Patent Publication Number: US-11049931-B2

Title: Semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 16/063,733, filed Jun. 19, 2018, which is a National Stage of PCT/JP2016/084570 filed Nov. 22, 2016, which claims the benefit of priority from Japanese Patent Application No. 2016-022668, filed Feb. 9, 2016, the contents of which is incorporated in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a semiconductor device, and particularly to a trench-gate semiconductor device. 
     BACKGROUND ART 
     In a power electronics apparatus, it is necessary to switch back and forth between execution and stop of power supply in order to drive a load such as an electric motor. To this end, a semiconductor switching element such as a metal-oxide-semiconductor field-effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT) is used. A current path of a semiconductor switching element is composed of a vertical structure or a lateral structure. In a vertical structure, a withstand voltage is ensured in a vertical direction, that is, a thickness direction, so that a high withstand voltage can be easily obtained. As a structure for a switching element, a structure having an insulated gate is employed in many cases, and there are two types of a planar switching element and a trench-gate switching element. A trench-gate switching element has an improved channel density, and so, can easily obtain low on-resistance. Such being the case, a vertical and trench-gate switching element is employed in many cases. As a semiconductor material for a switching element, silicon (Si) is typically used, and in recent years, also a wide-band-gap semiconductor typified by silicon carbide (SiC) is used. 
     International Application Laid-Open No. WO98/35390 (A1) (Patent Document 1) discloses a vertical and a trench-gate MOSFET using SiC. On a wall portion and a bottom portion of a trench of the MOSFET, a trench oxide (gate insulating film) is provided. In a region below the gate oxide film, a protective region (field relaxing region) of a conductivity type reverse to a conductivity type of each of a source and a drain, is provided. The foregoing document suggests that the protective region protects the gate oxide film from degradation or dielectric breakdown which is likely to be caused by a high voltage applied to the drain. 
     As described above, known is a technique of providing a field relaxing region for protecting a gate insulating film in a vertical and trench-gate switching element. Because of such a configuration, a depletion layer extends from a field relaxing region toward a drift layer in an off state of a switching element. This depletion layer can reduce an electric field applied to a gate insulating film on a bottom face of a gate trench. This behavior is particularly useful for a switching element using SiC, as compared to a switching element using Si. This is because, in a case where SiC is used, dielectric breakdown of a switching element is more likely to occur in a gate insulating film than in an SiC region being as a semiconductor region. The reason for this is that an avalanche field intensity of SiC is approximately ten times as high as that of Si. Thus, in a case where SiC is used as a semiconductor region, a gate insulating is likely to be subjected to dielectric breakdown earlier than a semiconductor region. 
     For a gate trench located inside an active region of a semiconductor element, the above-described field relaxing effect can be attained from not only a field relaxing region provided therein, but also a field relaxing region provided in another gate trench adjacent thereto. Meanwhile, for a gate trench located in an outermost edge of an active region, such additional effect as described above cannot be attained. Accordingly, there is vulnerability in that a gate insulating film is easily broken down in an outermost edge of an active region. 
     International Application Laid-Open No. WO2015/015808 (A1) (Patent Document 2) discusses a structure for a semiconductor device which has been devised considering the above-described problem. More specifically, a trench is provided in an active region, and further, a terminating trench is formed so as to surround the foregoing trench in a terminating region in a perimeter of the active region. A protective diffusion layer (field relaxing region) is provided below not only the trench in the active region, but also the terminating trench. As a result of this, the above-described vulnerability in a gate trench located in an outermost edge of an active region can be overcome. The protective diffusion layer may be connected to a source electrode via a contact hole, and effects in this case are described in Patent Document 2 as follows. 
     Upon a switching operation of a silicon-carbide semiconductor device, an on state and an off state alternate in a switching period. In an off state, carriers are diffused from a protective diffusion layer so that a depletion layer extends, while in an on state, the diffused carriers return back to an original state. If carriers are slow in returning at a time of switching to an on state, a switching speed is correspondingly reduced, to cause an increase of switching loss. Because of connection of the protective diffusion layer to the source electrode, the carriers are drawn back to the protective diffusion layer by virtue of a source potential, so that switching loss is reduced. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: International Application Laid-Open No. WO98/35390 (A1) 
     Patent Document 2: International Application Laid-Open No. WO2015/015808 (A1) 
     SUMMARY 
     Problem to be Solved by the Invention 
     During high-speed switching or a short circuit of a load, a transient current which charges a pn junction formed by a field relaxing region and a drift layer, flows. Because of voltage drop associated with a transient current, an electric field applied to an insulating film on a field relaxing region is instantaneously increased. Such a high electric field may possibly cause dielectric breakdown of a gate-drain insulating film, particularly a gate insulating film, on a field relaxing region. 
     The present invention has been devised in order to overcome the above-described problems, and it is an object of the present invention to provide a semiconductor device which can suppress dielectric breakdown of a gate-drain insulating film, particularly a gate insulating film, on a field relaxing region, during high-speed switching or a short circuit of a load. 
     Means to Solve the Problem 
     A semiconductor device according to the present invention includes a drift layer, a well region, a first impurity region, a gate trench, an outer trench, a gate insulating film, a gate electrode, a gate connection layer, a first main electrode, a second main electrode, a trench-bottom field relaxing region, and a trench-bottom high-concentration region. The drift layer lies over an active region and a region outside the active region and is of a first conductivity type. The well region is provided on the drift layer within the active region and is of a second conductivity type different from the first conductivity type. The first impurity region is provided on the well region, is separated from the drift layer by the well region and is of the first conductivity type. The gate trench is provided within the active region and includes a sidewall facing the first impurity region, the well region, and the drift layer. The outer trench is provided in the drift layer outside the active region. The gate insulating film is provided in the gate trench and the outer trench. The gate electrode is provided in the gate trench with the gate insulating film being interposed. The gate connection layer is in contact with the gate electrode and includes a portion placed on the outer trench with the gate insulating film being interposed. The first main electrode includes a main contact that is electrically connected to the well region and the first impurity region within the active region, and an outer contact that is spaced away from the active region and is in contact with a bottom face of the outer trench. The second main electrode is electrically connected to the drift layer, is separated from the well region by at least the drift layer, and faces the first main electrode with the drift layer being interposed. The trench-bottom field relaxing region is provided in the drift layer, is separated from the second main electrode by the drift layer, and is of the second conductivity type. The trench-bottom high-concentration region is of the second conductivity type, has an impurity concentration higher than an impurity concentration of the trench-bottom field relaxing region, is provided on the trench-bottom field relaxing region, and extends from a position where the trench-bottom high-concentration region faces the gate connection layer with the gate insulating film being interposed, to a position where the trench-bottom high-concentration region is in contact with the outer contact of the first main electrode. 
     Effects of the Invention 
     According to the present invention, the trench-bottom high-concentration region on the trench-bottom field relaxing region extends from a position where the trench-bottom high-concentration region faces the gate connection layer with the gate insulating film being interposed, to a position where the trench-bottom high-concentration region is in contact with the outer contact of the first main electrode. As a result of this, during high-speed switching or a short-circuit of a load, voltage drop caused by a transient current which charges a pn junction formed by the field relaxing region and the drift layer is reduced. Therefore, breakdown of an insulating film, particularly a gate insulating film, below the gate connection layer, can be suppressed. The objects, features, aspects, and advantages the present invention shall be further clarified by the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial plan view schematically showing a configuration of a semiconductor device according to a first embodiment of the present invention. 
         FIG. 2  is a partial cross-sectional view taken along a line II-II in  FIG. 1 . 
         FIG. 3  is a partial cross-sectional view taken along a line in  FIG. 1 . 
         FIG. 4  is a partial cross-sectional view taken along a line IV-IV in  FIG. 1 . 
         FIG. 5  is a partial plan view schematically showing a configuration of a semiconductor device according to a first modification of the first embodiment of the present invention. 
         FIG. 6  is a partial plan view schematically showing a configuration of a semiconductor device according to a second modification of the first embodiment of the present invention. 
         FIG. 7  is a partial cross-sectional view taken along a line VII-VII in  FIG. 6 . 
         FIG. 8  is a partial plan view schematically showing a configuration of a semiconductor device according to a third modification of the first embodiment of the present invention. 
         FIG. 9  is a partial cross-sectional view taken along a line IX-IX in  FIG. 8 . 
         FIG. 10  is a partial cross-sectional view taken along a line X-X in  FIG. 8 . 
         FIG. 11  is a partial cross-sectional view taken along a line XI-XI in  FIG. 8 . 
         FIG. 12  is a partial cross-sectional view taken along a line XII-XII in  FIG. 8 . 
         FIG. 13  is a partial plan view schematically showing a configuration of a semiconductor device according to a fourth modification of the first embodiment of the present invention. 
         FIG. 14  is a partial cross-sectional view taken along a line XIV-XIV in  FIG. 13 . 
         FIG. 15  is a partial cross-sectional view taken along a line XV-XV in  FIG. 13 . 
         FIG. 16  is a partial plan view schematically showing a configuration of a semiconductor device according to a second embodiment of the present invention. 
         FIG. 17  is a partial cross-sectional view taken along a line XVII-XVII in  FIG. 16 . 
         FIG. 18  is a partial cross-sectional view taken along a line XVIII-XVIII in  FIG. 16 . 
         FIG. 19  is a partial plan view schematically showing a configuration of a semiconductor device according to a first modification of the second embodiment of the present invention. 
         FIG. 20  is a partial plan view schematically showing a configuration of a semiconductor device according to a second modification of the second embodiment of the present invention. 
         FIG. 21  is a partial plan view schematically showing a configuration of a semiconductor device according to a third modification of the second embodiment of the present invention. 
         FIG. 22  is a partial cross-sectional view taken along a line XXII-XXII in  FIG. 21 . 
         FIG. 23  is a partial cross-sectional view taken along a line in  FIG. 21 . 
         FIG. 24  is a partial plan view schematically showing a configuration of a semiconductor device according to a third embodiment of the present invention. 
         FIG. 25  is a partial plan view schematically showing a configuration of a semiconductor device according to a first modification of the third embodiment of the present invention. 
         FIG. 26  is a partial plan view schematically showing a configuration of a semiconductor device according to a second modification of the third embodiment of the present invention. 
         FIG. 27  is a partial plan view schematically showing a configuration of a semiconductor device according to a third modification of the third embodiment of the present invention. 
         FIG. 28  is a partial cross-sectional view taken along a line XXVIII-XXVIII in  FIG. 27 . 
         FIG. 29  is a partial plan view schematically showing a configuration of a semiconductor device according to a fourth modification of the third embodiment of the present invention. 
         FIG. 30  is a partial plan view schematically showing a configuration of a semiconductor device according to a fourth embodiment of the present invention. 
         FIG. 31  is a partial cross-sectional view taken along a line XXXI-XXXI in  FIG. 30 . 
         FIG. 32  is a partial plan view schematically showing a configuration of a semiconductor device according to a first modification of the fourth embodiment of the present invention. 
         FIG. 33  is a partial cross-sectional view taken along a line XXXIII-XXXIII in  FIG. 32 . 
         FIG. 34  is a partial plan view schematically showing a configuration of a semiconductor device according to a fifth embodiment of the present invention. 
         FIG. 35  is a partial cross-sectional view taken along a line XXXV-XXXV in  FIG. 34 . 
         FIG. 36  is a partial cross-sectional view taken along a line XXXVI-XXXVI in  FIG. 34 . 
         FIG. 37  is a partial plan view schematically showing a configuration of a semiconductor device according to a first modification of the fifth embodiment of the present invention. 
         FIG. 38  is a partial cross-sectional view taken along a line XXXVIII-XXXVIII in  FIG. 37 . 
         FIG. 39  is a partial plan view schematically showing a configuration of a semiconductor device according to a second modification of the fifth embodiment of the present invention. 
         FIG. 40  is a partial cross-sectional view taken along a line XL-XL in  FIG. 39 . 
         FIG. 41  is a partial cross-sectional view schematically showing a configuration of a semiconductor device according to a sixth embodiment of the present invention. 
         FIG. 42  is a partial cross-sectional view schematically showing a configuration of a semiconductor device according to a seventh embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. It is noted that the drawings provide schematic representations, and respective sizes and positional interrelationships of figures in different drawings are not necessarily shown exactly and can be appropriately changed. Also, in the following description, the same components will be shown in the drawings while being denoted by the same reference symbols, and names and functions thereof shall be similar. Thus, detailed description for such components will be occasionally omitted. 
     Also, while the following description will use terms which mean specific positions and directions such as “upper”, “lower”, “side”, “bottom”, “front”, and “back” in some portions, those terms are used for the sake of convenience in order to facilitate understanding of contents of the embodiments, and are not pertinent to directions for actual implementation. 
     First Embodiment 
     (Configuration) 
       FIG. 1  is a partial plan view schematically showing a configuration of a MOSFET  101  as a semiconductor device according to a first embodiment.  FIG. 2  is a partial cross-sectional view taken along a line II-II in  FIG. 1 , and shows a periodic structure of a unit cell  31   a  in an active region  30 .  FIG. 3  is a partial cross-sectional view taken along a line in  FIG. 1 .  FIG. 4  is a partial cross-sectional view taken along a line IV-IV in  FIG. 1 . It is noted that a gate insulating film  7  and an interlayer insulating film  9  ( FIGS. 2 to 4 ) are omitted in a plan view of  FIG. 1 . Also, regarding a source electrode  10  ( FIGS. 2 to 4 ), only respective positions of a source contact CS and an outer contact CO which are included therein are shown. Also, regarding a gate wire portion  20  ( FIGS. 3 and 4 ), only a position of a gate contact CG included therein is shown. The same omissions will be made in the other plan views which will be later described in detail. 
     The MOSFET  101  includes a current path between the source electrode  10  and a drain electrode  11 , which runs along a vertical direction (thickness direction) in  FIG. 2 . That is, the MOSFET  101  is a vertical semiconductor device. 
     The MOSFET  101  includes the active region  30  (on the upper left in  FIG. 1 ) and a terminating region  40  located outside the active region  30  (on the lower right in  FIG. 1 ) in a plan layout (a layout seen from a point of view of a viewer of  FIG. 1 ). The active region  30  is a region where a MOS structure of the MOSFET  101  is placed. The terminating region  40  is placed in a terminating end of the MOSFET  101 , and has a structure which prevents withstand-voltage performance of the MOSFET  101  from being degraded in a terminating end. Accordingly, it is preferable that the terminating region  40  surrounds the active region  30 . 
     The active region  30  includes the unit cell  31   a  and an active-region end  31   b . The active-region end  31   b  is a cell placed in an end (outermost edge) of the active region  30 . The unit cell  31   a  is a cell other than the active-region end  31   b , and is periodically placed in a region inner than the active-region end  31   b . Accordingly, the active-region end  31   b  is placed between the unit cell  31   a  and the terminating region  40 . Though a width of the active-region end  31   b  (a lateral dimension in  FIG. 1 ) is not limited to any specific value, the width may be either identical to a width of the unit cell  31   a  or larger than a width of the unit cell  31   a . A channel controlled by a gate electrode  8  embedded in a gate trench TG is provided in each of the unit cells  31   a . That is, the MOSFET  101  is a trench-gate semiconductor device. Further, a similar channel may be placed also in the active-region end  31   b . The gate electrode  8  extending along a boundary between the above-described cells is embedded in the gate trench TG. 
     To put a configuration more specifically, the MOSFET  101  (semiconductor device) includes a semiconductor substrate  1 , a semiconductor layer provided on the semiconductor substrate  1 , the gate insulating film  7 , the gate electrode  8 , the interlayer insulating film  9 , a field insulating film  9   a , the source electrode  10  (first main electrode), the drain electrode  11  (second main electrode), a gate connection layer  14 , the gate wire portion  20 , and a protective insulating layer  21 . The semiconductor layer includes a drift layer  3 , a well region  4 , a source region  5  (first impurity region), a terminating field relaxing region  12 , a trench-bottom field relaxing region  13 , a well contact region  16 , and a trench-bottom high-concentration region  18 . The gate trench TG and an outer trench TO are provided in the semiconductor layer. 
     The semiconductor substrate  1  ( FIGS. 2 to 4 ) is of an n type (first conductivity type). The semiconductor substrate  1  lies over the active region  30  and the terminating region  40  ( FIG. 1 ). The semiconductor substrate  1  is formed of SiC. As SiC, a substance having a hexagonal crystal system is preferably used, and more preferably, a substance of 4H polytype is used. As an impurity providing an n type to SiC, nitride (N) or phosphor (P) can be used, for example. 
     The drift layer  3  is provided on the semiconductor substrate  1 , and lies over the active region  30  and the terminating region  40 . The drift layer  3  is of an n type (first conductivity type). It is preferable that the drift layer  3  has an impurity concentration which is lower than an impurity concentration of the semiconductor substrate  1 . In the present embodiment, the semiconductor layer including the drift layer  3  is a silicon carbide layer. The silicon carbide layer preferably has a hexagonal crystal structure, and more preferably, is of 4H polytype. 
     The well region  4  is provided on the drift layer  3  within the active region  30 . In other words, the well region  4  is formed in a surface portion of the drift layer  3 . The well region  4  extends to a sidewall of the outer trench TO. In a configuration shown in  FIG. 3 , the well region  4  extends to an open end of the outer trench TO (an upper-right corner of the active-region end  31   b  in  FIG. 3 ). The well region  4  is of a p type (second conductivity type different from the first conductivity type). As an impurity providing a p type to SiC, aluminum (Al) or boron (B) can be used, for example. 
     The source region  5  is provided on a part of the well region  4  within the active region  30 . The source region  5  is separated from the drift layer  3  by the well region  4 . The source region  5  is of an n type. 
     The well contact region  16  is provided on a part of the well region  4  within the active region  30 . The well contact region  16  is of a p type, and has an impurity concentration which is higher than an impurity concentration of the well region  4 . The well contact region  16  is surrounded by the well region  4  and the source region  5  in plan view (seen from a point of view of a viewer of  FIG. 1 ). It is noted that the well contact region  16  may be surrounded by only the source region  5  in plan view. In this case, the source region  5  is provided also in a portion where the well region  4  is shown in  FIG. 1 . 
     The gate trench TG ( FIG. 2 ) is provided in the semiconductor layer within the active region  30 . The gate trench TG includes a sidewall which faces the source region  5 , the well region  4 , and the drift layer  3 . In other words, the gate trench TG extends from a surface (upper face in  FIG. 2 ) of the source region  5 , and reaches an inside of the drift layer  3  passing through the source region  5  and the well region  4 . In the unit cell  31   a , the source region  5  is placed in an open end of the gate trench TG (in other words, a corner of an end of a surface in a mesa structure of the semiconductor layer between the gate trenches TG). 
     The outer trench TO is provided in the semiconductor layer including the drift layer  3  within the terminating region  40  (in other words, outside the active region  30 ). The outer trench TO ( FIG. 3 ) includes a sidewall adjacent to the well region  4  and the drift layer  3  within the active region  30 . This sidewall may be further adjacent to the source region  5 . A depth of the outer trench TO may be nearly identical to that of the gate trench TG, and in this case, the outer trench TO and the gate trench TG can be formed by one etching operation. 
     The gate insulating film  7  includes a portion provided in the gate trench TG and a portion provided in the outer trench TO. Also, the gate insulating film  7  includes a portion with which an open end of the gate trench TG ( FIG. 2 ) is covered and a portion with which an open end of the outer trench TO ( FIG. 3 ) is covered. The gate insulating film  7  is a silicon oxide film, for example. This silicon oxide film is typically a thermally-oxidized film. 
     The gate electrode  8  is embedded in the gate trench TG with the gate insulating film  7  being interposed. In a position spaced away from the gate connection layer  14  (later described in detail) (in other words, a position spaced away from the terminating region  40 ), an upper face of the gate electrode  8  (refer to  FIG. 2 ) is in a position deeper than a surface of the source region  5  (in other words, an open end of the gate trench TG). That is, an upper face of the gate electrode  8  is in a position deeper than an open end of the gate trench TG. In the foregoing position, an open end of the gate trench TG is covered with the interlayer insulating film  9  with the gate insulating film  7  being interposed. Accordingly, the gate electrode  8  in the foregoing position does not face an open end of the gate trench TG, while facing a sidewall of the gate trench TG with the gate insulating film  7  being interposed. Because of the above-described configuration, in the unit cell  31   a , the source region  5  in an open end of the gate trench TG faces the interlayer insulating film  9  with the gate insulating film  7  being interposed, and does not face the gate electrode  8 . 
     The gate connection layer  14  extends from an inside of the terminating region  40  to the active region  30 , and is in contact with the gate electrode  8  within the active region  30 . A portion where the gate connection layer  14  and the gate electrode  8  are in contact with each other may protrude over an open end of the gate trench TG ( FIG. 2 ) (refer to  FIG. 4 ). The gate connection layer  14  includes a portion placed on the outer trench TO with only the gate insulating film  7  being interposed. The gate connection layer  14  climbs onto an open end of the outer trench TO (an upper-right corner of the active-region end  31   b  in  FIG. 3 ), from an inside of the terminating region  40 . In an open end, the semiconductor layer and the gate connection layer  14  are insulated from each other by the gate insulating film  7 . In other words, in an open end, the semiconductor layer and the gate connection layer  14  face each other with the gate insulating film  7  being interposed. It is noted that though the well region  4  is placed in an open end in  FIG. 3 , the source region  5  ( FIG. 2 ) may be placed there. The gate connection layer  14  is formed of the same material as a material of the gate electrode  8 , so that the gate connection layer  14  and the gate electrode  8  can be formed of one conductor layer (typically, a doped polysilicon layer) in a manufacturing process of the MOSFET  101 . However, the gate connection layer  14  may be formed of a material different from that of the gate electrode  8 . For example, in a case where the gate contact CG (later described in detail) which electrically connects the gate connection layer  14  and the gate wire portion  20  to each other is formed of a metal silicide (nickel silicide, for example), also a part or a whole of the gate connection layer  14  may be formed of metal silicide. For example, a portion of the gate connection layer  14 , which is located closer to the gate contact CG, may be formed of metal oxide, and the other portion of the gate connection layer  14 , which is located closer to the gate electrode  8 , may be formed of the same material as a material of the gate electrode  8 . 
     The gate wire portion  20  is placed in the outer trench TO while being insulated from the semiconductor layer. In other words, the gate wire portion  20  is placed on the semiconductor layer with the field insulating film  9   a , the gate insulating film  7 , and the interlayer insulating film  9  being interposed. The gate wire portion  20  includes the gate contact CG. The gate contact CG is electrically connected to the gate connection layer  14  via a contact hole provided in the interlayer insulating film  9 . As a result of this, the gate wire portion  20  is electrically connected to the gate electrode  8  by the gate connection layer  14 . The gate wire portion  20  is connected to a gate pad (not shown) within the terminating region  40 . The gate pad is an external terminal for applying a gate potential to the MOSFET  101 . 
     A part of the semiconductor layer is covered with the field insulating film  9   a  in order to protect a terminating end within terminating region  40 . The field insulating film  9   a  includes a portion which is placed on the trench-bottom high-concentration region  18  (later described in detail) in the outer trench TO. The semiconductor layer provided with the field insulating film  9   a , the gate insulating film  7 , the gate electrode  8 , and the gate connection layer  14  is covered with the interlayer insulating film  9 . It is preferable that the field insulating film  9   a  is thicker than the gate insulating film  7 . The field insulating film  9   a  is placed away from a boundary between the active region  30  and the terminating region  40  in the present embodiment. It is noted that the field insulating film  9   a  may be omitted in the present embodiment. 
     The source electrode  10  includes the source contact CS (main contact) and an outer contact CO. The source contact CS is electrically connected to the well region  4  and the source region  5  via a contact hole provided in the interlayer insulating film  9 , within the active region  30 . The outer contact CO is spaced away from the active region  30 , and is in contact with a bottom face of the outer trench TO via a contact hole provided in the interlayer insulating film  9 , within the terminating region  40 . Each of the source contact CS and the outer contact CO includes a silicide region  19  in a portion thereof which is in contact with the semiconductor layer. 
     The drain electrode  11  is provided on a face of the semiconductor substrate  1 , which is opposite to a face on which the drift layer  3  is provided (a lower face in  FIG. 2 ). Accordingly, the drain electrode  11  is electrically connected to the drift layer  3  via the semiconductor substrate  1 . Also, the drain electrode  11  is separated from the well region  4  by at least the drift layer  3 , or by the well region  4  and the semiconductor substrate  1  in the present embodiment. Also, the drain electrode  11  faces the source electrode  10  with the drift layer  3  being interposed. Preferably, the drain electrode  11  includes a silicide region (not shown) as a portion which is in contact with the semiconductor substrate  1 . 
     The trench-bottom field relaxing region  13  is of a p type. The trench-bottom field relaxing region  13  is provided in the drift layer  3 . The trench-bottom field relaxing region  13  is separated from the drain electrode  11  by the drift layer  3 . The trench-bottom field relaxing region  13  includes a portion placed within the active region  30  and a portion placed within the terminating region  40 . 
     Within the active region  30  ( FIG. 2 ), it is preferable that the trench-bottom field relaxing region  13  includes a portion which is placed in a position deeper than a bottom face of the gate trench TG, in order to effectively relax an electric field in a bottom face of the gate trench TG. Also, it is preferable that the trench-bottom field relaxing region  13  includes a portion which overlaps the gate trench TG in a plan layout. From the above-described points of view, it is preferable that the trench-bottom field relaxing region  13  is placed on a bottom face of the gate trench TG as shown in  FIG. 2 . This placement is preferable also for easiness in a manufacturing process. However, this placement is a mere example, and the trench-bottom field relaxing region  13  may be placed away from a bottom face of the gate trench TG. Alternatively, the trench-bottom field relaxing region  13  may be placed between adjacent ones of the gate trenches TG in a plan layout. Further alternatively, the trench-bottom field relaxing region  13  may be either in contact with, or spaced away from, the well region  4 . 
     Within the terminating region  40 , the trench-bottom field relaxing region  13  is placed in a position deeper than a bottom face of the outer trench TO, and includes a portion which overlaps the outer trench TO in a plan layout. Preferably, as shown in  FIGS. 3 and 4 , the trench-bottom field relaxing region  13  is in contact with a bottom face of the outer trench TO, and more preferably, the trench-bottom field relaxing region  13  is in contact with a corner where a bottom face and a sidewall of the outer trench TO join each other. 
     The terminating field relaxing region  12  can be formed on a bottom face of the outer trench TO within the terminating region  40 . The terminating field relaxing region  12  is of a p type. The terminating field relaxing region  12  may be continuous with an outer edge of the trench-bottom field relaxing region  13  as shown in  FIGS. 3 and 4 . 
     The trench-bottom high-concentration region  18  is provided on the trench-bottom field relaxing region  13  within the terminating region  40 . In other words, the trench-bottom high-concentration region  18  is formed in a surface portion of the trench-bottom field relaxing region  13 . At least a part of the trench-bottom high-concentration region  18  is placed on a bottom face of the outer trench TO. However, the trench-bottom high-concentration region  18  may include a portion which is placed inside the trench-bottom field relaxing region  13 . The trench-bottom high-concentration region  18  extends outward (toward a right side in  FIGS. 3 and 4 ) from a position where the trench-bottom high-concentration region  18  faces the gate connection layer  14  with only the gate insulating film  7  being interposed (from a left side in  FIGS. 3 and 4 ), and continues to extend to a position where the trench-bottom high-concentration region  18  is contact with the outer contact CO of the source electrode  10 . Accordingly, the outer contact CO is connected to the trench-bottom field relaxing region  13  via the trench-bottom high-concentration region  18 . The trench-bottom high-concentration region  18  may extend further outward, in other words, toward the terminating field relaxing region  12 , from a position where the trench-bottom high-concentration region  18  is in contact with the outer contact CO. The trench-bottom high-concentration region  18  is of a p type. The trench-bottom high-concentration region  18  has an impurity concentration which is higher than an impurity concentration of the trench-bottom field relaxing region  13 . 
     It is noted that a region lying over the active region  30  and the terminating region  40  may include a portion having a different sectional structure, in addition to the sectional structure described above in detail ( FIGS. 3 and 4 ). 
     (Manufacturing Method) 
     Next, an example of a manufacturing method for the MOSFET  101  will be described below. 
     First, an n-type SiC substrate of 4H polytype is prepared as the semiconductor substrate  1 . By epitaxial growth of SiC on a surface of the semiconductor substrate  1 , an n-type (n − -type) semiconductor layer with relatively high resistance is formed. A part of the semiconductor layer is used as the drift layer  3  without being further processed. 
     A recess serving as an alignment mark is formed on a surface of the semiconductor layer by a reactive ion etching (RIE) process. With the alignment mark being used as a reference, the p-type well region  4  and the n-type (n + -type) source region  5  with low resistance are formed in a surface portion of the drift layer  3  by ion implantation. As an implantation mask for the source region  5 , a resist mask is used, for example. Preferably, the source region  5  is formed so as to have an n-type impurity concentration (donor concentration) which is equal to or higher than 5×10 18  [cm −3 ] and is equal to or lower than 5×10 20  [cm −3 ], and the well region  4  is formed so as to have a p-type impurity concentration (acceptor concentration) which is equal to or higher than 1×10 16  [cm −3 ] and is equal to or lower than 3×10 19  [cm −3 ]. It is noted that in a typical process, a region which is to serve as the source region  5  is subjected to not only n-type ion implantation for formation of the source region  5 , but also p-type ion implantation for formation of the well region  4 . In such a case, an n-type impurity concentration of the source region  5  is set to be higher than a p-type impurity concentration of the well region  4  so that the source region  5  can have an n-type conductivity type. An impurity concentration of the well region  4  may be either constant or non-constant along a depth thereof. For example, the well region  4  may have either impurity-concentration distribution in which a concentration becomes lower toward a surface thereof, or impurity-concentration distribution in which a concentration has a peak somewhere along a depth thereof. Subsequently, the p-type well contact region  16  is formed by ion implantation. Preferably, the well contact region  16  is formed so as to have a p-type impurity concentration which is equal to or higher than 1×10 19  [cm −3 ] and is equal to or lower than 1×10 22  [cm −3 ]. Incidentally, the sequence of the ion implantation processes can be appropriately altered. 
     Subsequently, an etching mask for forming the gate trench TG and the outer trench TO is formed on the foregoing semiconductor layer. The etching mask can be formed by patterning using a resist mask, for example. Thereafter, the gate trench TG and the outer trench TO which are deeper than the well region  4  and reach the drift layer  3  are formed by an RIE process. 
     Subsequently, the p-type trench-bottom field relaxing region  13  is formed on a bottom face of the gate trench TG with the etching mask being left as an implantation mask, and then, the p-type terminating field relaxing region  12  is formed on a bottom face of the outer trench TO. The trench-bottom field relaxing region  13  and the terminating field relaxing region  12  may be formed either simultaneously or at different times. Also, the terminating field relaxing region  12  may have lateral concentration distribution. That is, there may be provided concentration distribution in which a concentration becomes lower step by step toward an outside from an end of the terminating field relaxing region  12 , which end is closer to the active region  30 , for example. 
     Subsequently, the p-type trench-bottom high-concentration region  18  is formed by ion implantation. The trench-bottom high-concentration region  18  may be formed either simultaneously with, or at a time different from, at least one of the trench-bottom field relaxing region  13  and the terminating field relaxing region  12 . It is noted that the above-described well contact region  16  may be formed simultaneously with the trench-bottom high-concentration region  18  after the gate trench TG is formed. In a case where the trench-bottom high-concentration region  18  and the terminating field relaxing region  12  are formed by different processes, a p-type impurity concentration of the trench-bottom high-concentration region  18  may have either concentration distribution in which a concentration is constant along a depth, or concentration distribution in which a concentration has a specific peak value. Preferably, the trench-bottom high-concentration region  18  is formed so as to have a p-type impurity concentration which is equal to or higher than 1×10 18  [cm −3 ] and is equal to or lower than 1×10 22  [cm −3 ]. Terms “p-type impurity concentration”, referred to in this specification, of the trench-bottom high-concentration region  18  means a maximum value of a p-type impurity concentration of the trench-bottom high-concentration region  18 . 
     Subsequently, annealing is performed in order to activate an implanted ion. For example, a heat treatment is performed in a temperature range from 1500° C. to 2200° C. inclusive for a time which is equal to or more than 0.5 minutes and is equal to or more than 60 minutes. 
     Subsequently, the field insulating film  9   a  is formed by formation of an insulating film and patterning of the formed insulating film. Formation of the insulating film can be achieved by a thermal oxidation process or a chemical vapor deposition (CVD) process. Patterning can be achieved by wet etching or dry etching. At that time, it is preferable that the field insulating film  9   a  is provided on a portion of a bottom face of the outer trench TO, which portion is located closer to a terminating end than the active-region end  31   b , so that the active region  30  can be more effectively utilized. Before that time, the insulating film formed inside the gate trench TG within the active region  30  should be sufficiently removed by wet etching, dry etching, or the like. Further, the gate insulating film  7  is formed in and around the gate trench TG and the outer trench TO by a thermal oxidation process, a CVD process, or the like. 
     Subsequently, a conductor layer is formed on a whole of the semiconductor layer on which the gate insulating film  7 , the field insulating film  9   a , and the interlayer insulating film  9  are formed. In the present embodiment, a polysilicon film doped with an impurity is formed by a CVD process. At that time, a sufficient amount of polysilicon is embedded in the gate trench TG. In other words, polysilicon is deposited so as to have a thickness exceeding a depth of the gate trench TG in a position of the gate trench TG. Also, polysilicon is deposited in the outer trench TO. 
     In this regard, it is mentioned that CVD growth of polysilicon in the gate trench TG not only proceeds from a bottom face of the gate trench TG along a depth, but also proceeds laterally from a sidewall of the gate trench TG. Accordingly, polysilicon is relatively easily embedded in the gate trench TG which has a relatively small width. On the other hand, in the outer trench TO having a width which is larger than a width of the gate trench TG, growth from a sidewall of the outer trench TO hardly contributes except for a neighboring region of a sidewall. For example, in a position inside the outer trench TO, which position is laterally spaced away from a sidewall of the outer trench TO by a distance corresponding a depth of the outer trench TO or more, CVD growth of polysilicon from a sidewall hardly affects. Therefore, a thickness of polysilicon which grows on a position spaced away from a sidewall of the outer trench TO by a certain distance on a bottom face of the outer trench TO, is nearly identical to a thickness of polysilicon which grows on a surface of the semiconductor layer in the active region  30 . 
     As described above, growth in the gate trench TG is further promoted by contribution of CVD growth from a sidewall of the gate trench TG, as compared to growth outside the gate trench TG. Accordingly, a thickness of a polysilicon film on a bottom face of the gate trench TG is larger than a thickness of a polysilicon film outside the gate trench TG. Therefore, a surface of a polysilicon film within the active region  30 , which includes a somewhat recessed portion in a position of the gate trench TG, is flat to a certain extent. In other words, irregularities provided by the gate trench TG are made almost flat by a polysilicon film with which the irregularities are covered. 
     Subsequently, the gate electrode  8  and the gate connection layer  14  are formed from the above-described polysilicon film. More specifically, the polysilicon film is etched back in such a manner that those components can remain. As described above, since the polysilicon film has a large thickness on a bottom face of the gate trench TG, polysilicon can be left to serve as the gate electrode  8  in the gate trench TG without protecting the gate trench TG with a mask. On the other hand, in order to leave the polysilicon film for the gate connection layer  14 , it is necessary to previously form a mask corresponding to a pattern of the gate connection layer  14  at a time of etching-back. In this regard, it is mentioned that the gate connection layer  14  should be connected with the gate electrode  8  located outside the outer trench TO for the sake of a function thereof as wiring. For this reason, a mask having a pattern corresponding to the gate connection layer  14  should reach a sidewall of the outer trench TO, which is located closer to the active region  30  in a plan layout. In this regard, it is difficult in an actual process to form a mask so that a position of an end of the mask is completely aligned with a position of a sidewall of the outer trench TO because of an error in overlapping patterns. Thus, a certain amount of process margin should be provided. Specifically, a mask for etching-back is formed so as to invade the active region  30  from a sidewall of the outer trench TO, which is located closer to the active region  30 . That is, in order to surely prevent disconnection between the gate electrode  8  and the gate connection layer  14 , a mask for etching-back is formed so that a bottom face of the outer trench TO and a corner of an open end of the outer trench TO, which end is located closer to the active region  30 , are covered therewith. 
     A length of a portion of a mask for etching-back, which portion invades the active region  30  from a position of a sidewall of the outer trench TO, is preferably equal to or larger than 0.1 μm and is equal to or smaller than 3 μm. If the length is too small, disconnection between the gate electrode  8  and the gate connection layer  14  is more likely to occur for the above-described reasons. To provide a margin of approximately 3 μm could almost certainly avoid such a problem as described above. Additionally, if the foregoing length is unnecessarily large, an area which can be effectively used in the active region  30  is decreased. Thus, in order to ensure predetermined performance such as on-resistance, a size of the MOSFET  101  shall be increased. Also, a width of the active-region end  31   b  should be increased, and to this end, an interval between portions of the trench-bottom field relaxing regions  13  which are adjacent to each other with the active-region end  31   b  being interposed may become larger than an interval between portions of the trench-bottom field relaxing regions  13  which are adjacent to each other with the unit cell  31   a  being interposed. As a result of this, a field relaxing effect produced by the trench-bottom field relaxing region  13  may be unsatisfactorily exerted near the active-region end  31   b.    
     For the above-described reasons, a mask for etching-back which includes a portion lying over the terminating region  40  and the active region  30  is formed by patterning. A resist mask may be used as the foregoing mask for etching-back, and to use a resist mask makes patterning easy. With the use of the thus formed mask for etching-back, etching-back is performed on a polysilicon film until the gate insulating film  7  on the semiconductor layer is exposed outside the gate trench TG. In order to surely remove an unnecessary portion of polysilicon, over-etching should be performed. By selecting an etching condition which allows a ratio of an etching rate of polysilicon to a material of the gate insulating film  7  to be sufficiently large, it is possible to achieve satisfactory over-etching. 
     In etching-back, polysilicon embedded in the gate trench TG can remain as the gate electrode  8  because a thickness thereof is large as described above. At that time, under the influence of etching-back, a surface of a polysilicon film remaining as the gate electrode  8  in the gate trench TG is located deeper than an open end of the gate trench TG ( FIG. 2 ). 
     Subsequently, the mask for etching-back is removed. Then, the interlayer insulating film  9  is formed so that the terminating region  40  and the active region  30  are covered therewith. Thereafter, contact holes for the source contact CS and the outer contact CO are formed by dry etching, for example. Also, a contact hole for the gate contact CG is formed by dry etching, for example. After that, the source electrode  10  including the source contact CS and the outer contact CO is formed. Also, the gate wire portion  20  including the gate contact CG is formed. Then, the protective insulating layer  21  is formed. 
     Subsequently, the drain electrode  11  is formed on a back face of the semiconductor substrate  1 . As a result of this, the MOSFET  101  is obtained. 
     Comparative Example 
     It is assumed that a MOSFET of a comparative example is a MOSFET in which the trench-bottom field relaxing region  13  is provided over a portion where the trench-bottom high-concentration region  18  ( FIGS. 3 and 4 ) is provided and the trench-bottom field relaxing region  13  is omitted. 
     During high-speed switching of the MOSFET, a transient current for charging a pn junction capacitance of the trench-bottom field relaxing region  13  flows into the outer contact CO of the source electrode  10  through the trench-bottom field relaxing region  13 . At that time, voltage drop corresponding to a product of the foregoing transient current and parasitic resistance of the trench-bottom field relaxing region  13  occurs in the trench-bottom field relaxing region  13 . As electric resistance of the trench-bottom field relaxing region  13  becomes higher, the voltage drop is increased. While an increase of an impurity concentration of the trench-bottom field relaxing region  13  can reduce electric resistance, such increase of an impurity concentration may possibly degrade an intrinsic function of the trench-bottom field relaxing region  13 . Especially, at a time of a drive at high dV/dt (for example, a high-speed switching drive at 10 V/ns or higher), voltage drop caused by a transient current is increased, so that dielectric breakdown of the gate insulating film  7  directly above a path of the transient current may possibly occur. Also, even though such breakdown as described above may not occur, switching loss of the MOSFET is increased. 
     Also, during a short circuit of a load of the MOSFET, a high voltage is instantaneously applied to the drain electrode  11 . Accordingly, from the trench-bottom field relaxing region  13  to the drift layer  3 , a reverse voltage is applied and a depletion layer extends. Also at that time, in the same manner as described above, a charging current for the depletion layer which is caused by application of a high dV/dt flows through the trench-bottom field relaxing region  13 . Because of resultant voltage drop, a concern about reliability of the gate insulating film  7  is raised for the same reasons as described above. 
     Further, during a short circuit of a load of the MOSFET, a short-circuit current flowing from the drain electrode  11  concentrates on a region where the depletion layer does not greatly extends. A response speed in extension of the depletion layer at that time is rate-determined by a time constant which depends on a product C×R of a depletion-layer capacitance C and resistance R of a path of a charging current. Accordingly, a depletion layer cannot extend at a high speed from a portion of the trench-bottom field relaxing region  13 , which portion has high resistance R. As a result of this, before the depletion layer fully extends, a short-circuit current concentrates locally on a neighborhood of the foregoing portion of the trench-bottom field relaxing region  13 . Therefore, the foregoing portion is more susceptible to short-circuit breakdown than the other portions. Presence of such the more susceptible portion could reduce a short-circuit tolerance of the MOSFET. 
     (Effects) 
     Unlike the above-described comparative example, according to the present embodiment, the trench-bottom high-concentration region  18  on the trench-bottom field relaxing region  13  extends from a position where it faces the gate connection layer  14  with only the gate insulating film  7  being interposed, to a position where it is in contact with the outer contact CO of the source electrode  10 . As a result of this, during high-speed switching or a short circuit of a load, voltage drop caused by a transient current for charging a pn junction formed by the field relaxing region and the drift layer  3  is reduced. Therefore, breakdown of the gate insulating film  7  below the gate connection layer  14  can be suppressed. 
     Also, the foregoing charging/discharging current flows through the trench-bottom high-concentration region  18 , which has low resistance because of a high impurity concentration thereof. As a result of this, switching loss caused by a charging/discharging current can be reduced. Also, a response speed in extension of a depletion layer during a short circuit of a load is increased, so that a short-circuit tolerance of the MOSFET  101  can be improved. 
     The field insulating film  9   a  on the trench-bottom high-concentration region  18  allows insulation between the trench-bottom high-concentration region  18  and the gate connection layer  14  to be more certainly ensured. 
     In the present embodiment, the field insulating film  9   a  is placed away from a boundary between the active region  30  and the terminating region  40 . If the field insulating film  9   a  invades the active region  30  beyond the foregoing boundary, an area of a portion which can be effectively used by providing a transistor element (effective area) in the active region  30  is reduced. According to the present embodiment, reduction in an effective area can be avoided. 
     An avalanche field intensity of the drift layer  3  is enhanced because the drift layer  3  is a silicon carbide layer. In this case, dielectric breakdown of a switching element is likely to occur particularly in the gate insulating film  7 , not in a semiconductor region. According to the present embodiment, such breakdown can be effectively suppressed. Also, it is known that electron trap is more likely to be formed between a semiconductor layer and a gate insulating film in a case where the semiconductor layer is formed of SiC, as compared to a case where Si, a semiconductor which is more generally used, is used. For this reason, in a silicon-carbide semiconductor device, a concern about reliability of a gate insulating film is intensified. Therefore, an effect produced by the present embodiment which allows reduction of an electric field applied to a gate insulating film, is more significant. 
     It is noted that a surface of a silicon carbide layer (upper face in  FIG. 2 ) is preferably a face (0001) in a hexagonal crystal system. It is preferable that the face (0001) has an off angle of approximately 10° or smaller, being not a face (0001) in a strict sense. An orientation of an off angle (off orientation) is in a [11-20]-axis direction, for example. It is noted that a face (000-1) may be used, and the same off angle as described above may be provided in a case where a face (000-1) is used. Also, as an alternative to the above-described faces, a face (1-100) or a face (03-38) may be used. 
     The drift layer  3  may be made of a wide-bandgap semiconductor other than SiC. A wide-bandgap semiconductor other than SiC includes Ga 2 O 3 , GaN (gallium nitride), or diamond, for example. Generally, a semiconductor device using a wide-bandgap semiconductor is expected to be put to a use at a high temperature and at a high voltage, in particular. At a high temperature, reliability of an insulating film tends to be degraded, and so the effects produced by applying the present embodiment are significant. Also, along with an increase of a withstand voltage, a voltage which is to be applied to an insulating film is increased, and so, the effects produced by applying the present embodiment are significant. 
     Also, the drift layer  3  may be made of a non-wide-bandgap semiconductor such as silicon, in place of a wide-bandgap semiconductor. Also in this case, the above-described basic effects of the present embodiment can be attained. 
     Next, effects produced by the terminating field relaxing region  12  will be described. Generally, a terminating field relaxing region is a region including a p-type impurity which has a field relaxing effect, such as a junction termination extension (JTE) region or a field limiting ring (FLR) region. Those regions are formed in an outer edge of an active region where a MOSFET cell is placed, and suppress breakdown of a semiconductor device due to concentration of an electric field on an outermost edge of the active region. In a case of a planar semiconductor device, a terminating field relaxing region such as a JTE region or an FLR region is formed on a surface of a drift layer in which an outer trench is not formed. On the other hand, in a case of a trench-gate semiconductor device, to form a terminating field relaxing region on a surface of a drift layer outside a trench would not allow satisfactory relaxing of field concentration in a bottom portion of a gate trench in an end of an active region, in an off state. In other words, an electric field concentrates on a pn junction formed by a trench-bottom field relaxing region formed on a bottom face of a gate trench in an end of an active region, and a drift layer. As a result of this, avalanche breakdown may possibly occur at a drain voltage which is lower than a withstand voltage estimated from a impurity concentration and a thickness of a drift layer. In contrast thereto, in a case where the terminating field relaxing region  12  is formed on a bottom face of the outer trench TO according to the present embodiment, local electric-field concentration on the trench-bottom field relaxing region  13  located in an outermost edge of the active region  30  is relaxed, so that adequate withstandability to avalanche can be ensured. It is noted that respective depths of the gate trench TG and the outer trench TO are not necessarily nearly identical to each other in order to attain the above-described effects, and it is sufficient if the terminating field relaxing region  12  is formed at about the same depth as the trench-bottom field relaxing region  13 . 
     (Modifications) 
       FIG. 5  is a partial plan view schematically showing a configuration of a MOSFET  101   a  (semiconductor device) according to a first modification of the present embodiment. Whereas a cell structure in the active region  30  is in a shape of a stripe in plan view in the MOSFET  101  ( FIG. 1 ), a cell structure is in a shape which forms a grid pattern in the present modification. It is noted that a shape of a cell structure is not limited to a shape of a stripe or a shape which forms a grid pattern. For example, a cell structure may have a shape of a polygon or waves. 
       FIG. 6  is a partial plan view schematically showing a configuration of a MOSFET  101   b  (semiconductor device) according to a second modification of the present embodiment.  FIG. 7  is a partial cross-sectional view taken along a line VII-VII in  FIG. 6 . Whereas the active-region end  31   b  has a configuration analogous to that of the unit cell  31   a  in the MOSFET  101  ( FIG. 1 ), the above-described two components considerably differ from each other in the present modification. More specifically, in the present modification, the active-region end  31   b  does not include the source contact CS. For this reason, the active-region end  31   b  does not function as a MOSFET element, and mainly functions to be connected to the gate connection layer  14 . Thus, in the present modification, the active-region end  31   b  can be regarded as a dummy cell. Omission of the source contact CS can make a width of the active-region end  31   b  smaller than a width of the unit cell  31   a . According to the present modification, a distance between adjacent portions of the trench-bottom field relaxing region  13  near the active-region end  31   b  being a dummy cell is reduced. As a result of this, reliability of the gate insulating film  7  in the active-region end  31   b  in an off state can be further improved. Also, a corner of an open end of the outer trench TO, which corner is located closer to the active region  30 , is formed by the well region  4 . The foregoing corner is not necessarily required to be formed by the source region  5  because the active-region end  31   b  is a dummy cell in the present modification. The foregoing corner is formed by the well region  4  having resistance which is higher than resistance of the source region  5 , so that reliability of the gate insulating film  7  in the corner is improved. 
       FIG. 8  is a partial plan view schematically showing a configuration of a MOSFET  101   c  (semiconductor device) according to a third modification of the present embodiment.  FIG. 9  is a partial cross-sectional view taken along a line IX-IX in  FIG. 8 .  FIG. 10  is a partial cross-sectional view taken along a line X-X in  FIG. 8 .  FIG. 11  is a partial cross-sectional view taken along a line XI-XI in  FIG. 8 .  FIG. 12  is a partial cross-sectional view taken along a line XII-XII in  FIG. 8 . Whereas the trench-bottom field relaxing region  13  is provided over a bottom face of the gate trench TG within the active region  30  in the MOSFET  101  ( FIG. 1 ), the trench-bottom field relaxing region  13  is provided in a form of stripes within the active region  30  in the present modification. A periodic direction in which presence and absence of the trench-bottom field relaxing region  13  repeatedly alternate in a form of strips is along a direction in which the gate trench TG extends (a vertical direction in  FIG. 8 ). Accordingly, as shown in  FIGS. 10 and 11 , a bottom face of the gate trench TG includes a portion covered with the trench-bottom field relaxing region  13  and a portion not covered with the trench-bottom field relaxing region  13 . Even though a bottom face of the gate trench TG includes a portion not covered with the trench-bottom field relaxing region  13 , serious inconvenience is not caused as long as a field relaxing effect can be satisfactorily produced by the trench-bottom field relaxing region  13  located near the portion. According to the present modification, the trench-bottom field relaxing region  13  is not formed on all portions of a bottom face of the gate trench TG in plan view. Thus, an on current which flows through an inverted channel in an on state hardly suffers from current constriction under the influence of a pseudo junction-field effect transistor (J-FET) between the trench-bottom field relaxing region  13  and the well region  4 . Therefore, not only switching loss, but also conduction loss, can be reduced. Additionally, it is preferable that the trench-bottom field relaxing region  13  includes a portion placed on a bottom face of the gate trench TG and a portion placed on a bottom face of the outer trench TO and the two portions are continuous with each other, as shown in  FIG. 8 . As a result of this, the trench-bottom field relaxing region  13  within the active region  30  and the trench-bottom field relaxing region  13  within the terminating region  40  are electrically connected to each other. 
       FIG. 13  is a partial plan view schematically showing a configuration of a MOSFET  101   d  (semiconductor device) according to a fourth modification of the present embodiment.  FIG. 14  is a partial cross-sectional view taken along a line XIV-XIV in  FIG. 13 .  FIG. 15  is a partial cross-sectional view taken along a line XV-XV in  FIG. 13 . According to the present modification, the trench-bottom high-concentration region  18  is connected to the well contact region  16  via a trench-sidewall high-concentration well  18   a  provided on a sidewall of the outer trench TO. This configuration can be easily obtained by using an ion beam travelling along a direction inclined with respect to the semiconductor substrate  1  (an ion beam travelling from the upper right to the lower left in  FIGS. 14 and 15 ) at a time of ion implantation for forming the trench-bottom high-concentration region  18 , for example. According to the present modification, resistance of a current path of a charging/discharging current for a pn junction of the trench-bottom field relaxing region  13  during switching is further reduced. Accordingly, the above-described effects produced by the present embodiment can be further enhanced. 
     In each of the above-described modifications, the other components than specifically described can be omitted as appropriate. Conversely, also in a case where the above-described arbitrary component is appropriately added, the above-described effects can be produced. 
     Second Embodiment 
     (Configuration) 
       FIG. 16  is a partial plan view schematically showing a configuration of a MOSFET  102  (semiconductor device) according to a second embodiment.  FIG. 17  is a partial cross-sectional view taken along a line XVII-XVII in  FIG. 16 .  FIG. 18  is a partial cross-sectional view taken along a line XVIII-XVIII in  FIG. 16 . 
     In the MOSFET  102 , a source electrode  10  includes an inner contact CI which is in contact with a bottom face of a gate trench TG within an active region  30 . A portion of the gate trench TG where the inner contact CI is placed in plan view locally has a large width. A bottom face of that portion of the gate trench TG is formed by the trench-bottom field relaxing region  13  and the trench-bottom high-concentration region  18  provided on the trench-bottom field relaxing region  13 . Thus, the trench-bottom high-concentration region  18  includes a portion placed on a bottom face of the gate trench TG, and this portion is in contact with the inner contact CI. The inner contact CI includes a silicide region  19  in a portion being in contact with a semiconductor layer. The inner contact CI may be placed in either an end of the active region  30  or a portion inner than the end of the active region  30 . 
     It is noted that since the configuration is nearly identical to the above-described configuration of the first embodiment in the other respects than described above, the same or corresponding components are denoted by the same reference numerals and description of such components will not be repeated. 
     (Effects) 
     According to the present embodiment, the inner contact CI which is in contact with a bottom face of the gate trench TG is provided. As a result of this, below a gate insulating film  7  on a bottom face of the gate trench TG, voltage drop caused by a charging/discharging current for a pn junction formed by the trench-bottom high-concentration region  18  and a drift layer  3  can be suppressed. Accordingly, reliability of the gate insulating film  7  within the active region  30  where the gate trench TG is provided can be enhanced. Also, switching loss caused by a charging/discharging current can be reduced. Also, a response speed in extension of a depletion layer during a short circuit of a load is increased, so that a short-circuit tolerance of the MOSFET  102  can be improved. 
     A charging/discharging current from the trench-bottom field relaxing region  13  near the active-region end  31   b  can be divided into the inner contact CI within the active region  30  and the outer contact CO within the terminating region  40 . As a result of this, voltage drop can be suppressed particularly near the active-region end  31   b.    
     (Modifications) 
       FIG. 19  is a partial plan view schematically showing a configuration of a MOSFET  102   a  (semiconductor device) according to a first modification of the present embodiment. In the present modification, unlike the MOSFET  102  ( FIG. 16 ), the gate electrode  8  extends so as to form a grid pattern. In other words, cell structures are arranged in a grid pattern. 
       FIG. 20  is a partial plan view schematically showing a configuration of a MOSFET  102   b  (semiconductor device) according to a second modification of the present embodiment. In the present modification, like the MOSFET  101   b  ( FIG. 6 ), the active-region end  31   b  serving as a dummy cell is provided. 
       FIG. 21  is a partial plan view schematically showing a configuration of a MOSFET  102   c  (semiconductor device) according to a third modification of the present embodiment.  FIG. 22  is a partial cross-sectional view taken along a line XXII-XXII in  FIG. 21 .  FIG. 23  is a partial cross-sectional view taken along a line XXIII-XXIII in  FIG. 21 . In the present modification, the unit cell  31   a  which includes the source contact CS and can function as a transistor element and the unit cell  31   a  including the inner contact CI are arranged in a form of stripes. 
     As above, modifications of a cell structure within the active region  30  have been described. However, a cell structure within the active region  30  is not limited to those structures. 
     Third Embodiment 
     (Configuration and Effects) 
       FIG. 24  is a partial plan view schematically showing a configuration of a MOSFET  103  (semiconductor device) according to a third embodiment. In the MOSFET  103 , an active region  30  includes an end region RE located in an end of the active region  30  and an inner region RI located inner than the end region RE. An area which is occupied by an inner contact CI per unit area in the end region RE is larger than an area which is occupied by the inner contact CI per unit area in the inner region RI. It is noted that since the configuration is nearly identical to the above-described configuration of the second embodiment in the other respects than described above, the same or corresponding components are denoted by the same reference numerals and description of such components will not be repeated. 
     A charging/discharging current for a pn junction formed by a trench-bottom field relaxing region  13  between the end region RE and the outer contact CO is divided into an outer contact CO and the inner contact CI, and thus, is reduced. Nonetheless, the trench-bottom field relaxing region  13  is provided over a wide range between the end region RE and the outer contact CO, as shown in the drawings. A current flows from such a wide range toward each inner contact CI within the end region RE. Accordingly, in a configuration of the MOSFET  102  ( FIG. 16 : second embodiment), for example, a high current tends to flow toward each inner contact CI. In a path through which such a high current flows, a decrease of reliability of a gate insulating film  7 , an increase of switching loss, or reduction in a short-circuit tolerance, as described above, are likely to be caused. 
     In contrast thereto, according to the present embodiment, an area which is occupied by the inner contact CI per unit area in the end region RE is larger than an area which is occupied by the inner contact CI per unit area in the inner region RI. Accordingly, a magnitude of a current flowing toward each inner contact CI within the end region RE can be reduced. Therefore, a decrease of reliability of the gate insulating film  7 , an increase of switching loss, or reduction in a short-circuit tolerance can be further suppressed. 
     (Modifications) 
       FIG. 25  is a partial plan view schematically showing a configuration of a MOSFET  103   a  (semiconductor device) according to a first modification of the present embodiment. In the present modification, unlike the MOSFET  103  ( FIG. 24 ), the gate electrode  8  extends so as to form a grid pattern. In other words, cell structures are arranged in a grid pattern. 
       FIG. 26  is a partial plan view schematically showing a configuration of a MOSFET  103   b  (semiconductor device) according to a second modification of the present embodiment. In the present modification, a dummy cell described above with regard to the MOSFET  101   b  ( FIG. 6 ) is provided in a part of the end region RE. 
       FIG. 27  is a partial plan view schematically showing a configuration of a MOSFET  103   c  (semiconductor device) according to a third modification of the present embodiment.  FIG. 28  is a partial cross-sectional view taken along a line XXVIII-XXVIII in  FIG. 27 . In the present modification, a unit cell which includes a source contact CS and can function as a transistor element and a unit cell including the inner contact CI are arranged in a form of stripes within the active region  30 . Arrangement in a form of stripes is designed so that a unit cell including the inner contact CI is placed in the end region RE. 
       FIG. 29  is a partial plan view schematically showing a configuration of a MOSFET  103   d  (semiconductor device) according to a fourth modification of the present embodiment. In the present modification, a unit cell which includes the source contact CS and can function as a transistor element and a unit cell including the inner contact CI are arranged in a form of stripes within the active region  30 . One of unit cells repeatedly placed in a form of stripes, which is placed in an endmost portion, is set to be a unit cell including the inner contact CI. In a portion inner than the endmost portion, a unit cell including the source contact CS is repeatedly placed. 
     As above, modifications of a cell structure in the active region  30  have been described. However, a cell structure in the active region  30  is not limited to those structures. 
     Fourth Embodiment 
     (Configuration and Effects) 
       FIG. 30  is a partial plan view schematically showing a configuration of a MOSFET  104  (semiconductor device) according to a fourth embodiment.  FIG. 31  is a partial cross-sectional view taken along a line XXXI-XXXI in  FIG. 30 . In the MOSFET  104 , a trench-bottom high-concentration region  18  extends from an outer contact CO to an inner contact CI. It is noted that since the configuration is nearly identical to the above-described configuration of the second or third embodiment in the other respects than described above, the same or corresponding components are denoted by the same reference numerals and description of such components will not be repeated. 
     A charging/discharging current for a pn junction which is formed by a trench-bottom field relaxing region  13  between an active-region end  31   b  and the outer contact CO is divided into the outer contact CO and the inner contact CI, and thus, is reduced. Nonetheless, the trench-bottom field relaxing region  13  is provided over a wide range between the active-region end  31   b  and the outer contact CO, as shown in the drawings. A current flows from such a wide range toward each inner contact CI within the active-region end  31   b . Accordingly, in a configuration of the MOSFET  102  ( FIG. 16 : second embodiment), for example, a high current tends to flow toward each inner contact CI. In a path through which such a high current flows, a decrease of reliability of a gate insulating film  7 , an increase of switching loss, or reduction in a short-circuit tolerance, as described above, are likely to be caused. 
     In contrast thereto, according to the present embodiment, the trench-bottom high-concentration region  18  extends from the outer contact CO to the inner contact CI. In other words, the trench-bottom high-concentration region  18  extends along a path through which the above-described high charging/discharging current flows. Accordingly, resistance of a current path through which a high charging/discharging current flows is reduced. Therefore, a decrease of reliability of the gate insulating film  7 , an increase of switching loss, or reduction in a short-circuit tolerance can be further suppressed. 
     (Modification) 
       FIG. 32  is a partial plan view schematically showing a configuration of a MOSFET  104   a  (semiconductor device) according to a first modification of the present embodiment.  FIG. 33  is a partial cross-sectional view taken along a line XXXIII-XXXIII in  FIG. 32 . In the present modification, a unit cell which includes a source contact CS and can function as a transistor element and a unit cell including the inner contact CI are arranged in a form of stripes within an active region  30 . Arrangement in a form of stripes is designed so that a unit cell including the inner contact CI is placed in an end of the active region  30 . Additionally, a cell structure in the active region  30  is not limited to those in the present embodiment and its modification. 
     Fifth Embodiment 
     (Configuration) 
       FIG. 34  is a partial plan view schematically showing a configuration of a MOSFET  105  (semiconductor device) according to a fifth embodiment.  FIG. 35  is a partial cross-sectional view taken along a line XXXV-XXXV in  FIG. 34 .  FIG. 36  is a partial cross-sectional view taken along a line XXXVI-XXXVI in  FIG. 34 . 
     In the MOSFET  105 , a field insulating film  9   a  reaches an open end of an outer trench TO. As shown in  FIG. 35 , a gate connection layer  14  extends from an inside of the outer trench TO toward an active region  30  to an outside of the outer trench TO (from the right side toward the left side in  FIG. 35 ), and climbs onto an open end of the outer trench TO (an upper-right corner of an active-region end  31   b  in  FIG. 35 ) with the field insulating film  9   a  and a gate insulating film  7  on the field insulating film  9   a  being interposed. Accordingly, unlike the MOSFET  101  ( FIG. 3 : first embodiment), a configuration in which the gate connection layer  14  and an open end of the outer trench TO face each other with only the gate insulating film  7  being interposed, is not provided. The field insulating film  9   a  in addition to the gate insulating film  7  is placed between the gate connection layer  14  and an open end of the outer trench TO, without fail. Preferably, the well region  4  is provided in an open end of the outer trench TO. 
     Further, the field insulating film  9   a  completely insulates the gate connection layer  14  and a trench-bottom high-concentration region  18  from each other. Accordingly, unlike the MOSFET  101  ( FIG. 3 ), a configuration in which the gate connection layer  14  and the trench-bottom high-concentration region  18  face each other with only the gate insulating film  7  being interposed, is not provided. The field insulating film  9   a  in addition to the gate insulating film  7  is provided between the gate connection layer  14  and the trench-bottom high-concentration region  18 , without fail. 
     It is noted that since the configuration is nearly identical to the above-described configurations of the first to fourth embodiments in the other respects than described above, the same or corresponding components are denoted by the same reference numerals and description of such components will not be repeated. 
     (Effects) 
     According to the present embodiment, as shown in  FIG. 35 , the field insulating film  9   a  in addition to the gate insulating film  7  is provided between the gate connection layer  14  and an open end of the outer trench TO, without fail. As a result of this, on an open end of the outer trench TO, reliability of the gate insulating film  7  can be improved. 
     The gate connection layer  14  is insulated from the trench-bottom high-concentration region  18  which serves as a main path of a charging/discharging current for a pn junction of the trench-bottom field relaxing region  13  during switching, by not only the gate insulating film  7 , but also the field insulating film  9   a . As a result of this, dielectric breakdown is prevented from occurring between the trench-bottom high-concentration region  18  and the gate connection layer  14  due to voltage drop caused by the foregoing charging/discharging current. 
     (Modifications) 
       FIG. 37  is a partial plan view schematically showing a configuration of a MOSFET  105   a  (semiconductor device) according to a first modification of the present embodiment.  FIG. 38  is a partial cross-sectional view taken along a line XXXVIII-XXXVIII in  FIG. 37 . It is noted that a cross-sectional view taken along line XXXIII-XXXVIII in  FIG. 37  is similar to  FIG. 33  (the first modification of the fourth embodiment). Also in the present modification, as shown in  FIG. 38 , the field insulating film  9   a  in addition to the gate insulating film  7  is provided between the gate connection layer  14  and an open end of the outer trench TO, without fail. In the meantime, in the present modification, the gate connection layer  14  includes a portion which is insulated from the trench-bottom high-concentration region  18  by only the gate insulating film  7  as shown in  FIG. 33 . 
       FIG. 39  is a partial plan view schematically showing a configuration of a MOSFET  105   b  (semiconductor device) according to a second modification of the present embodiment.  FIG. 40  is a partial cross-sectional view taken along a line XL-XL in  FIG. 39 . In the present modification, unlike the above-described first modification, the field insulating film  9   a  completely insulates the gate connection layer  14  and the trench-bottom high-concentration region  18  from each other. 
     Sixth Embodiment 
     While a MOSFET has been described as a semiconductor device in the above-described first to fifth embodiments, a semiconductor device is not limited to a MOSFET. A sixth embodiment will describe a case where a semiconductor device is an IGBT. 
       FIG. 41  is a partial cross-sectional view schematically showing a configuration of an IGBT  106  (semiconductor device) according to the present embodiment. The IGBT  106  includes a semiconductor substrate  1 A (a second impurity region) in place of the semiconductor substrate  1  of the MOSFET  101  ( FIG. 2 ). Whereas the semiconductor substrate  1  is of an n type, the semiconductor substrate  1 A is of a p type. Thus, the semiconductor substrate  1 A can function as a collector region in the IGBT  106 . Also, the IGBT  106  includes an emitter region  5 A (first impurity region), an emitter electrode  10 A (first main electrode), and a collector electrode  11 A (second main electrode) as components corresponding to the source region  5 , the source electrode  10 , and the drain electrode  11  of the MOSFET  101  ( FIG. 2 ), respectively. In other words, in a manufacturing process of the MOSFET  101  ( FIG. 2 ), by using the p-type semiconductor substrate  1 A in place of the n-type semiconductor substrate  1 , it is possible to obtain the IGBT  106 . In the IGBT  106 , the semiconductor substrate  1 A is electrically connected to a drift layer  3  and the collector electrode  11 A. Accordingly, the collector electrode  11 A is electrically connected to the drift layer  3  via the semiconductor substrate  1 A. Also, the semiconductor substrate  1 A is separated from a well region  4  by the drift layer  3 . The emitter electrode  10 A includes an emitter contact CE (main contact) as a component corresponding to the source contact CS. 
     According to the present embodiment, in an IGBT, effects similar to those produced in the above-described first to fifth embodiments can be produced. It is noted that since the configuration is nearly identical to the above-described configurations of the first to fifth embodiments in the other respects than described above, the same or corresponding components are denoted by the same reference numerals and description of such components will not be repeated. 
     Besides, an IGBT can be obtained by other processes than described above. For example, an IGBT can be obtained by the following process. 
     Referring to  FIG. 2 , first, a structure shown therein is provided on an upper face of the semiconductor substrate  1 . Secondly, the semiconductor substrate  1  is removed. Thirdly, a p-type collector region (second impurity region) is formed on a lower face of the drift layer  3  which is exposed as a result of removal of the semiconductor substrate  1 . Formation of the collector region can be achieved by ion implantation into a lower face of the drift layer  3 , for example. Then, the collector electrode  11 A is formed on the collector region. 
     Seventh Embodiment 
       FIG. 42  is a partial cross-sectional view schematically showing a configuration of a MOSFET  101   e  (semiconductor device) according to a seventh embodiment. The MOSFET  101   e  includes a trench-bottom high-concentration region  18 V in place of the trench-bottom high-concentration region  18  ( FIG. 3 : first embodiment). The trench-bottom high-concentration region  18 V includes a first portion  18   i  which is in contact with an outer contact CO of a source electrode  10 , and a second portion  18   j  having a p-type impurity concentration which is lower than a p-type impurity concentration of the first portion  18   i . The second portion  18   j  is in contact with a gate connection layer  14  with only a gate insulating film  7  being interposed. The first portion  18   i  is not in contact with the gate insulating film  7 . In other words, the first portion  18   i  is separated from the gate insulating film  7 . Not the first portion  18   i  but the second portion  18   j  is in contact with the gate insulating film  7 . The second portion  18   j  is placed within a terminating region  40 , and forms a part of an outer trench TO. 
     It is noted that the first portion  18   i  and the second portion  18   j  can be formed in the same manner as the trench-bottom high-concentration region  18  ( FIG. 3 ). More specifically, the first portion  18   i  and the second portion  18   j  can be formed either by the same process or by different processes. All of the first portion  18   i , the second portion  18   j , and a terminating field relaxing region  12  may be formed by the same process. 
     In the MOSFET  101  ( FIG. 3 : first embodiment), the trench-bottom high-concentration region  18  should have a sufficiently high impurity concentration in order to ensure that the outer contact CO has a p-type ohmic property with sufficiently low resistance. Such a high impurity concentration would probably make a surface of a semiconductor rougher after annealing for activation. When a surface of a portion of the trench-bottom high-concentration region  18 , which portion is in contact with the gate connection layer  14  with only the gate insulating film  7  being interposed, is made rough to a great degree as described above, long-term reliability of the gate insulating film  7  on the foregoing portion may possibly be adversely affected. 
     In contrast thereto, according to the present embodiment, the trench-bottom high-concentration region  18 V includes the first portion  18   i  and the second portion  18   j . Since the first portion  18   i  having a relatively high impurity concentration is in contact with the outer contact CO, it is ensured that the outer contact CO has a p-type ohmic property with sufficiently low resistance. Further, not the first portion  18   i  having a relatively high impurity concentration, but the second portion  18   j  having a relatively low impurity concentration is in contact with the gate connection layer  14  with only the gate insulating film  7  being interposed, so that a portion having a rough surface in the trench-bottom high-concentration region  18 V is prevented from being in contact with the gate connection layer  14  with only the gate insulating film  7  being interposed. As a result of this, the above-described decrease of long-term reliability of the gate insulating film  7  can be avoided. 
     While a semiconductor device having a MOS structure has been described in detail in each of the above embodiments, a metal insulator semiconductor (MIS) structure may be employed, other than a MOS structure. In other words, an insulating film other than an oxide film may be employed as a gate insulating film. Also, while a case where a first conductivity type is an n type and a second conductivity type is a p type has been described in detail, those conductivity types may be interchanged. 
     While properties, materials, dimensions, shapes, positional interrelationships, conditions for implementation, or the like of respective components have been described in some portions in the above-described embodiments, those are mere examples in all aspects, and each of the embodiments is not limited to those described. Accordingly, numerous modifications which are not illustratively described are conceivable within the scope of each of the embodiments. For example, there are included a case where an arbitrary component is transformed, is added, or is omitted, and further a case where at least one component in at least one embodiment is extracted and is combined with a component in another embodiment. Also, unless contradiction arises, a component which has been described as being “one” in each of the above-described embodiments may be “one or more”. Further, each of components forming the invention is a conceptual unit, and there are included a case where one component includes a plurality of structures, a case where one component corresponds to a part of a certain structure. Also, description in the present specification should be referred to for all objects of the present invention, and no portion therein acknowledges being conventional arts. 
     EXPLANATION OF REFERENCE SIGNS 
     CE: emitter contact (main contact), CG: gate contact, CI: inner contact, CO: outer contact, CS: source contact (main contact), RE: end region, TG: gate trench, RI: inner region, TO: outer trench,  1 : semiconductor substrate,  1 A: semiconductor substrate (second impurity region),  3 : drift layer,  4 : well region,  5 : source region (first impurity region),  5 A: emitter region (first impurity region),  7 : gate insulating film,  8 : gate electrode,  9 : interlayer insulating film,  9   a : field insulating film,  10 : source electrode (first main electrode),  10 A: emitter electrode (first main electrode),  11 : drain electrode (second main electrode),  11 A: collector electrode (second main electrode),  12 : terminating field relaxing region,  13 : trench-bottom field relaxing region,  14 : gate connection layer,  16 : well contact region,  18 ,  18 V: trench-bottom high-concentration region,  18   a : trench-sidewall high-concentration well,  18   i : first portion,  18   j : second portion,  19 : silicide region,  20 : gate wire portion,  21 : protective insulating layer,  30 : active region,  31   a : unit cell,  31   b : active-region end,  40 : terminating region,  101  to  105 ,  101   a  to  101   e ,  102   a  to  102   c ,  103   a  to  103   d ,  104   a ,  105   a ,  105   b : MOSFET (semiconductor device),  106 : IGBT (semiconductor device)