Patent Publication Number: US-2023154986-A1

Title: Insulated-gate semiconductor device and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 17/487,563, filed on Sep. 28, 2021, which is a divisional of U.S. application Ser. No. 16/844,113, filed Apr. 9, 2020, which is a divisional of U.S. application Ser. No. 16/661,925, filed Oct. 23, 2019, which is a divisional of U.S. application Ser. No. 16/170,530, filed Oct. 25, 2018, which claims benefit of priority under 35 USC 119 based on Japanese Patent Application No. 2017-237033 filed on Dec. 11, 2017, the entire contents of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an insulated-gate semiconductor device having an insulated-gate electrode structure in a trench and a method of manufacturing the same. 
     BACKGROUND 
     In a trench gate MOS field effect transistor (MOSFET), as compared with a planer gate MOSFET, reduction of on-resistance due to reduction of cell pitch can be expected. In a trench gate MOSFET using a wide band gap semiconductor such as silicon carbide (SiC) as a material, structures using an a-plane (11−20) on a sidewall surface of a trench have been proposed (refer to US 2017/0,077,251 A (Patent Literature 1), JP 6105032 B2 (Patent Literature 2), and JP 2016-163047 A (Patent Literature 3)). In Patent Literatures 1 to 3, an n-type source region and a p-type base region are provided on one sidewall surface of a trench, and the sidewall surface is used as a current path. 
     In an insulated-gate semiconductor device such as a trench gate MOSFET using a wide band gap semiconductor as a material, further improvement of the structure of the insulated-gate semiconductor device and the method of manufacturing the insulated-gate semiconductor device is required. 
     SUMMARY 
     In view of the above problems, an object of the invention is to provide an insulated-gate semiconductor device capable of further improving the insulated-gate semiconductor device and a method of manufacturing the same. 
     An aspect of the present invention inheres in an insulated-gate semiconductor device having a plurality of trenches arranged in a chip structure, the plurality of trenches defining both sidewalls in a first sidewall surface having a first inclination angle with respect to a reference plane of the chip structure and a second sidewall surface facing the first sidewall surface and having a second inclination angle different from the first inclination angle with respect to the reference plane, including: a first unit cell provided with an insulated-gate electrode structure in a first trench included in the plurality of trenches and including a first conductivity type main-electrode region being in contact with a first sidewall surface of the first trench, a second conductivity type base region being in contact with a bottom surface of the main-electrode region and the first sidewall surface, a first conductive type drift layer being in contact with a bottom surface of the base region and the first sidewall surface and having an impurity concentration lower than that of the main-electrode region, and a second conductivity type gate protection-region being in contact with the second sidewall surface and a bottom surface of the first trench and having an impurity concentration higher than that of the base region; and a second unit cell provided with an insulated-gate electrode structure in a second trench included in the plurality of trenches and including a second conductivity type operation suppression region being buried in an upper portion of the drift layer, being in contact with a first sidewall surface and a second sidewall surface of the second trench, and having an impurity concentration higher than that of the base region, wherein the second unit cell is arranged so as to include the second trench located at one end of an array of the plurality of trenches. 
     Another aspect of the present invention inheres in a method of an insulated-gate semiconductor device encompassing: a plurality of unit cells, the unit cell including: an insulated-gate electrode structure arranged inside a trench defining both sidewalls in a first sidewall surface having a first inclination angle with respect to a reference plane of a chip structure and a second sidewall surface facing the first sidewall surface and having a second inclination angle different from the first inclination angle with respect to the reference plane; a first conductivity type main-electrode region being in contact with a first sidewall surface of the trench; a second conductivity type base region being in contact with a bottom surface of the main-electrode region and the first sidewall surface; a first conductivity type drift layer being in contact with a bottom surface of the base region and the first sidewall surface and having an impurity concentration lower than that of the main-electrode region; a second conductivity type gate protection-region being in contact with the second sidewall surface and a bottom surface of the trench and having an impurity concentration higher than that of the base region; and a main electrode being in contact with the main-electrode region, wherein a Schottky barrier diode implemented by the drift layer and the main electrode located between the adjacent unit cells is embedded. 
     Further aspect of the present invention inheres in a method of an insulated-gate semiconductor device encompassing: an insulated-gate electrode structure arranged inside a trench defining both sidewalls in a first sidewall surface having a first inclination angle with respect to a reference plane of a chip structure and a second sidewall surface facing the first sidewall surface and having a second inclination angle different from the first inclination angle with respect to the reference plane; a first conductivity type main-electrode region being in contact with a first sidewall surface of the trench and having a high impurity concentration; a second conductivity type base region being in contact with a bottom surface of the main-electrode region and the first sidewall surface; a first conductivity type drift layer being in contact with a bottom surface of the base region and the first sidewall surface and having an impurity concentration lower than that of the main-electrode region; a second conductivity type gate protection-region being in contact with the second sidewall surface and a bottom surface of the trench and having an impurity concentration higher than that of the base region; and a second conductivity type base-contact region being in contact with the gate protection-region while being separated from the trench and having an impurity concentration higher than that of the base region. 
     Still further aspect of the present invention inheres in a method of an insulated-gate semiconductor device encompassing a plurality of stripe-shaped unit cells arranged, the unit cell including: a first conductivity type drift layer; a second conductivity type base region provided on the drift layer; a first conductivity type main-electrode region provided in an upper portion of the base region and having an impurity concentration higher than that of the drift layer; an insulated-gate electrode structure provided inside a stripe-shaped trench so that one sidewall surface is in contact with the main-electrode region and the base region; and a second conductivity type gate protection-region provided in a stripe shape on the drift layer so as to be in contact with a bottom surface and the other sidewall surface of the trench and having an impurity concentration higher than that of the base region, wherein a structure of interposing the common base region between the trenches of adjacent unit cells and a structure of interposing the common gate protection-region between the trenches of adjacent unit cells are alternately repeated, and the gate protection-regions are intermittently arranged along a longitudinal direction of the trench. 
     Still further aspect of the present invention inheres in a method of an insulated-gate semiconductor device encompassing: forming a second conductivity type base region on a first conductivity type drift layer; forming a first conductivity type main-electrode region having an impurity concentration higher than that of the drift layer in an upper portion of the base region; forming a trench defining both sidewalls in a first sidewall surface having a first inclination angle with respect to a reference plane of a chip structure with the main-electrode region being formed and a second sidewall surface facing the first sidewall surface and having a second inclination angle different from the first inclination angle with respect to the reference plane until reaching the drift layer and exposing the main-electrode region and the base region to the first sidewall surface; forming a second conductivity type gate protection-region being in contact with a bottom surface and the first sidewall surface of the trench by performing ion implantation obliquely on a bottom surface and the first sidewall surface of the trench; and forming an insulated-gate electrode structure inside the trench. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a plan view illustrating an example of an insulated-gate semiconductor device according to a first embodiment; 
         FIG.  2    is a vertical cross-sectional view of the insulated-gate semiconductor device according to the first embodiment taken from the A-A direction in  FIG.  1   ; 
         FIG.  3    is a vertical cross-sectional view of the insulated-gate semiconductor device according to the first embodiment taken from the B-B direction in  FIG.  1   ; 
         FIG.  4    is a schematic view for describing a plane orientation of a sidewall surface of a trench; 
         FIG.  5    is a schematic view for describing the plane orientation of the sidewall surface of the trench; 
         FIG.  6    is a graph illustrating a relationship among the plane orientation of the sidewall surface of the trench, a gate voltage, and mobility; 
         FIG.  7    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the first embodiment; 
         FIG.  8    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the first embodiment; 
         FIG.  9    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the first embodiment; 
         FIG.  10    is a cross-sectional view of main components illustrating an example of an insulated-gate semiconductor device according to a second embodiment; 
         FIG.  11    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the second embodiment; 
         FIG.  12    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the second embodiment; 
         FIG.  13    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the second embodiment; 
         FIG.  14    is a cross-sectional view of main components illustrating an example of an insulated-gate semiconductor device according to a third embodiment; 
         FIG.  15    is a process cross-sectional view for describing an example of a method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  16    is a process cross-sectional view continued from  FIG.  15    for describing the example of the method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  17    is a process cross-sectional view continued from  FIG.  16    for describing the example of the method of manufacturing an insulated-gate semiconductor device according to a third embodiment; 
         FIG.  18    is a process cross-sectional view continued from  FIG.  17    for describing the example of the method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  19    is a process cross-sectional view continued from  FIG.  18    for describing the example of the method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  20    is a process cross-sectional view continued from  FIG.  19    for describing the example of the method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  21    is a process cross-sectional view continued from  FIG.  20    for describing the example of the method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  22    is a process cross-sectional view continued from  FIG.  21    for describing the example of the method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  23    is a process cross-sectional view continued from  FIG.  22    for describing the example of the method of manufacturing the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  24    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  25    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  26    is a cross-sectional view of main components illustrating another example of the insulated-gate semiconductor device according to the third embodiment; 
         FIG.  27    is a cross-sectional view of main components illustrating an example of an insulated-gate semiconductor device according to a fourth embodiment; 
         FIG.  28    is a horizontal cross-sectional view of the insulated-gate semiconductor device according to the fourth embodiment taken from the A-A direction in  FIG.  27   ; 
         FIG.  29    is a vertical cross-sectional view of the insulated-gate semiconductor device according to the fourth embodiment taken from the C-C direction in  FIG.  28   ; 
         FIG.  30    is another horizontal cross-sectional view of the insulated-gate semiconductor device according to Comparative Example taken from the A-A direction in  FIG.  27   ; 
         FIG.  31    is another horizontal cross cross-sectional view of the insulated-gate semiconductor device according to the fourth embodiment taken from the A-A direction in  FIG.  27   ; and 
         FIG.  32    is another horizontal cross cross-sectional view of the insulated-gate semiconductor device according to the fourth embodiment taken from the A-A direction in  FIG.  27   . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the Drawings, a first to fourth embodiments of the present invention will be described below. In the Specification and the Drawings, the same or similar elements are indicated by the same or similar reference numerals. The drawings are schematic, and it should be noted that the relationship between thickness and planer dimensions, the thickness proportion of each layer, and the like are different from real ones. Accordingly, specific thicknesses or dimensions should be determined with reference to the following description. Moreover, in some drawings, portions are illustrated with different dimensional relationships and proportions. The embodiments described below merely illustrate schematically semiconductor devices for specifying and giving shapes to the technical idea of the present invention, and the span of the technical idea is not limited to materials, shapes, structures, and relative positions of elements described herein. 
     In the Specification, a “first main-electrode region” is assigned to a semiconductor region which will be a source region or a drain region in a field-effect transistor (FET) or a static induction transistor (SIT), an emitter region or a collector region in an insulated-gate bipolar transistor (IGBT) and an anode region or a cathode region in a static induction (SI) thyristor or a gate turn-off (GTO) thyristor. A “second main-electrode region” is assigned to a semiconductor region which will not be the first main-electrode region and will be the source region or the drain region in the FET or the SIT, the emitter region or the collector region in the IGBT, and the anode region or the cathode region in the SI thyristor or the GTO thyristor. That is, when the first main-electrode region is the source region, the second main-electrode region means the drain region. When the first main-electrode region is the emitter region, the second main-electrode region means the collector region. When the first main-electrode region is the anode region, the second main-electrode region means the cathode region. In some appropriate cases such as MOSFET, a function of the first main-electrode region and a function of the second main-electrode region are exchangeable each other by exchanging a bias relationship if the structure of the subject semiconductor device is symmetric. Furthermore, a “main-electrode region” is described in the Specification, the main-electrode region comprehensively means any one of the first main-electrode region and the second main-electrode region. 
     In addition, in the following description, the definition of the directions such as up and down is merely provided for the convenience of description and does not limit the technical idea of the invention. For example, when an object is rotated by 90° and observed, the direction will be read by changing the up and down to the left and right; and when an object is rotated by 180° and observed, the direction will be read by inverting the up and down. In addition, in the following description, a case where the first conductivity type is n-type and the second conductivity type is p-type will be exemplarily described. However, the conductivity types may be selected in an inverse relationship, so that the first conductivity type may be p-type and the second conductivity type may be n-type. In addition, “+” or “−” attached to “n” or “p” denote that a semiconductor region has a relatively high or low impurity concentration as compared with a semiconductor region to which “+” and “−” are not attached. However, even semiconductor regions attached with the same “n” and “n” do not denote that the impurity densities of the respective semiconductor regions are strictly the same. In addition, in the following description, it is technically and logically obvious that members or regions doped with limitations of the “first conductivity type” and “second conductivity type” denote members or regions made of semiconductor materials without particular limitation. In addition, in this specification, in the Miller index notation, “−” denotes a bar attached to the index following the Miller index, and a negative sign is represented by attaching “−” before the index. 
     As illustrated in  FIG.  1   , the insulated-gate semiconductor device (MISFET) according to the first embodiment has a chip structure including an array of a plurality of unit cells C 1  to C 3 , . . . C 4  to C 6 , each of which includes a plurality of trenches  10   a  to  10   c , . . . ,  10   d  to  10   f  having the same shape. In addition, in the insulated-gate semiconductor device according to the first embodiment, the number of trenches and the number of unit cells are not particularly limited. The insulated-gate semiconductor device according to the first embodiment can be implemented as a power semiconductor device (power device) for flowing a large current by forming a multi-channel structure by further arranging a plurality of unit cells. 
     In  FIG.  1   , the planar patterns of the trenches  10   a  to  10   c , . . . ,  10   d  to  10   f  are schematically illustrated by broken lines. The trenches  10   a  to  10   c , . . . ,  10   d  to  10   f  are in a stripe shape and extend parallel to each other. In a direction (parallel direction of the trenches  10   a  to  10   c , . . . ,  10   d  to  100  perpendicular to the longitudinal direction of the trenches  10   a  to  10   c , . . . ,  10   d  to  10   f , the unit cells C 1  to C 3  are located in the left peripheral portion of the array structure, and the unit cell C 1  is located at one end of the array structure. In addition, the unit cells C 4  to C 6  are located in the right peripheral portion of the array structure, and the unit cell C 6  is located at the other end of the array structure. 
     A vertical cross-sectional view of the unit cells C 1  to C 3  in the left peripheral portion illustrated in  FIG.  1    taken from the A-A direction corresponds to  FIG.  2   . As illustrated in  FIG.  2   , the insulated-gate semiconductor device according to the first embodiment includes a first conductivity type (n-type) drift layer  1  and second conductivity type (p-type) base regions  3   a  and  3   b  arranged on the drift layer  1 . Each of the drift layer  1  and the base regions  3   a  and  3   b  is implemented by an epitaxial growth layer made of SiC. 
     n + -type first main-electrode regions (source regions)  4   a  to  4   d  having an impurity concentration higher than that of the drift layer  1  are selectively provided in upper portions of the base regions  3   a  and  3   b . The source regions  4   a  and  4   c  are in contact with the base regions  3   a  and  3   b , respectively. In addition, since the source regions  4   b  and  4   d  are not used as current paths, the source regions  4   b  and  4   d  may not be provided. p + -type gate protection-regions  2   a  and  2   b  having an impurity concentration higher than that of the base regions  3   a  and  3   b  are selectively provided on the drift layer  1 . The upper surfaces of the gate protection-regions  2   a  and  2   b  are located at the same horizontal level as the upper surfaces of the source regions  4   a  to  4   d . The gate protection-region  2   a  is in contact with the source regions  4   b  and  4   c  and the base region  3   b.    
     Trenches  10   a  to  10   c  are provided so as to extend from the upper surfaces of the source regions  4   a  to  4   d  and reach the drift layer  1 . In  FIG.  2   , the case where both sidewall surfaces of the trenches  10   a  to  10   c  are parallel in the vertical direction is exemplified, but the sidewall surfaces of the trenches  10   a  to  10   c  are not limited to be parallel in the vertical direction. For example, both sidewall surfaces of the trenches  10   a  to  10   c  may be inclined so as to be tapered downward. In addition, although the case where the bottom surfaces of the trenches  10   a  to  10   c  are flat is exemplified, but the bottom surfaces may be curved surfaces or corner portions of the bottom surfaces may have curvatures. 
     One sidewall surface (first sidewall surface described later) of the trench  10   b  is in contact with the source region  4   a  and the base region  3   a , and the other sidewall surface (second sidewall surface described later) is in contact with the source region  4   b  and the gate protection-region  2   b . One sidewall surface of the trench  10   d  is in contact with the source region  4   c  and the base region  3   b , and the other sidewall surface is in contact with the source region  4   d  and the gate protection-region  2   b . Although the case where the bottom surfaces of the trenches  10   b  and  10   d  are in contact with the drift layer  1  and the gate protection-regions  2   a  and  2   b , respectively, is exemplified, all the bottom surfaces of the trenches  10   b  and  10   d  may be covered with the gate protection-regions  2   a  and  2   b , respectively. On the other hand, both sidewall surfaces and the bottom surface of the trench  10   a  are covered with a p + -type operation suppression region  2   x . The operation suppression region  2   x  is in contact with the source region  4   a  and the base region  3   a . The operation suppression region  2   x  is provided at the same depth as the gate protection-regions  2   a  and  2   b.    
     Gate insulating films  5   a  to  5   c  are provided on the bottom surfaces and the sidewall surfaces of the trenches  10   a  to  10   c . As the gate insulating films  5   a  to  5   c , in addition to a silicon oxide film (SiO 2  film), there may be adopted a single layer film of any one of a silicon oxynitride (SiON) film, a strontium oxide (SrO) film, a silicon nitride (Si 3 N 4 ) film, an aluminum oxide (Al 2 O 3 ) film, a magnesium oxide (MgO) film, an yttrium oxide (Y 2 O 3 ) film, a hafnium oxide (HfO 2 ) film, a zirconium oxide (ZrO 2 ) film, a tantalum oxide (Ta 2 O 5 ) film, and a bismuth oxide (Bi 2 O 3 ) film or a composite film obtained by laminating a plurality of these films. 
     Gate electrodes  6   a  to  6   c  are buried inside the trenches  10   a  to  10   c  with gate insulating films  5   a  to  5   c  interposed, so that an insulated-gate electrode structure ( 5   a ,  6   a ;  5   b ,  6   b ;  5   c ,  6   c ) is implemented. As the material of the gate electrodes  6   a  to  6   c , for example, a polysilicon layer (doped polysilicon layer) which is doped with impurities such as phosphorus (P) at a high impurity concentration may be used. 
     A first main electrode (source electrode)  8  is arranged on the gate electrodes  6   a  to  6   c  with an interlayer insulating film  7  interposed. As the interlayer insulating film  7 , there may be adopted a non-doped silicon oxide film (SiO 2  film) not containing phosphorus (P) or boron (B), which is called “NSG”. However, as the interlayer insulating film  7 , there may be adopted a phosphosilicate glass (PSG) film, a borosilicate glass (BSG) film, a borophosphosilicate glass (BPSG) film, a silicon nitride (Si 3 N 4 ) film, or the like. The source electrode  8  is electrically connected to the source regions  4   a  and  4   c  and the gate protection-regions  2   a  and  2   b . The source electrode  8  is arranged separately from the gate surface electrode (not illustrated) located at the back of the paper surface. 
     For example, the source electrode  8  may be implemented by an aluminum (Al) film. As the gate surface electrode, the same material as the source electrode  8  may be used. Although not illustrated, a source contact layer and a barrier metallic layer serving as underlying metal may be arranged under the source electrode  8 . For example, the source contact layer may be implemented by a nickel silicide (NiSi x ) film, and the barrier metallic layer may be implemented by a titanium nitride (TiN) film. 
     On the bottom surface of the drift layer  1 , an n + -type second main-electrode region (drain region)  9  is arranged so as to be in contact with the drift layer  1 . The drain region  9  is implemented by a semiconductor substrate (SiC substrate) made of SiC. A second main electrode (drain electrode)  11  is arranged on the bottom surface of the drain region  9 . As the drain electrode  11 , there may be used, for example, a single layer film made of gold (Au) or a metal film laminated in the order of Al, nickel (Ni), and Au, and a metal film such as molybdenum (Mo) or tungsten (W) or an alloy layer obtained by depositing nickel (Ni) and titanium (Ti) and reacting with SiC may be further laminated at the lowermost layer of the single layer film or the metal film. 
     On the other hand, a vertical cross cross-sectional view of the unit cells C 4  to C 6  located in the right peripheral portion illustrated in  FIG.  1    taken from the B-B direction corresponds to  FIG.  3   . As illustrated in  FIG.  3   , p-type base regions  3   c  to  3   e  are arranged on the n-type drift layer  1 . n + -type source regions  4   e  to  4   j  are selectively provided in upper portions of the base regions  3   c  to  3   e . The source regions  4   e ,  4   g , and  4   h  are in contact with the base regions  3   c  to  3   e , respectively. In addition, since the source regions  4   f ,  4   h , and  4   j  are not used as current paths, the source regions  4   f ,  4   h , and  4   j  may not be provided. p + -type gate protection-regions  2   c  to  2   e  are selectively provided on the drift layer  1 . The upper surfaces of the gate protection-regions  2   c  to  2   e  are located at the same horizontal level as the upper surfaces of the source regions  4   e  to  4   j . The gate protection-region  2   c  is in contact with the source region  4   g  and the base region  3   d . The gate protection-region  2   d  is in contact with the source region  4   i  and the base region  3   e.    
     Trenches  10   d  to  10   f  are provided so as to extend from the upper surfaces of the source regions  4   e  to  4   j  and reach the drift layer  1 . One sidewall surface of the trench  10   d  is in contact with the source region  4   e  and the base region  3   c , and the other sidewall surface is in contact with the source region  4   f  and the gate protection-region  2   c . One sidewall surface of the trench  10   e  is in contact with the source region  4   g  and the base region  3   d , and the other sidewall surface is in contact with the source region  4   h  and the gate protection-region  2   d . One sidewall surface of the trench  10   f  is in contact with the source region  4   i  and the base region  3   e , and the other sidewall surface is in contact with the source region  4   j  and the gate protection-region  2   e . Although the case where the bottom surfaces of the trenches  10   d  to  10   f  are in contact with the drift layer  1  and the gate protection-regions  2   c  to  2   e , respectively, is exemplified, all the bottom surfaces of the trenches  10   d  to  10   f  may be covered with the gate protection-regions  2   c  to  2   e , respectively. 
     The gate electrodes  6   d  to  6   f  are buried inside the trenches  10   d  to  10   f  through the gate insulating films  5   d  to  5   f  to implement insulated-gate electrode structures ( 5   d ,  6   d ), ( 5   e ,  6   e ), and ( 5   f ,  6   f ). A source electrode  8  is arranged on the gate electrodes  6   d  to  6   f  with an interlayer insulating film  7  interposed. The source electrode  8  is electrically connected to the source regions  4   e ,  4   g , and  4   h  and the gate protection-regions  2   c  to  2   e . An n + -type drain region  9  is arranged on the bottom surface of the drift layer  1  so as to be in contact with the drift layer  1 . A drain electrode  11  is arranged on the bottom surface of the drain region  9 . 
     The plane orientation to be used for the sidewall surfaces of the trenches  10   a  to  10   c , . . . ,  10   d  to  10   f  illustrated in  FIGS.  1  to  3    will be described with reference to  FIGS.  4  to  6   . As illustrated in  FIG.  4   , the chip structure in which the trenches  10   a  to  10   c , . . . ,  10   d  to  10   f  illustrated in  FIGS.  1  to  3    are formed has, for example, an off angle θ 1  of about 4° to 8° in the &lt;11−20&gt; direction with respect to &lt;0001&gt; direction (c-axis). The off angle θ 1  is an angle defined by a plane (base plane) perpendicular to the c-axis which is the (0001) plane (Si plane) or (000−1) plane (C plane) and a reference plane of the chip structure. A straight line L 1  indicated by a plurality of solid lines on the side surface of the chip structure schematically illustrates an Si plane. It is considered that a trench T 1  and a trench T 2  perpendicular to the trench T 1  are provided in this chip structure. As the sidewall surfaces S 1  and S 2  of the trench T 1 , an m-plane which is a (1−100) plane perpendicular to the (0001) plane is used. Since the sidewall surfaces S 1  and S 2  of the trench T 1  are actually formed in a tapered shape, all the sidewall surfaces S 1  and S 2  of the trench T 1  are m-planes inclined by about 9° to the Si plane side. 
       FIG.  5    illustrates a case where the trench T 2  is provided in the chip structure. As illustrated in  FIG.  5   , as all the facing sidewall surfaces S 3  and S 4  of the trench T 2 , an a-plane which is the (11−20) plane is used. In  FIG.  5   , broken lines L 2  and L 3  parallel to the a-plane are schematically illustrated. In this case, since the semiconductor wafer has the off angle θ 1 , the inclination angle θ 2  of one sidewall surface S 3  of the trench T 2  with respect to the a-plane is different from the inclination angle θ 3  of the other sidewall surface S 4  with respect to the a-plane. For example, in a case where the off angle θ 1  is 4°, the inclination angle θ 2  of the sidewall surface S 3  of the trench T 2  on the Si plane side with respect to the a-plane is 5°, and the inclination angle θ 4  of the sidewall surface S 4  of the trench T 2  on the Si plane side with respect to the a-plane is 13°.  FIG.  6    illustrates the relationship between the gate voltage and the electron mobility for the m-plane inclined by 9° to the Si plane side, the a-plane inclined by 5° to the Si plane side, and the a-plane inclined by 13° to the Si plane side. Referring to  FIG.  6   , the electron mobility is heightened in the order of the a-plane inclined by 5° to the Si plane side, the m-plane inclined by 9° to the Si plane side, and the a-plane inclined by 13° to the Si plane side. 
     In the insulated-gate semiconductor device according to the first embodiment, as the sidewall surfaces of the trenches  10   b  to  10   c , . . . ,  10   d  to  10   f  illustrated in  FIGS.  1  to  3    which are in contact with the source regions  4   a ,  4   c ,  4   e ,  4   g , and  4   i  and the base regions  3   a  to  3   e , the a-plane having a relatively small inclination angle θ 2  to the Si plane side and a high electron mobility is used as a current path. This a-plane is defined as a “first sidewall surface” having a first inclination angle θ 2  with respect to the reference plane (a-plane) of the chip structure. 
     On the other hand, as the sidewall surfaces of the trenches  10   b  to  10   c , . . . ,  10   d  to  10   f  illustrated in  FIGS.  1  to  3    which are in contact with the source regions  4   b ,  4   d ,  4   f ,  4   h  and  4   j  and the gate protection-regions  2   a  to  2   e , the a-plane having a relatively large inclination angle θ 3  to the Si plane side and a low electron mobility is used. The a-plane is defined as a low “second sidewall surface” facing the first sidewall surface, having a second inclination angle θ 3  different from the first inclination angle θ 2  with respect to the reference plane (a-plane), having an electron mobility lower than that of the first sidewall surface. In this manner, both sidewalls of the trenches  10   b  to  10   c , . . . ,  10   d  to  10   f  are defined as a “first sidewall surface” and a “second sidewall surface”. 
     At the time of the operation of the insulated-gate semiconductor device according to the first embodiment, a positive voltage is applied to the drain electrode  11 , and a positive voltage equal to or higher than a threshold value is applied to the gate electrodes  6   a  to  6   f . As a result, in the unit cells C 2  to C 6  excluding the unit cell C 1  located at one end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , inversion layers (channels) are formed on the sides of the base regions  3   a  to  3   e  closer to the gate electrodes  6   b  to  6   f , so that the transistor is be turned on. In the turned-on state, a current flows from the drain electrode  11  to the source electrode  8  through the drain region  9 , the drift layer  1 , the inversion layers of the base regions  3   a  to  3   e , and the source regions  4   a ,  4   c ,  4   e ,  4   g  and  4   i . On the other hand, in a case where the voltage applied to the gate electrodes  6   a  to  6   f  is lower than the threshold value, since no inversion layer is formed in the base regions  3   a  to  3   e , the transistor is turned off, and thus, no current flows from the drain electrode  11  to the source electrode  8 . On the other hand, in the unit cell C 1  located at one end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , the both sidewall surfaces and the bottom surface of the trench  10   a  are covered with the operation suppression region  2   x . Therefore, at the time of operation of the insulated-gate semiconductor device according to the first embodiment, the operation of the unit cell C 1  is suppressed. 
     In the unit cell C 1  located at the end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , the pattern of the trench  10   a  is relatively easily collapsed. On the other hand, according to the insulated-gate semiconductor device of the first embodiment, in the unit cell C 1  located at the end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , the bottom and both sidewall surfaces of the trench  10   a  and are covered with the p + -type operation suppression region  2   x . As a result, even in a case where the pattern of the trench  10   a  is collapsed, since the operation of the unit cell C 1  is suppressed, the reliability can be improved. 
     In addition, as illustrated in  FIG.  7   , also in the unit cell C 6  located at the other end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , the bottom and both sidewall surfaces of the trench  10   f  may be covered with a p + -type operation suppression region  2   y . The operation suppression region  2   y  is in contact with the bottom and the sidewall surface of the trench  10   e  of the unit cell C 5  adjacent to the unit cell C 6 . The operation suppression region  2   y  is a semiconductor region common to the gate protection-region of the unit cell C 5  and also functions as a gate protection-region of the unit cell C 5 . According to the structure illustrated in  FIG.  7   , even in a case where the patterns of the trenches  10   a  and  10   f  of the unit cells C 1  and C 6  located at both ends of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6  are collapsed, since the operations of the unit cells C 1  and C 6  are suppressed, the reliability can be improved. 
     In addition, as illustrated in  FIG.  8   , in the two unit cells C 1  and C 2  located at the ends of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , the bottom and both sidewall surfaces of the trenches  10   a  and  10   b  may be covered with the p + -type operation suppression region  2   x . As a result, even in a case where the patterns of the trenches  10   a  and  10   b  of the two unit cells C 1  and C 2  located at the end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6  are collapsed, the operations of the unit cells C 1  and C 2  are suppressed, so that reliability can be improved. 
     In addition, as illustrated in  FIG.  9   , also in the two unit cells C 5  and C 6  located at the other ends of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , the bottom and both sidewall surfaces of the trenches  10   e  and  10   f  may be covered with the p + -type operation suppression region  2   y . The operation suppression region  2   y  is in contact with the bottom and the sidewall surface of the trench  10   d  of the unit cell C 4  adjacent to the unit cell C 5 . The operation suppression region  2   y  is a semiconductor region common to the gate protection-region of the unit cell C 4  and also functions as a gate protection-region of the unit cell C 4 . According to the structure illustrated in  FIGS.  8  and  9   , in the unit cells C 1  and C 2  and the unit cells C 5  and C 6  located at both ends of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6 , even in a case where the patterns of the trenches  10   a  and  10   b  and the trench  10   e  and  10   f  are collapsed, the operations of the unit cells C 1  and C 2  and the unit cells C 5  and C 6  are suppressed, so that the reliability can be improved. 
     In addition, the structure in which the trench  10   a  of one unit cell C 0  at one end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6  illustrated in  FIG.  1    is covered with the p + -type operation suppression region  2   x  and the structure in which the trenches  10   e  and  10   f  of the two unit cells C 5  and C 6  at the other ends of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6  illustrated in  FIG.  9    are covered with the p + -type operation suppression region  2   y  may be combined. In addition, the structure in which the trenches  10   a  and  10   b  of the two unit cells C 0  at one end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6  illustrated in  FIG.  8    are covered with the p + -type operation suppression region  2   x , and the structure in which the trench  10   f  of one unit cell C 6  at the other end of the array structure of the unit cells C 1  to C 3 , . . . , C 4  to C 6  illustrated in  FIG.  7    is covered with the p + -type operation suppression region  2   y  may be combined. 
     Second Embodiment 
     As illustrated in  FIG.  10   , the insulated-gate semiconductor device according to the second embodiment includes a first conductivity type (n-type) drift layer  1  and second conductivity type (p + -type) base regions  3   a  and  3   b  selectively provided on the drift layer  1 . First conductivity type main-electrode regions (source regions)  4   a  to  4   d  having an impurity concentration higher than that of the drift layer  1  are provided in upper portions of the base regions  3   a  and  3   b . In addition, since the main-electrode regions  4   b  and  4   d  are not used as current paths, the main-electrode regions may not be provided. 
     Trenches  10   a  and  10   b  are provided so as to penetrate the source regions  4   a  to  4   d . One sidewall surface of the trench  10   a  is in contact with the source region  4   a  and the base region  3   a , and the other sidewall surface is in contact with the source region  4   b . One sidewall surface of the trench  10   b  is in contact with the source region  4   c  and the base region  3   b , and the other sidewall surface is in contact with the source region  4   d.    
     In the insulated-gate semiconductor device according to the second embodiment, the a-plane having relatively high electron mobility is used as the sidewall surfaces of the trenches  10   a  and  10   b  closer to the source regions  4   a  and  4   c  and the base regions  3   a  and  3   b . On the other hand, the a-plane having relatively low electron mobility is used as the sidewall surfaces of the trenches  10   a  and  10   b  closer to the source regions  4   b  and  4   d . That is, the sidewall surfaces of the trenches  10   a  and  10   b  closer to the source regions  4   a  and  4   c  and the base regions  3   a  and  3   b  become the first sidewall surfaces, and the sidewall surfaces of the trenches  10   a  and  10   b  closer to the source regions  4   b  and  4   d  become the second sidewall surfaces. 
     Insulated gate type electrode structures ( 5   a ,  6   a ) and ( 5   b ,  6   b ) implemented by gate insulating films  5   a  and  5   b  and gate electrodes  6   a  and  6   b , respectively, are provided inside the trenches  10   a  and  10   b . Second conductivity type (p + -type) gate protection-regions  2   a  to  2   c  having an impurity concentration higher than that of the base regions  3   a  and  3   b  are selectively provided on the drift layer  1 . The gate protection-region  2   a  is in contact with the source region  4   a  and the base region  3   a . The gate protection-region  2   b  is in contact with the bottom surface and the sidewall surface of the trench  10   a  and in contact with the source region  4   b . The gate protection-region  2   c  is in contact with the bottom surface and the sidewall surface of the trench  10   b  and is in contact with the source region  4   d.    
     A first main electrode (source electrode)  8  is arranged on the gate electrodes  6   a  and  6   b  with an interlayer insulating film  7  interposed. The source electrode  8  is in contact with the source regions  4   a  and  4   c  and the gate protection-regions  2   a  to  2   c . An n + -type second main-electrode region (drain region)  9  is arranged on the bottom surface of the drift layer  1  so as to be in contact with the drift layer  1 . A second main electrode (drain electrode)  11  is arranged on the bottom surface of the drain region  9 . 
     In the insulated-gate semiconductor device according to the second embodiment, a Schottky barrier diode D 1  implemented by the drift layer  1  and the source electrode  8  (schematically indicated by circuit symbols in  FIG.  10   ) is provided between unit cells including the trenches  10   a  and  10   b . The Schottky barrier diode D 1  functions as a freewheeling diode (FWD). In the structure illustrated in  FIG.  10   , the Schottky junction of the Schottky barrier diode D 1  is formed by the upper surface of the drift layer  1  located at the same horizontal level as the upper surfaces of the source regions  4   a  to  4   d  and the source electrode  8 . Other structures and basic operations of the insulated-gate semiconductor device according to the second embodiment are the same as those of the insulated-gate semiconductor device according to the first embodiment, and thus, redundant description will be omitted. 
     According to the insulated-gate semiconductor device pertaining to the second embodiment, by incorporating the Schottky barrier diode D 1 , externally attached FWD becomes unnecessary, so that the number of parts can be reduced. 
     In Modified Example of the insulated-gate semiconductor device according to the second embodiment illustrated in  FIG.  11   , the structure of a Schottky barrier diode D 2  is different from that of the insulated-gate semiconductor device according to the second embodiment illustrated in  FIG.  10   . p + -type gate protection-regions  2   a  to  2   c  and a p + -type base-contact region  2   f  are selectively provided on the drift layer  1 . The source electrode  8  has a protrusion  8   a  buried down to the same depth as the bottom surfaces of the trenches  10   a  and  10   b  so as to be interposed between the gate protection-region  2   b  and the base-contact region  2   f . A Schottky junction is formed by the bottom surface of the protrusion  8   a  of the source electrode  8  and the drift layer  1 , so that the Schottky barrier diode D 2  is implemented. 
     In addition, in Modified Example of the insulated-gate semiconductor device according to the second embodiment illustrated in  FIG.  12   , the structure of a Schottky barrier diode D 3  is different from that of the insulated-gate semiconductor device according to the second embodiment illustrated in  FIG.  10   . p + -type gate protection-regions  2   a  to  2   c  are selectively provided on the drift layer  1 . The source electrode  8  has a protrusion  8   a  buried down to the same depth as the bottom surfaces of the trenches  10   a  and  10   b  so as to be interposed between the gate protection-region  2   b  and the base region  3   b . A Schottky junction is formed by the side surface of the protrusion  8   a  of the source electrode  8  and the drift layer  1 , so that the Schottky barrier diode D 3  is implemented. According to Modified Example of the insulated-gate semiconductor device pertaining to the second embodiment illustrated in  FIG.  12   , a Schottky junction is formed by the side surface of the protrusion  8   a  of the source electrode  8  and the drift layer  1 . Therefore, while maintaining the area of the Schottky barrier diode D 3 , the width W 1  of the gate protection-region  2   b  can be reduced, so that the chip size can be reduced. 
     In Modified Example of the insulated-gate semiconductor device according to the second embodiment illustrated in  FIG.  13   , the structure of the Schottky barrier diode D 3  is different from that of the insulated-gate semiconductor device according to the second embodiment illustrated in  FIG.  10   . p + -type gate protection-regions  2   a  to  2   c  and a p + -type base-contact region  2   f  are selectively provided on the drift layer  1 . The source electrode  8  has a protrusion  8   a  buried in the drift layer  1  down to the same depth as the bottom surfaces of the trenches  10   a  and  10   b  so as to be interposed between the drift layer  1  and the base-contact region  2   f . The bottom surface of the protrusion  8   a  is in contact with the drift layer  1  and the base-contact region  2   f . A Schottky junction is formed by the region extending from the bottom surface of the source electrode  8  to the side surface and the bottom surface of the protrusion  8   a  of the source electrode  8  and the drift layer  1  in a stepwise manner. According to Modified Example of the insulated-gate semiconductor device pertaining to the second embodiment illustrated in  FIG.  13   , the area of the Schottky barrier diode D 4  can be increased, and the forward voltage can be reduced. 
     Third Embodiment 
     As illustrated in  FIG.  14   , the insulated-gate semiconductor device according to the third embodiment include a first conductivity type (n-type) drift layer  1  and a second conductivity type (p-type) base regions  3   a  and  3   b  selectively provided on the drift layer  1 . First conductivity type (n + -type) main-electrode regions (source regions)  4   a  to  4   c  having an impurity concentration higher than that of the drift layer  1  are provided in upper portions of the base regions  3   a  and  3   b . Since the source region  4   b  is not used as a current path, the source region  4   b  may not be provided. A trench  10  penetrating the source regions  4   a  and  4   b  and reaching the drift layer  1  is provided from the upper surfaces of the source regions  4   a  to  4   c . One sidewall surface of the trench  10  is in contact with the source region  4   a  and the base region  3   a , and the other sidewall surface is in contact with the source region  4   b.    
     In the insulated-gate semiconductor device according to the third embodiment, the a-plane having a relatively high electron mobility is used as the sidewall surface of the trench  10  closer to the source region  4   a  and the base region  3   a . That is, the sidewall surface of the trench  10  closer to the source region  4   a  side and the base region  3   a  become the first sidewall surface, and the sidewall surface of the trench  10  closer to the source region  4   b  becomes the second sidewall surface. 
     An insulated-gate electrode structure ( 5 ,  6 ) is provided inside the trench  10 . A first main electrode (source electrode)  8  is arranged on the gate electrode  6  with an interlayer insulating film  7  interposed. The source electrode  8  is in contact with the source regions  4   a  and  4   c . An n + -type second main-electrode region (drain region)  9  is arranged on the bottom surface of the drift layer  1  so as to be in contact with the drift layer  1 . A second main electrode (drain electrode)  11  is arranged on the bottom surface of the drain region  9 . 
     A second conductivity type (p + -type) gate protection-region  2  having an impurity concentration higher than that of the base regions  3   a  and  3   b  is selectively provided on the drift layer  1 . The gate protection-region  2  is a region formed in a self-aligned manner by obliquely implanting p-type impurity ions into the sidewall surface and the bottom surface of the trench  10  at the time of manufacturing the insulated-gate semiconductor device according to the third embodiment. The gate protection-region  2  has an L-shaped cross-sectional pattern to be in contact with the bottom surface and the sidewall surface of the trench  10 . 
     Second conductivity type (p + -type) base-contact regions  2   h  and  2   i  having an impurity concentration higher than that of the base regions  3   a  and  3   b  are selectively provided on the drift layer  1 . The base-contact region  2   h  is in contact with the source region  4   a  and the base region  3   a . The base-contact region  2   i  is in contact with the source regions  4   b  and  4   c , the base region  3   b , and the gate protection-region  2 . For example, the impurity concentration of the gate protection-region  2  may be higher than the impurity concentration of the base-contact regions  2   h  and  2   i  or may be the same as the impurity concentration of the base-contact regions  2   h  and  2   i . Other structures and basic operations of the insulated-gate semiconductor device according to the third embodiment are similar to those of the insulated-gate semiconductor device according to the first embodiment, and thus, redundant description will be omitted. 
     According to the insulated-gate semiconductor device pertaining to the third embodiment, by providing the gate protection-region  2  so as to be in contact with the bottom surface and the sidewall surface of the trench  10 , electric field concentration on the bottom of the trench  10  can be suppressed, so that the gate insulating film  5  of the bottom of the trench  10  can be protected. 
     Next, the method of manufacturing the insulated-gate semiconductor device according to the third embodiment will be described with reference to  FIGS.  15  to  23    by exemplifying the case of a trench gate MISFET. In addition, the method of manufacturing the trench gate MISFET described below is merely an example and can be realized by various other manufacturing methods including this modified example within the spirit described in the claims. 
     First, an n + -type semiconductor substrate (SiC substrate) doped with n-type impurities such as nitrogen (N) or the like is prepared. For example, the SiC substrate is a 4H—SiC substrate and has an off angle of 4°. By using the n + -type SiC substrate as the drain region  9 , as illustrated in  FIG.  15   , the n-type drift layer  1  and the p-type base region  3  are sequentially epitaxially grown on the upper surface of the drain region  9 . 
     Next, a photoresist film is coated on the upper surface of the base region  3 , and the photoresist film is delineated by a photolithography technique. By using the delineated photoresist film as a mask for ion implantation, n-type impurity ions such as N are implanted with multiple acceleration energies. After removal of the mask for ion implantation, a photoresist film is newly coated on the base region  3 , and the photoresist film is delineated by a photolithography technique. By using the delineated photoresist film as a mask for ion implantation, p-type impurity ions such as Al are implanted with multiple acceleration energies. After removal of the mask for ion implantation, annealing is performed to activate the implanted n-type impurity ions and p-type impurity ions. As a result, as illustrated in  FIG.  16   , p + -type base-contact regions  2   h  and  2   i  are selectively formed in an upper portion of the drift layer  1  so as to be exposed to the upper surfaces of the base regions  3   a  and  3   b . In addition, n + -type source regions  4  and  4   c  are selectively formed in an upper portion of the base regions  3   a  and  3   b.    
     Next, a photoresist film  31  is coated on the upper surfaces of the source regions  4  and  4   c  and the base-contact regions  2   h  and  2   i , and the photoresist film  31  is delineated by a photolithography technique. By using the delineated photoresist film  31  as a mask for etching, portions of the source region  4 , the base region  3   a , the base-contact region  2   i , and the drift layer  1  are selectively removed by dry etching such as reactive ion etching (RIE). As a result, as illustrated in  FIG.  17   , the trench  10  is selectively formed so as to reach the upper portion of the drift layer  1 . One sidewall surface of the trench  10  is the first sidewall surface and exposes the source region  4   a  and the base region  3   a . The other sidewall surface of the trench  10  is the second sidewall surface and exposes the base-contact region  2   i . The bottom surface of the trench  10  exposes the drift layer  1  and the base-contact region  2   i.    
     Next, as illustrated in  FIG.  18   , by using the photoresist film  31  as a mask for ion implantation, p-type impurity ions are obliquely implanted into the sidewall surface and the bottom surface of the trench  10  closer to the base-contact region  2   i . After removal of the photoresist film  31  as a mask for ion implantation, annealing is performed to activate the implanted p-type impurity ions. As a result, as illustrated in  FIG.  19   , a gate protection-region  2  having an L-shaped cross-sectional pattern is formed in a self-aligning manner so as to be exposed to the sidewall surface and the bottom surface of the trench  10 . 
     Next, as illustrated in  FIG.  20   , by a thermal oxidation method, a chemical vapor deposition (CVD), or the like, a gate insulating film  5  such as an SiO 2  film is formed on the bottom surface and the sidewall surface of the trench  10  and the upper surfaces of the source regions  4   a  to  4   c  and the base-contact regions  2   h  and  2   i  of the trench  10 . Next, a polysilicon layer (doped polysilicon layer) doped with impurities such as phosphorus (P) at a high impurity concentration is deposited by a CVD method or the like so as to bury the trench  10 . After that, the polysilicon layer and the gate insulating film  5  on the upper surfaces of the source regions  4   a  to  4   c  and the base-contact regions  2   h  and  2   i  are removed by etch back, chemical mechanical polishing (CMP), or the like. As a result, as illustrated in  FIG.  21   , a gate electrode  6  made of a polysilicon layer is buried in the trench  10 , so that an insulated-gate electrode structure ( 5 ,  6 ) is formed. 
     Next, an interlayer insulating film  7  is deposited on the upper surface of the insulated-gate electrode structure ( 5 ,  6 ) by the CVD method or the like. Then, as illustrated in  FIG.  22   , a portion of the interlayer insulating film  7  is selectively removed by a photolithography technique and dry etching. As a result, a source contact hole is cut in the interlayer insulating film  7 . Although not illustrated, a gate contact hole is also cut in the interlayer insulating film  7  so that a portion of the gate surface electrode connected to the gate electrode  6  is exposed at a position different from the source contact hole. 
     Next, a metallic layer such as an Al film is deposited by a sputtering method or the like. As illustrated in  FIG.  23   , a pattern of a source electrode  8  and a gate surface electrode (not illustrated) is formed by delineating the metallic layer such as an Al film by a photolithography technique and RIE or the like. As a result, the pattern of the source electrode  8  and the pattern of the gate surface electrode are separated. Next, as illustrated in  FIG.  14   , a drain electrode  11  made of Au or the like is formed on the entire bottom surface of the drain region  9  by a sputtering method, a vapor deposition method, or the like. In this manner, the insulated-gate semiconductor device according to the third embodiment of the present invention is completed. 
     According to the method of manufacturing the insulated-gate semiconductor device pertaining to the third embodiment, after forming the trench  10 , by obliquely implanting the p-type impurity ions, the gate protection-region  2  in contact with the bottom surface and the sidewall surface of the trench  10  can be formed in a self-aligned manner. Therefore, the insulated-gate semiconductor device illustrated in  FIG.  14    can be easily realized. 
     In addition, Modified Example of the insulated-gate semiconductor device according to the third embodiment illustrated in  FIG.  24    is different from the insulated-gate semiconductor device according to the third embodiment illustrated in  FIG.  14    in that the side surface of the end of the gate protection-region  2  in contact with the bottom surface of the trench  10  is inclined with respect to the vertical direction. The end of the gate protection-region  2  in contact with the bottom surface of the trench  10  is inclined in parallel to a straight line connecting a position P 1  of the upper end of the trench  10  and a position P 2  of the end of the gate protection-region  2  in contact with the bottom surface of the trench  10 . 
     In a manufacturing method of Modified Example of the insulated-gate semiconductor device according to the third embodiment illustrated in  FIG.  24   , for example, the trench  10  is formed so as to be separated from the base-contact region  2   i  and expose the drift layer  1  to the sidewall surface and the bottom surface. After that, ion implantation is performed obliquely on the sidewall surface and the bottom surface of the trench  10  so that the region where the end of the gate protection-region  2  is formed does not overlap with the base-contact region  2   i . Therefore, the gate protection-region  2  of which the side surface of the end in contact with the bottom surface of the trench  10  is inclined can be formed in a self-aligned manner. 
     In addition, Modified Example of the insulated-gate semiconductor device according to the third embodiment illustrated in  FIG.  25    is different from the insulated-gate semiconductor device illustrated in  FIG.  14    in that the bottom surface of the gate protection-region  2  is shallower than the bottom surface of the base-contact region  2   i . In a manufacturing method of Modified Example of the insulated-gate semiconductor device according to the third embodiment illustrated in  FIG.  25   , for example, the trench  10  is formed so as to be separated from the base-contact region  2   i  and expose the drift layer  1  to the sidewall surface and the bottom surface. After that, by performing ion implantation obliquely on the sidewall surface and the bottom surface of the trench  10 , the gate protection-region  2  can be formed in a self-aligned manner at a position shallower than the bottom surface of the base-contact region  2   i.    
       FIG.  26    illustrates Modified Example of the insulated-gate semiconductor device according to the third embodiment. The insulated-gate semiconductor device illustrated in  FIG.  26    is different from the insulated-gate semiconductor device illustrated in  FIG.  14    in that n + -type current spreading layers (CSL)  12   a  and  12   b  are provided on the bottom surfaces of the base-contact regions  2   h  and  2   i  and the base region  3   a . By providing the current spreading layers  12   a  and  12   b , the on-resistance can be reduced. The current spreading layers  12   a  and  12   b  can be formed by implanting n-type impurity ions such as nitrogen (N) into the drift layer  1 . In addition, current spreading layers may be provided only on the bottom surfaces of the base-contact regions  2   h  and  2   i.    
     Fourth Embodiment 
     As illustrated in  FIG.  27   , an insulated-gate semiconductor device according to a fourth embodiment has an array structure of a plurality of stripe-shaped unit cells C 1  to C 4  which have four striped trenches  10   a  to  10   d , respectively. The insulated-gate semiconductor device according to the fourth embodiment includes a first conductivity type (n-type) drift layer  1  and second conductivity type (p-type) base regions  3   a  and  3   b  arranged on the drift layer  1 . First conductivity type (n + -type) main-electrode regions (source regions)  41  and  43  having an impurity concentration higher than that of the drift layer  1  are provided in upper portions of the base regions  3   a  and  3   b.    
     Trenches  10   a  to  10   d  are provided so as to extend from the upper surfaces of the source regions  41  and  43  and reach the drift layer  1 . The trenches  10   a  and  10   b  are in contact with both ends of the source region  41  and the base region  3   a , respectively, with the source region  41  and the base region  3   a  interposed. The trenches  10   c  and  10   d  are in contact with both ends of the source region  43  and the base region  3   b , respectively, with the source region  43  and the base region  3   b  interposed. 
     Insulated gate type electrode structures ( 5   a ,  6   a ), ( 5   b ,  6   b ), ( 5   c ,  6   c ), and ( 5   d ,  6   d ) implemented by gate insulating films  5   a  to  5   d  and gate electrodes  6   a  to  6   d  are provided on inner sides of the trenches  10   a  to  10   d . A first main electrode (source electrode)  8  is arranged on the gate electrodes  6   a  to  6   d  with interlayer insulating films  7  interposed. An n + -type second main-electrode region (drain region)  9  is arranged on the bottom surface of the drift layer  1  so as to be in contact with the drift layer  1 . A second main electrode (drain electrode)  11  is arranged on the bottom surface of the drain region  9 . 
     Second conductivity type (p + -type) gate protection-regions  21 ,  22   a , and  23  having an impurity concentration higher than that of the base regions  3   a  and  3   b  are selectively provided on the drift layer  1 . The gate protection-region  21  is in contact with the bottom surface and the sidewall surface of the trench  10   a . The gate protection-region  22   a  is in contact with the bottom surface and the sidewall surface of the trench  10   b  and is in contact with the bottom surface and the sidewall surface of the trench  10   c . The gate protection-region  23  is in contact with the bottom surface and the sidewall surface of the trench  10   d.    
     In the insulated-gate semiconductor device according to the fourth embodiment, the a-plane is used as both sidewall surfaces of the trenches  10   a  to  10   d . For example, the a-plane having a relatively high electron mobility is used as the sidewall surface of the trench  10   a  closer to the gate protection-region  21 , the sidewall surface of the trench  10   b  closer to the source region  41  and the base region  3   a , the sidewall surface of the trench  10   c  closer to the gate protection-region  22   a , and the sidewall surface of the trench  10   d  closer to the source region  42  and the base region  3   b  and is defined as a first sidewall surface. On the other hand, a-plane having a relatively low electron mobility is used as the sidewall surface of the trench  10   a  closer to the source region  41  and the base region  3   a , the sidewall surface of the trench  10   b  closer to the gate protection-region  22   a , the sidewall surface of the trench  10   c  closer to the source region  42  side and the base region  3   b , and the sidewall surface of the trench  10   d  closer to the gate protection-region  23  and is defined as a second sidewall surface. Alternatively, the first sidewall surface and the second sidewall surface may be reversed as described above as both sidewall surfaces of the trenches  10   a  to  10   d.    
     In addition, in the insulated-gate semiconductor device according to the fourth embodiment, an m-plane which is (1−100) plane may be used as both sidewall surfaces of the trenches  10   a  to  10   d . In addition, in a case where the m-plane is used, since the inclination angles of the both sidewall surfaces of the trenches  10   a  to  10   d  with respect to the reference plane (m-plane) are the same, so that the electron mobilities on both sidewall surfaces of the trenches  10   a  to  10   d  are the same. 
     In the insulated-gate semiconductor device according to the fourth embodiment, a structure in which common base region  3   a  and source regions  41  and  43  are interposed between the trenches  10   a  and  10   b  of the adjacent unit cells C 1  and C 2  and between the trenches  10   c  and  10   d  of the adjacent unit cells C 3  and C 4  and a structure in which a common gate protection-region  22   a  is interposed between the trenches  10   b  and  10   c  of the adjacent unit cells C 2  and C 3  are alternately repeated. Other structures of the insulated-gate semiconductor device according to the fourth embodiment are similar to those of the insulated-gate semiconductor device according to the first embodiment, and thus, redundant description will be omitted. 
     At the time of operation of the insulated-gate semiconductor device according to the fourth embodiment, a positive voltage is applied to the drain electrode  11 , and a positive voltage equal to or higher than a threshold value is applied to the gate electrodes  6   a  to  6   d . Therefore, inversion layers (channels) are formed on both side surface of the base regions  3   a  and  3   b , and thus, the transistor is turned on. In the turned-on state, a current flows from the drain electrode  11  to the source electrode  8  through the drain region  9 , the drift layer  1 , the inversion layers on both side surfaces of the base regions  3   a  and  3   b , and the source regions  41  and  43 . On the other hand, in a case where the voltage applied to the gate electrodes  6   a  to  6   d  is lower than the threshold value, since no inversion layer is formed on both side surfaces of the base regions  3   a  and  3   b , the transistor is turned off, and thus, and no current flows from the drain electrode  11  to the source electrode  8 . 
       FIG.  28    illustrates a plan layout taken from the A-A direction in which the source regions  41  and  43  in  FIG.  27    are horizontally cut.  FIG.  27    is a cross-sectional view taken from the B-B direction in  FIG.  28   . As illustrated in  FIG.  28   , the planar patterns of the source regions  41  and  43  and the gate electrodes  6   a  to  6   d  are each formed in a stripe shape and extend parallel to each other. In addition, the gate protection-regions  22   a  and  22   b  are intermittently provided at predetermined intervals along the longitudinal direction of the source regions  41  and  43  and the gate electrodes  6   a  to  6   d . Source regions  42   a  and  42   b  are provided between the gate protection-regions  22   a  and  22   b . It is preferable that the interval W 3  between the gate protection-regions  22   a  and  22   b  is equal to or less than (equal to or narrower than) the interval (JFT width) W 2  of the junction field effect transistor (JFET) region interposed by the gate protection-regions  21  and  22   a  illustrated in  FIG.  27   .  FIG.  29    is a cross-sectional view taken from the C-C direction in  FIG.  28   . As illustrated in  FIG.  29   , a base region  3   c  is provided on the bottom surface of the source region  42   a.    
     Herein, an insulated-gate semiconductor device according to Comparative Example will be described. In the insulated-gate semiconductor device according to Comparative Example, as illustrated in  FIG.  30   , the gate protection-region  22  constitutes a planar pattern extending along the longitudinal direction of the trenches  10   a  to  10   d . On the other hand, according to the insulated-gate semiconductor device pertaining to the fourth embodiment, the gate protection-regions  22   a  and  22   b  are provided intermittently, and the space between the gate protection-regions  22   a  and  22   b  is used as the source regions  42   a  and  42   b . Therefore, the channel can be increased, and the on-resistance can be reduced. 
     In addition, in the structure illustrated in  FIG.  29   , the base region  3   c  on the bottom surface of the source region  42   a  may not be provided, and the Schottky barrier diode may be implemented by the source region  42   a  and the source electrode  8  in the region of the planar pattern of the source region  42   a . That is, the Schottky barrier diode may be provided in each region between the gate protection-regions  22   a  and  22   b.    
     In addition, as illustrated in  FIG.  31   , the gate protection-regions  21   a  and  21   b  interposed between the trenches  10   a  and  10   b  (refer to  FIG.  27   ) may be provided intermittently along the longitudinal direction of the trenches  10   a  and  10   b . The gate protection-regions  21   a  and  21   b  and the source regions  41   a  and  41   b  are alternately provided. In addition, the gate protection-regions  23   a  and  23   b  interposed between the trenches  10   c  and  10   d  (refer to  FIG.  27   ) may be provided intermittently along the longitudinal direction of the trenches  10   c  and  10   d . The gate protection-regions  23   a  and  23   b  and the source regions  43   a  and  43   b  are alternately provided. 
     In addition, as illustrated in  FIG.  31   , the array of the gate protection-regions  21   a  and  21   b , the array of the gate protection-regions  22   a  and  22   b , and the array of the gate protection-regions  23   a  and  23   b  may be provided at the same positions in the direction (parallel direction of the trenches  10   a  to  10   d ) perpendicular to the longitudinal direction of the trenches  10   a  to  10   d  (refer to  FIG.  27   ). In the parallel direction of the trenches  10   a  to  10   d , the gate protection-regions  21   a ,  22   a , and  23   a  and the gate protection-regions  21   a ,  22   a , and  23   a  are arranged at the same positions. In addition, in the parallel direction of the trenches  10   a  to  10   d , the source regions  41   a ,  42   a , and  43   a  and the source regions  41   b ,  42   b , and  43   b  are arranged at the same positions. 
     In addition, as illustrated in  FIG.  32   , the array of the gate protection-regions  21   a  and  21   b , the array of the gate protection-regions  22   a  and  22   b , and the array of the gate protection-regions  23   a  and  23   b  may be arranged so as to be shifted from each other in the parallel direction of the trenches  10   a  to  10   d . In the parallel direction of the trenches  10   a  to  10   d , the gate protection-regions  21   a ,  22   a , and  23   a  and the gate protection-regions  21   b ,  22   b , and  23   b  are arranged so as to be shifted from each other. In addition, in the parallel direction of the trenches  10   a  to  10   d , the source regions  41   a ,  42   a , and  43   a  and the source regions  41   b ,  42   b , and  43   b  are arranged shifted from each other. 
     Other Embodiments 
     As described above, the invention has been described according to the first to fourth embodiments, but it should not be understood that the description and drawings constituting a portion of this disclosure limit the invention. From this disclosure, various alternative embodiments, examples, and operational techniques will be apparent to those skilled in the art. 
     In the first to fourth embodiments of the invention, the MISFET having the insulated-gate electrode structure in the trench is exemplified, but the invention is not limited to the MISFET. The invention can be applied to insulated-gate semiconductor devices having various insulated-gate electrode structures such as IGBTs having insulated-gate electrode structures in the trenches. As the trench gate IGBT, there may employed a structure in which the n + -type source regions  4   a  to  4   j  of the MISFET illustrated in  FIGS.  2  and  3    are used as emitter regions, and a p + -type collector region is provided on the bottom surface side of the drift layer  1  instead of the n + -type drain region  9 . 
     In addition, in the embodiment of the invention, the insulated-gate semiconductor device using SiC is exemplified. However, the invention can be applied to an insulated-gate semiconductor device using a semiconductor (wide band gap semiconductor) having a forbidden band width larger than that of SiC and having a hexagonal system such as gallium nitride (GaN), aluminum nitride (GaN), or indium nitride (InN).