the gate insulating layer at the corner of the trench. In other words, a large number of equipotential lines extend into the well region. Therefore, the electric field is concentrated on the gate insulating layer at the corner of the trench and electric field strength increases. 
       FIGS. 13A and 13B  are diagrams illustrating the simulation results of the potential distribution of the MOSFET  100  according to this embodiment when the MOSFET  100  is turned off.  FIG. 13A  illustrates the potential distribution of a MOSFET without a high-concentration p region for comparison.  FIG. 13A  is the same as  FIG. 12B . 
       FIG. 13B  illustrates the potential distribution of the MOSFET  100  according to this embodiment. As can be seen from the comparison between  FIG. 13A  and  FIG. 13B , since the high-concentration p region is provided in the MOSFET  100 , the extension of the equipotential lines from the drift region to the p well region is substantially completely prevented. Therefore, the interval between the equipotential lines in the gate insulating layer at the corner of the trench is less than that in  FIG. 13B . 
       FIGS. 14A and 14B  are diagrams illustrating the simulation results of the electric field strength distribution of the MOSFET  100  according to this embodiment when the MOSFET  100  is turned off.  FIG. 14A  illustrates the electric field strength distribution of a MOSFET without a high-concentration p region for comparison.  FIG. 14B  illustrates the electric field strength distribution of the MOSFET  100  according to this embodiment. 
     In the MOSFET without a high-concentration p region, the maximum electric field strength of the gate insulating layer is 9.6 MV/cm. In contrast, in the MOSFET  100  according to this embodiment, the maximum electric field strength of the gate insulating layer is 7.3 MV/cm. The results prove that, when the high-concentration p region is provided, the maximum electric field strength of the gate insulating layer at the corner of the trench is reduced. 
       FIG. 15  is a diagram illustrating the simulation results of the relationship between the p-type impurity concentration of the high-concentration p region and the maximum electric field strength of the gate insulating layer in the MOSFET  100  according to this embodiment. The p-type impurity concentration of the p well region is fixed to 1×10 17  cm −3 . The distance (“T” in  FIG. 1 ) between the high-concentration p region and the gate insulating layer is fixed to 0.7 μm. 
     In  FIG. 15 , a black circle indicates the maximum electric field strength when the high-concentration p region is not provided. As can be seen from  FIG. 15 , the maximum electric field strength is reduced when the p-type impurity concentration of the high-concentration p region is equal to or greater than 5×10 17  cm −3 . Therefore, the p-type impurity concentration of the high-concentration p region is preferably equal to or greater than 5×10 17  cm −3 , more preferably equal to or greater than 1×10 18  cm −3 , and most preferably equal to or greater than 4×10 18  cm −3 . 
     The p-type impurity concentration of the first high-concentration p region  32   a  and the second high-concentration p region  32   b  is preferably higher than the p-type impurity concentration of the first p well region  26   a  and the second p well region  26   b  by one digit or more, in order to reduce the maximum electric field strength of the gate insulating layer  16 . 
       FIG. 16  is a diagram illustrating the simulation results of the distance (“T” in  FIG. 1 ) between the high-concentration p region and the gate insulating layer, on-resistance, and the maximum electric field strength in the MOSFET  100  according to this embodiment. In  FIG. 16 , a black rectangle indicates on-resistance and a white circle indicates the maximum electric field strength. 
     The p-type impurity concentration of the p well region is fixed to 1×10 17  cm −3 . The p-type impurity concentration of the high-concentration p region is fixed to 1×10 18  cm −3 . 
     When the high-concentration p region is not provided, on-resistance is 1.2 mΩcm 2 . In addition, when the high-concentration p region is not provided, the maximum electric field strength is 9.6 MV/cm. 
     As can be seen from  FIG. 16 , when the distance between the high-concentration p region and the gate insulating layer decreases, on-resistance increases. The reason is that p-type impurity concentration in the vicinity of the gate insulating layer increases and the channel resistance of the MOSFET  100  increases. 
     When the distance between the high-concentration p region and the gate insulating layer is equal to or greater than 0.05 μm, on-resistance is reduced suddenly and is substantially equal to the value when the high-concentration p region is not provided. Therefore, the distance between the high-concentration p region and the gate insulating layer is preferably equal to or greater than 0.05 μm and more preferably equal to or greater than 0.1 μm. 
     As can be seen from  FIG. 16 , when the distance between the high-concentration p region and the gate insulating layer increases, the maximum electric field strength of the gate insulating layer increases. Then, the equipotential lines extend into the p well region between the high-concentration p region and the gate insulating layer. However, when distance between the high-concentration p region and the gate insulating layer is equal to or less than 0.7 μm, the maximum electric field strength is lower than that when the high-concentration p region is not provided. Therefore, the distance between the high-concentration p region and the gate insulating layer is preferably equal to or less than 0.7 μm and more preferably equal to or less than 0.4 μm. 
       FIGS. 17A and 17B  are diagrams illustrating the simulation results of the relationship between the distance between the high-concentration p region and the drift region and the maximum electric field strength of the gate insulating layer in the MOSFET  100  according to this embodiment.  FIG. 17A  is a diagram illustrating simulation parameters.  FIG. 17B  is a diagram illustrating the simulation results. 
     In the simulation, the p-type impurity concentration of the p well region is fixed to 1×10 17  cm −3 . The p-type impurity concentration of the high-concentration p region is fixed to 1×10 18  cm −3 . The distance (“T” in  FIG. 17A ) between the high-concentration p region and the gate insulating layer is fixed to 0.7 μm. The distance (“S” in  FIG. 17A ) between the high-concentration p region and the drift region is a variable. 
     As can be seen from  FIG. 17B , when the distance between the high-concentration p region and the drift region increases, the maximum electric field strength of the gate insulating layer increases. This is because the maximum electric field strength is affected by the equipotential lines extending into the p well region between the high-concentration p region and the drift region. 
     When the distance between the high-concentration p region and the drift region is equal to or less than 0.1 μm, the maximum electric field strength is sufficiently lower than that when the high-concentration p region is not provided. Therefore, the distance between the high-concentration p region and the drift region is preferably equal to or less than 0.1 μm and more preferably equal to or less than 0.05 μm. 
     When the thickness of the high-concentration p region in the depth direction is too large, there is a concern that channel resistance will increase and on-resistance will be too high. Therefore, the thickness of the high-concentration p region in the depth direction is preferably equal to or less than 0.2 μm and more preferably equal to or less than 0.15 μm. 
     On the other hand, when the thickness of the high-concentration p region in the depth direction is too small, there is a concern that the effect of preventing the equipotential lines from extending into the p well region will be insufficient. Therefore, the thickness of the high-concentration p region in the depth direction is preferably equal to or greater than 0.05 μm and more preferably equal to or greater than 0.1 μm. 
     In the MOSFET  100  according to this embodiment, as described with reference to  FIG. 16 , when the distance (“T” in  FIG. 1 ) between the high-concentration p region and the gate insulating layer varies, the on-resistance and the maximum electric field strength vary. Therefore, it is preferable to prevent a variation in the distance between the high-concentration p region and the gate insulating layer when the MOSFET  100  is manufactured, in order to stabilize the characteristics of the MOSFET  100 . 
     In the method for manufacturing the MOSFET  100  according to this embodiment, the distance between the high-concentration p region and the gate insulating layer, that is, the distance between the high-concentration p region and the trench is determined in a self-alignment manner. Therefore, a variation in the distance between the high-concentration p region and the gate insulating layer is prevented. As a result, the MOSFET  100  with stable characteristics is achieved. 
       FIG. 18  is a cross-sectional view schematically illustrating a modification example of the semiconductor device according to this embodiment. A MOSFET  110  according to the modification example differs from the MOSFET  100  in that the width of the first high-concentration p region  32   a  and the second high-concentration p region  32   b  is smaller than that in the MOSFET  100 . The MOSFET  110  has the same function and effect as the MOSFET  100 . 
     As described above, according to this embodiment, the maximum electric field strength of the gate insulating layer  16  is reduced and the MOSFET  100  that can improve the breakdown voltage of the gate insulating layer is achieved. In addition, according to this embodiment, the MOSFET  100  with stable characteristics can be manufactured. 
     Second Embodiment 
     A semiconductor device according to this embodiment is the same as the semiconductor device according to the first embodiment except that a distance between the second plane and the sixth silicon carbide region is less than a distance between the second plane and the second silicon carbide region and a distance between the second plane and a seventh silicon carbide region is less than a distance between the second plane and the third silicon carbide region. Hereinafter, the description of the same content as that in the first embodiment will not be repeated. 
       FIG. 19  is a cross-sectional view schematically illustrating the semiconductor device according to this embodiment. A MOSFET  200  is a trench gate MOSFET. 
     In the MOSFET  200 , a distance (“d 1 ” in  FIG. 19 ) between the second plane and the first high-concentration p region (sixth silicon carbide region)  32   a  is less than a distance (“d 2 ” in  FIG. 19 ) between the second plane and the first p well region (second silicon carbide region)  26   a . In other words, d 2 −d 1  is a positive value. That is, the first high-concentration p region  32   a  is deeper than the first p well region  26   a.    
     A distance between the second plane and the second high-concentration p region (seventh silicon carbide region)  32   b  is less than a distance between the second plane and the second p well region  26   b . In other words, the second high-concentration p region  32   b  is deeper than the second p well region  26   b.    
       FIG. 20  is a diagram illustrating the function and effect of the semiconductor device according to this embodiment.  FIG. 20  is a diagram illustrating the simulation results of the relationship between on-resistance and the difference (d 2 −d 1 ) between the depth of the high-concentration p region and the depth of the p well region in the MOSFET  200  according to this embodiment. 
     As can be seen from  FIG. 20 , on-resistance increases as the difference (d 2 −d 1 ) between the depth of the high-concentration p region and the depth of the p well region increases. The reason is that the area (or volume) of a portion of the drift region which is interposed between the high-concentration p region and the gate insulating layer increases and the parasitic resistance of the MOSFET  200  increases. 
     As can be seen from  FIG. 20 , when the difference (d 2 −d 1 ) between the depth of the high-concentration p region and the depth of the p well region is equal to or less than 0.1 μm, an increment in the on-resistance is equal to or less than 10%. Therefore, the difference (d 2 −d 1 ) between the depth of the first high-concentration p region  32   a  and the depth of the first p well region  26   a , that is, the difference (d 2 −d 1 ) between the distance between the second plane and the first p well region  26   a  and the distance between the second plane and the first high-concentration p region  32   a  is preferably equal to or less than 0.1 μm and more preferably equal to or less than 0.05 μm. 
     The difference between the depth of the second high-concentration p region  32   b  and the depth of the second p well region  26   b , that is, the difference between the distance between the second plane and the second p well region  26   b  and the distance between the second plane and the second high-concentration p region  32   b  is preferably equal to or less than 0.1 μm and more preferably equal to or less than 0.05 μm. 
     It is preferable that the distance (“d 1 ” in  FIG. 19 ) between the second plane and the first high-concentration p region  32   a  be greater than the distance (“d 3 ” in  FIG. 19 ) between the second plane and the gate insulating layer  16  in order to reduce the area of a portion of the drift region which is interposed between the high-concentration p region and the gate insulating layer. In other words, it is preferable that the depth of the first high-concentration p region  32   a  be less than the depth of the gate insulating layer  16 . It is preferable that the depth of the first high-concentration p region  32   a  be less than the depth of the trench  50 . 
     Similarly, it is preferable that the distance between the second plane and the second high-concentration p region  32   b  be greater than the distance between the second plane and the gate insulating layer  16 . In other words, it is preferable that the depth of the second high-concentration p region  32   b  be less than the depth of the gate insulating layer  16 . It is preferable that the depth of the second high-concentration p region  32   b  be less than the depth of the trench  50 . 
     As described above, according to this embodiment, similarly to the first embodiment, the maximum electric field strength of the gate insulating layer  16  is reduced and the MOSFET  200  that can improve the breakdown voltage of the gate insulating layer is achieved. 
     Third Embodiment 
     A semiconductor device according to this embodiment is the same as the semiconductor device according to the first embodiment except that the eleventh silicon carbide region comes into contact with the sixth silicon carbide region and the twelfth silicon carbide region comes into contact with the seventh silicon carbide region. Hereinafter, the description of the same content as that in the first embodiment will not be repeated. 
       FIG. 21  is a cross-sectional view schematically illustrating the semiconductor device according to this embodiment. A MOSFET  300  is a trench gate MOSFET. 
     The first contact region (eleventh silicon carbide region)  34   a  comes into contact with the first high-concentration p region (sixth silicon carbide region)  32   a . In addition, the second contact region (twelfth silicon carbide region)  34   b  comes into contact with the second high-concentration p region (seventh silicon carbide region)  32   b.    
     In the MOSFET  300 , a high-concentration contact region comes into contact with a high-concentration p-type region. Therefore, when avalanche breakdown occurs, the resistance of a path through which holes are drawn from the source electrode  12  is reduced. As a result, the turn-on operation of a parasitic bipolar transistor is prevented and the resistance of the MOSFET  300  to secondary breakdown is improved. 
     As described above, according to this embodiment, similarly to the first embodiment, the maximum electric field strength of the gate insulating layer  16  is reduced and the MOSFET  300  that can improve the breakdown voltage of the gate insulating layer is achieved. In addition, the MOSFET  300  having high resistance to secondary breakdown can be achieved. 
     Fourth Embodiment 
     A semiconductor device according to this embodiment is the same as the semiconductor device according to the first embodiment except that it further includes: an eighth silicon carbide region of a second conductivity type which is provided such that a distance between the eighth silicon carbide region and the second plane is less than a distance between the second plane and the sixth silicon carbide region, a distance between the eighth silicon carbide region and the gate insulating layer is greater than a distance between the gate insulating layer and the sixth silicon carbide region, comes into contact with the second silicon carbide region or the sixth silicon carbide region, and has a lower second-conductivity-type impurity concentration than the sixth silicon carbide region; and a ninth silicon carbide region of the second conductivity type which is provided such that a distance between the ninth silicon carbide region and the second plane is less than a distance between the second plane and the seventh silicon carbide region, a distance between the ninth silicon carbide region and the gate insulating layer is greater than a distance between the gate insulating layer and the seventh silicon carbide region, comes into contact with the third silicon carbide region or the seventh silicon carbide region, and has a lower second-conductivity-type impurity concentration than the seventh silicon carbide region. Hereinafter, the description of the same content as that in the first embodiment will not be repeated. 
       FIG. 22  is a cross-sectional view schematically illustrating the semiconductor device according to this embodiment. A MOSFET  400  is a trench gate MOSFET. 
     The MOSFET  400  includes a first deep p region (eighth silicon carbide region)  36   a  and a second deep p region (ninth silicon carbide region)  36   b  which are provided in the SiC layer  10 . 
     A distance (“d 4 ” in  FIG. 22 ) between the second plane and the first deep p region  36   a  is less than a distance (“d 1 ” in  FIG. 22 ) between the second plane and the first high-concentration p region  32   a . In other words, the depth of the first deep p region  36   a  is greater than the depth of the first high-concentration p region  32   a.    
     A distance between the first deep p region  36   a  and the gate insulating layer  16  is greater than a distance between the first high-concentration p region  32   a  and the gate insulating layer  16 . In other words, the first deep p region  36   a  is provided so as to be farther from the trench  50  than the first high-concentration p region  32   a.    
     The first deep p region  36   a  is provided so as to come into contact with the first p well region  26   a  or the first high-concentration p region  32   a.    
     A distance between the second plane and the second deep p region  36   b  is less than a distance between the second plane and the second high-concentration p region  32   b . In other words, the depth of the second deep p region  36   b  is greater than the depth of the second high-concentration p region  32   b.    
     A distance between the second deep p region  36   b  and the gate insulating layer  16  is greater than a distance between the second high-concentration p region  32   b  and the gate insulating layer  16 . In other words, the second deep p region  36   b  is provided so as to be farther from the trench  50  than the second high-concentration p region  32   b.    
     The second deep p region  36   b  is provided so as to come into contact with the second p well region  26   b  or the second high-concentration p region  32   b.    
     The first deep p region  36   a  and the second deep p region  36   b  are made of p-type SiC. The first deep p region  36   a  and the second deep p region  36   b  include, for example, aluminum (Al) as p-type impurities. 
     The p-type impurity concentration of the first deep p region  36   a  and the second deep p region  36   b  is lower than the p-type impurity concentration of the first high-concentration p region  32   a  and the second high-concentration p region  32   b . The p-type impurity concentration of the first deep p region  36   a  and the second deep p region  36   b  is, for example, equal to or greater than 5×10′ 5  cm −3  and equal to or less than 1×10 18  cm −3 . 
     The depth of the first deep p region  36   a  and the second deep p region  36   b  is, for example, equal to or greater than 0.6 μm and equal to or less than 2.0 μm. 
     When the MOSFET  400  is turned off, a portion in which electric field strength is high is formed at the boundary between the first deep p region  36   a  and the drift region  24  and the boundary between the second deep p region  36   b  and the drift region  24 . Therefore, the concentration of the electric field at the corner of the trench is reduced and the maximum electric field strength of the gate insulating layer  16  is reduced. As a result, the maximum electric field strength of the gate insulating layer  16  is lower than that in the first embodiment. 
     According to the simulation results, when the first p well region  26   a  and the second p well region  26   b  are not combined with the first deep p region  36   a  and the second deep p region  36   b , the maximum electric field strength of the gate insulating layer  16  is 3.7 MV/cm. In contrast, when the first p well region  26   a  and the second p well region  26   b  are combined with the first deep p region  36   a  and the second deep p region  36   b , the maximum electric field strength of the gate insulating layer  16  is 3.3 MV/cm. 
     It is preferable that the distance (“d 4 ” in  FIG. 22 ) between the second plane and the first deep p region  36   a  be less than the distance (“d 3 ” in  FIG. 22 ) between the second plane and the gate insulating layer  16 , in order to reduce the maximum electric field strength of the gate insulating layer  16 . In other words, it is preferable that the depth of the first deep p region  36   a  be greater than the depth of the trench  50 . 
     Similarly, it is preferable that the distance between the second plane and the second deep p region  36   b  be less than the distance (“d 3 ” in  FIG. 22 ) between the second plane and the gate insulating layer  16 . In other words, it is preferable that the depth of the second deep p region  36   b  is greater than the depth of the trench  50 . 
     As described above, according to this embodiment, the maximum electric field strength of the gate insulating layer  16  is lower than that in the first embodiment and the MOSFET  400  that can further improve the breakdown voltage of the gate insulating layer is achieved. 
     Fifth Embodiment 
     A semiconductor device according to this embodiment is the same as the semiconductor device according to the fourth embodiment except that the eighth silicon carbide region and the eleventh silicon carbide region are provided at the bottom of a contact trench and the ninth silicon carbide region and the twelfth silicon carbide region are provided at the bottom of a contact trench. The description of the same content as that in the first embodiment and the fourth embodiment will not be repeated. 
       FIG. 23  is a cross-sectional view schematically illustrating the semiconductor device according to this embodiment. A MOSFET  500  is a trench gate MOSFET. The MOSFET  500  is a so-called double trench MOSFET. 
     The first contact region (eleventh silicon carbide region)  34   a  and the first deep p region  36   a  are provided at the bottom of a first contact trench  52   a . The second contact region (twelfth silicon carbide region)  34   b  and the second deep p region  36   b  are provided at the bottom of a second contact trench  52   b.    
     The MOSFET  500  includes the first deep p region  36   a  and the second deep p region  36   b . Therefore, similarly to the fourth embodiment, the maximum electric field strength of the gate insulating layer  16  is lower than that in the first embodiment and it is possible to further improve the breakdown voltage of the gate insulating layer. 
     Since the MOSFET  500  includes the first contact trench  52   a  and the second contact trench  52   b , ion implantation with high acceleration energy for forming the first deep p region  36   a  and the second deep p region  36   b  is not required. In addition, it is possible to reduce the number of ion implantation processes. Therefore, it is easier to manufacture the semiconductor device than the semiconductor device according to the fourth embodiment. 
     Sixth Embodiment 
     A semiconductor device according to this embodiment is the same as the semiconductor device according to the first embodiment except that it further includes a tenth silicon carbide region of a second conductivity type which is provided between the first silicon carbide region and the gate insulating layer and is separated from the second silicon carbide region and the third silicon carbide region. Hereinafter, the description of the same content as that in the first embodiment will not be repeated. 
       FIG. 24  is a cross-sectional view schematically illustrating the semiconductor device according to this embodiment. A MOSFET  600  is a trench gate MOSFET. 
     The MOSFET  600  includes a trench bottom p region  38 . The trench bottom p region (tenth silicon carbide region)  38  is provided between the drift region  24  and the gate insulating layer  16 . In other words, the trench bottom p region  38  is provided at the bottom of the trench  50 . The trench bottom p region  38  is provided so as to be separated from the first p well region (second silicon carbide region)  26   a  and the second p well region (third silicon carbide region)  26   b.    
     The trench bottom p region  38  is made of p-type SiC. The trench bottom p region  38  includes, for example, aluminum (Al) as p-type impurities. The p-type impurity concentration of the trench bottom p region  38  is, for example, equal to or greater than 5×10 15  cm −3  and equal to or less than 5×10 18  cm −3 . 
     When the MOSFET  600  is turned off, a portion in which electric field strength is high is formed at the boundary between the trench bottom p region  38  and the drift region. Therefore, the concentration of the electric field on the corner of the trench is reduced and the maximum electric field strength of the gate insulating layer  16  is reduced. As a result, the maximum electric field strength of the gate insulating layer  16  is lower than that in the first embodiment. 
     According to the simulation results, when the first p well region  26   a  and the second p well region  26   b  are not combined with the trench bottom p region  38 , the maximum electric field strength of the gate insulating layer  16  is 2.6 MV/cm. In contrast, when the first p well region  26   a  and the second p well region  26   b  are combined with the trench bottom p region  38 , the maximum electric field strength of the gate insulating layer  16  is 1.8 MV/cm. 
     As described above, according to this embodiment, the maximum electric field strength of the gate insulating layer  16  is lower than that in the first embodiment and the MOSFET  600  that can further improve the breakdown voltage of the gate insulating layer is achieved. 
     Seventh Embodiment 
     A semiconductor device according to this embodiment is the same as the semiconductor device according to the first embodiment except that the thickness of the gate insulating layer between the first silicon carbide region and the gate electrode is greater than the thickness of the gate insulating layer between the second silicon carbide region and the gate electrode and the thickness of the gate insulating layer between the third silicon carbide region and the gate electrode. Hereinafter, the description of the same content as that in the first embodiment will not be repeated. 
       FIG. 25  is a cross-sectional view schematically illustrating the semiconductor device according to this embodiment. A MOSFET  700  is a trench gate MOSFET. 
     In the MOSFET  700 , the thickness of the gate insulating layer  16  between the drift region (first silicon carbide region)  24  and the gate electrode  18  is greater than the thickness of the gate insulating layer  16  between the first p well region (second silicon carbide region)  26   a  and the gate electrode  18 . In addition, the thickness of the gate insulating layer  16  between the drift region (first silicon carbide region)  24  and the gate electrode  18  is greater than the thickness of the gate insulating layer  16  between the second p well region (third silicon carbide region)  26   b  and the gate electrode  18 . 
     In other words, the thickness of a portion of the gate insulating layer  16  which comes into contact with the bottom of the trench  50  is greater than the thickness of a portion of the gate insulating layer  16  which comes into contact with the side surface of the trench  50 . 
     When the MOSFET  700  is turned off, the concentration of the electric field on the corner of the trench is reduced by the thick portion of the gate insulating layer  16  at the bottom of the trench  50  and the maximum electric field strength of the gate insulating layer  16  is reduced. Therefore, the maximum electric field strength of the gate insulating layer  16  is lower than that in the first embodiment. 
     As described above, according to this embodiment, the maximum electric field strength of the gate insulating layer  16  is lower than that in the first embodiment and the MOSFET  700  that can further improve the breakdown voltage of the gate insulating layer is achieved. 
     Eighth Embodiment 
     An inverter circuit and a driving device according to this embodiment includes the semiconductor device according to the first embodiment. 
       FIG. 26  is a diagram schematically illustrating the driving device according to this embodiment. A driving device  800  includes a motor  140  and an inverter circuit  150 . 
     The inverter circuit  150  includes three semiconductor modules  150   a ,  150   b , and  150   c  having the MOSFET  100  according to the first embodiment as a switching element. The three semiconductor modules  150   a ,  150   b , and  150   c  are connected in parallel to each other to form the three-phase inverter circuit  150  including three AC voltage output terminals U, V, and W. The motor  140  is drive by an AC voltage which is output from the inverter circuit  150 . 
     According to this embodiment, since the inverter circuit includes the MOSFET  100  with improved characteristics, it is possible to improve the characteristics of the inverter circuit  150  and the driving device  800 . 
     Ninth Embodiment 
     A vehicle according to this embodiment includes the semiconductor device according to the first embodiment. 
       FIG. 27  is a diagram schematically illustrating the vehicle according to this embodiment. A vehicle  900  according to this embodiment is a railway vehicle. The vehicle  900  includes a motor  140  and an inverter circuit  150 . 
     The inverter circuit  150  includes three semiconductor modules having the MOSFET  100  according to the first embodiment as a switching element. The three semiconductor modules are connected in parallel to each other to form the three-phase inverter circuit  150  having three AC voltage output terminals U, V, and W. The motor  140  is driven by an AC voltage which is output from the inverter circuit  150 . Wheels  90  of the vehicle  900  are rotated by the motor  140 . 
     According to this embodiment, since the vehicle  900  includes the MOSFET  100  with improved characteristics, the characteristics of the vehicle  900  are improved. 
     Tenth Embodiment 
     A vehicle according to this embodiment includes the semiconductor device according to the fifth embodiment. 
       FIG. 28  is a diagram schematically illustrating the vehicle according to this embodiment. A vehicle  1000  according to this embodiment is a car. The vehicle  1000  includes a motor  140  and an inverter circuit  150 . 
     The inverter circuit  150  includes three semiconductor modules having the MOSFET  100  according to the first embodiment as a switching element. The three semiconductor modules are connected in parallel to each other to form the three-phase inverter circuit  150  having three AC voltage output terminals U, V, and W. 
     The motor  140  is driven by an AC voltage which is output from the inverter circuit  150 . Wheels  90  of the vehicle  1000  are rotated by the motor  140 . 
     According to this embodiment, since the vehicle  1000  includes the MOSFET  100  with improved characteristics, the characteristics of the vehicle  1000  are improved. 
     Eleventh Embodiment 
     An elevator according to this embodiment includes the semiconductor device according to the first embodiment. 
       FIG. 29  is a diagram schematically illustrating the elevator according to this embodiment. An elevator  1100  according to this embodiment includes a basket  610 , a counter weight  612 , a wire rope  614 , a hoist  616 , a motor  140 , and an inverter circuit  150 . 
     The inverter circuit  150  includes three semiconductor modules having the MOSFET  100  according to the first embodiment as a switching element. The three semiconductor modules are connected in parallel to each other to form the three-phase inverter circuit  150  having three AC voltage output terminals U, V, and W. 
     The motor  140  is driven by an AC voltage which is output from the inverter circuit  150 . The hoist  616  is rotated by the motor  140  to move up and down the basket  610 . 
     According to this embodiment, since the elevator  1100  includes the MOSFET  100  with improved characteristics, the characteristics of the elevator  1100  are improved. 
     In the first to seventh embodiments, the MOSFET is given as an example of the semiconductor device. However, the invention may be applied to an insulated gate bipolar transistor (IGBT). 
     In the first to seventh embodiments, an example in which the first conductivity type is an n type and the second conductivity type is a p type has been described. However, the first conductivity type may be a p type and the second conductivity type may be an n type. 
     In the first to seventh embodiments, an example in which the SiC layer is made of 4H—SiC has been described above. However, the invention can be applied to other crystal forms such as 3C—SiC and 6H—SiC. 
     In the first to seventh embodiments, it is preferable that the n-type impurity be, for example, nitrogen (N) or phosphorus (P). However, the n-type impurity may be, for example, arsenic (As) or antimony (Sb). It is preferable that the p-type impurity be, for example, aluminum (Al). However, the p-type impurity may be, for example, boron (B), gallium (Ga), or indium (In). 
     For example, the components described in the first to seventh embodiments may be combined with each other in order to reduce the maximum electric field strength of the gate insulating layer  16 . 
     In the ninth to eleventh embodiments, an example in which the semiconductor device according to the invention is applied to the vehicle or the elevator has been described. However, the semiconductor device according to the invention may be applied to, for example, a power conditioner of a photovoltaic power generation system. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the semiconductor device, the semiconductor device manufacturing method, the inverter circuit, the driving device, the vehicle, and the elevator described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the devices and methods described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.