Patent Publication Number: US-2023138658-A1

Title: Semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Patent Application No. PCT/JP2021/022464 filed on Jun. 14, 2021, which designated the U.S. and claims the benefit of priority from Japanese Patent Application No. 2020-124638 filed on Jul. 21, 2020. The entire disclosures of all of the above applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a semiconductor device. 
     BACKGROUND 
     A semiconductor device with a semiconductor element may be configured to form an upper-lower arm circuit of a power converter. 
     SUMMARY 
     The present disclosure describes a semiconductor device having a semiconductor element configured to form an upper-lower arm circuit of a power converter, and further describes the semiconductor element having a control electrode, a high-potential electrode and a low-potential electrode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a power converter to which a semiconductor device according to a first embodiment is applied. 
         FIG.  2    illustrates a characteristic of Vds-Cgd of a semiconductor element in the semiconductor device according to the first embodiment. 
         FIG.  3    illustrates a switching waveform at a time of turn-off. 
         FIG.  4    illustrates two switching waveforms with different ratios C 1 /C 2 . 
         FIG.  5    illustrates the relationship between the ratio C 1 /C 2  and turn-off loss Eoff. 
         FIG.  6    is a cross-sectional view that illustrates a structure of the semiconductor element included in the semiconductor device. 
         FIG.  7    illustrates a turn-off operation and a change in a capacitance Cgd. 
         FIG.  8    illustrates the relationship between a breakdown voltage BV and a width of a JFET portion. 
         FIG.  9    is a partial cross-sectional view of a modification of the semiconductor element. 
         FIG.  10    illustrates a power converter according to a second embodiment. 
         FIG.  11    illustrates a switching waveform when an input voltage is relatively high. 
         FIG.  12    illustrates a switching waveform when the input voltage is relatively low. 
     
    
    
     DETAILED DESCRIPTION 
     In order to reduce a surge voltage that occurs at the time of turning off a semiconductor element, an external capacitor and a capacitance adjustment switching element may be required in addition to the semiconductor element. The capacitor may be connected in parallel with the parasitic capacitor between a control electrode and a high-potential main electrode of the semiconductor element. The capacitance adjustment switching element may be connected between the capacitor and the control electrode. However, this structure may lead to, for example, an increase of the physical size of the semiconductor device and the complexity of control. 
     According to a first aspect of the present disclosure, a semiconductor device includes a semiconductor element. The semiconductor element is configured to form an upper-lower arm circuit of a power conversion device. The semiconductor element has a control electrode, a high-potential electrode and a low-potential electrode. The high-potential electrode is at a higher potential than the low-potential electrode. Each of the high-potential electrode and the low-potential electrode is a main electrode. A parasitic capacitance between the control electrode and the high-potential electrode changes according to a potential difference between the high-potential electrode and the low-potential electrode. A value of the parasitic capacitance at a time when the potential difference is equal to 80 percent of a breakdown voltage of the semiconductor element is defined as a first capacitance value. An arbitrary value of the parasitic capacitance at a time when the potential difference is in an inclusive range of 20 percent to 40 percent of the breakdown voltage is defined as a second capacitance value. The first capacitance value is larger than the second capacitance value. 
     According to the semiconductor device disclosed herein, when the potential difference is equal to 80 percent of the breakdown voltage, the parasitic capacitance between the control electrode and the high-potential electrode has a value larger than any value at a time where the potential difference is in the inclusive range of 20 percent to 40 percent of the breakdown voltage. In other words, the value of the parasitic capacitance becomes larger in a region where the potential difference is larger, and becomes smaller in a region where the potential difference is smaller. Therefore, it is possible to reduce the turn-off loss while reducing the surge voltage with a simple structure. 
     According to a second aspect of the present disclosure, a semiconductor device includes a semiconductor element. The semiconductor element is included in an upper-lower arm circuit of a power conversion device. The semiconductor element includes a drain region, a low-concentration layer, a junction-gate field-effect transistor (JFET) portion, a current dispersion layer, a second-conductivity-type region, a source region, a gate electrode, a source electrode, and a drain electrode. The drain region is a first conductivity type. The low-concentration layer is located on the drain region and being the first conductivity type, and has a lower impurity concentration than the drain region. The JFET portion is located on the low-concentration layer and is the first conductivity type. The JFET portion extends in a first direction orthogonal to a thickness direction of the semiconductor element. The JFET portion has a width smaller than a width of the low-concentration layer in a second direction orthogonal to the thickness direction and the first direction. The current dispersion layer is located on the JFET portion and is the first conductivity type. The current dispersion layer has a width larger than the width of the JFET portion in the second direction. The second-conductivity-type region is a semiconductor region located on the low-concentration layer. The second-conductivity-type region is a second conductivity type and sandwiches the JFET portion in the second direction. The second-conductivity-type region is also located on the current dispersion layer while surrounding the current dispersion layer. The source region is located on the second-conductivity-type region and is the first conductivity type. The source region has a higher impurity concentration than the low-concentration layer. The gate electrode has a trench structure penetrating the source region and the second-conductivity-type region and reaching the current dispersion layer. The source electrode is electrically connected to the source region. The drain electrode is electrically connected to the drain region. The width of the JFET portion is equal to or larger than 591.53×BV −0997  and smaller than or equal to 278.52×BV −0.767 , where BV denotes the breakdown voltage. 
     According to the semiconductor device disclosed herein, the JFET portion includes a structure having a smaller width and being sandwiched by the second-conductivity-type region. When the width of the JFET portion is set in the above-mentioned range, the portion of the second-conductivity-type region sandwiching the JFET portion is completely depleted based on a condition that the potential difference between the drain electrode and the source electrode is in the inclusive range of 40 percent to 80 percent of the breakdown voltage. Through the depletion, the parasitic capacitance between the gate electrode and the drain electrode suddenly increases. In other words, when the potential difference is equal to 80 percent of the breakdown voltage, the parasitic capacitance between the control electrode and the high-potential electrode has a value larger than any value at a time where the potential difference is in the inclusive range of 20 percent to 40 percent of the breakdown voltage. Therefore, it is possible to reduce the turn-off loss while reducing the surge voltage with a simple structure. 
     Hereinafter, multiple embodiments will be described with reference to the drawings. In some embodiments, parts that are functionally and/or structurally corresponding to each other and/or associated with each other are given the same reference numerals. For corresponding parts and/or associated parts, additional explanations can be made to the description of other embodiments. 
     A semiconductor device according to the present embodiment is applied to a power converter in a movable body adopting, for example, a rotating electrical machine as a drive source. The movable body is, for example, an electrically powered vehicle such as an electric vehicle (EV), a hybrid vehicle (HV), or a fuel cell vehicle (FCV), a flying body such as a drone, a ship, a construction machine, or an agricultural machine. In the following, an example applied to a vehicle will be described. 
     First Embodiment 
     Firstly, a schematic configuration of a vehicle drive system to which a power converter is applied will be described with reference to  FIG.  1   . 
     (Vehicle Drive System) 
     As shown in  FIG.  1   , a vehicle drive system  1  is provided with a direct current power supply  2 , a motor generator  3 , and a power converter  4 . 
     The direct current power supply  2  is a direct current voltage source including a chargeable/dischargeable secondary battery. The secondary battery is, for example, a lithium ion battery or a nickel hydride battery. The motor generator  3  is a three-phase alternating current type rotating electric machine. The motor generator  3  functions as a vehicle driving power source, that is, an electric motor. The motor generator  3  functions also as a generator during regeneration. The power converter  4  performs electric power conversion between the direct current power supply  2  and the motor generator  3 . 
     (Circuitry Structure of Power Converter) 
     Next, a circuit structure of the power converter  4  will be described with reference to  FIG.  1   . As shown in  FIG.  1   , the power converter  4  includes a smoothing capacitor  5 , an inverter  6  as a power conversion device, in other words, a power conversion circuit. 
     The smoothing capacitor  5  mainly smoothens the DC voltage supplied from the direct current power supply  2 . The smoothing capacitor  5  is connected between a P line  7  which is a power line on a high potential side and an N line  8  which is a power line on a low potential side. The P line  7  is connected to the positive electrode of the direct current power supply  2 , and the N line  8  is connected to the negative electrode of the direct current power supply  2 . The positive electrode of the smoothing capacitor  5  is connected to the P line  7  between the direct current power supply  2  and the inverter  6 . Similarly, the negative electrode is connected to the N line  8  between the direct current power supply  2  and the inverter  6 . The smoothing capacitor  5  is connected to the direct current power supply  2  in parallel. 
     The inverter  6  is a DC-AC converter. The inverter  6  converts the direct current voltage into a three-phase alternating current voltage according to the switching control by the control circuit (not shown) and outputs the three-phase alternating current voltage to the motor generator  3 . Thereby, the motor generator  3  is driven to generate a predetermined torque. At the time of regenerative braking of the vehicle, the inverter  6  converts the three-phase alternating current voltage generated by the motor generator  3  by receiving the rotational force from wheels into a direct current voltage according to the switching control by the control circuit, and outputs the direct current voltage to the P line  7 . In this way, the inverter  6  performs bidirectional power conversion between the direct current power supply  2  and the motor generator  3 . 
     The inverter  6  includes upper-lower arm circuits  9  respectively for three phases. The upper-lower arm circuits  9  are also referred to as legs. Each of the upper-lower arm circuits  9  includes a semiconductor device  11  provided with at least one semiconductor element  10 . Each of the upper-lower arm circuit  9  includes a semiconductor element  10 H as the semiconductor element  10  at an upper arm side and a semiconductor element  10 L as the semiconductor element  10  at a lower arm side. The semiconductor element  10 H is connected at the P line  7  side while the semiconductor elements  10 H,  10 L are connected in series between the P line  7  and the N line  8 . The connection node between the semiconductor elements  10 H and  10 L is connected to a winding  3   a  of a corresponding phase in the motor generator  3  via an output line  12 . At least a part of the P line  7 , the N line  8  and the output line  12  is made of a conductive member such as a bus bar. 
     In the present embodiment, the inverter  6  includes six semiconductor elements  10 . The inverter  6  includes three semiconductor elements  10 H and three semiconductor elements  10 L. One semiconductor element  10  is included in one of six arms in the inverter  6 . 
     The semiconductor element  10  includes a metal-oxide-semiconductor field-effect transistor (MOSFET)  13  as a switching element formed on a semiconductor substrate made of material such as silicon (Si) and a wide bandgap semiconductor having a wider bandgap than silicon. The MOSFET  13  is made such that a main current flows in a thickness direction of the semiconductor substrate. A main electrode is formed on each of the respective plate surfaces of the semiconductor substrates. Examples of wide bandgap semiconductors include silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga 2 O 3 ) and diamond. 
     In the present embodiment, the semiconductor element  10  is formed with the n-channel MOSFET  13  on a SiC substrate. The semiconductor element  10  includes a gate electrode  13   g , a drain electrode  13   d  and a source electrode  13   s . The gate electrode  13   g  is a control electrode, and each of the drain electrode  13   d  and the source electrode  13   s  is a main electrode. The drain electrode  13   d  corresponds to a high-potential electrode, and the source electrode  13   s  corresponds to a low-potential electrode. A freewheeling diode  14  is connected in anti-parallel to each of the MOSFETs  13 . The diode  14  may be a parasitic diode (i.e., a body diode) of the MOSFET  13  or may be provided separately from the parasitic diode. The anode of the diode  14  is connected to the source electrode  13   s  of the corresponding MOSFET  13 , and the cathode is connected to the drain electrode  13   d.    
     In the semiconductor element  10 H at the upper arm side, the drain electrode  13   d  is connected to the P line  7 . In the semiconductor element  10 L at the lower arm side, the source electrode  13   d  is connected to the N line  8 . The source electrode  13   s  of the semiconductor element  10 H and the drain electrode  13   d  of the semiconductor element  10 L are mutually connected. 
     In the present embodiment, one semiconductor device  11  includes one semiconductor element  10 . The power converter  4  includes six semiconductor devices  11  included in the inverter  6 . Alternatively, the semiconductor device  11  may include two semiconductor elements  10 H,  10 L mutually connected in series in the upper-lower arm circuit  9 . The semiconductor device  11  may include all semiconductor elements  10  in the inverter  6 . The semiconductor device  11  may include multiple semiconductor elements  10  mutually connected in parallel in one arm. 
     The power converter  4  may include a drive circuit of the MOSFET  13  in the inverter  6 . The drive circuit supplies a drive voltage to the gate of the MOSFET  13  of the corresponding arm based on the drive command of the control circuit. The drive circuit drives the corresponding MOSFET  13  by applying a drive voltage to turn on and off the drive of the corresponding MOSFET  13 . The drive circuit may also be referred to as a driver. 
     The power converter  4  may include a control circuit of the MOSFET  13 . The control circuit generates a drive command for operating the MOSFET  13  and outputs the drive command to the drive circuit. The control circuit generates the drive command based on a torque request input from a higher-level ECU (not shown) or signals detected by various sensors. 
     Various sensors include, for example, a current sensor, a rotation angle sensor, and a voltage sensor. The current sensor detects the phase current flowing through the winding  3   a  of each phase. The rotation angle sensor detects the rotation angle of the rotor of the motor generator  3 . The voltage sensor detects the voltage across the smoothing capacitor  5 . The control circuit outputs, for example, a PWM signal as the drive command. The control circuit includes, for example, a microcomputer. ECU is an abbreviation for Electronic Control Unit. PWM is an abbreviation for Pulse Width Modulation. 
     (Characteristic of Element) 
     The following describes the characteristics of the semiconductor element  10 , in other words, the MOSFET  13  included in the semiconductor device  11  according to the present embodiment with reference to  FIGS.  2  to  5   .  FIG.  2    illustrates a Vds-Cgd characteristic.  FIG.  3    illustrates a switching waveform at the time of turn-off.  FIG.  4    illustrates the relationship between a ratio C 1 /C 2  and a switching waveform.  FIG.  5    illustrates the relationship between the ratio C 1 /C 2  and turn-off loss Eoff. 
     Vds illustrated in the drawing is the voltage between the drain and the source. The voltage Vds corresponds to a potential difference between a high-potential electrode and a low-potential electrode. Cgd indicates the capacitance, in other words, the parasitic capacitance between the gate and the drain. The capacitance Cgd is also referred to as a feedback capacitance. Id indicates the drain current. ΔV indicates the surge voltage. Each of  FIGS.  2 ,  3  and  5    illustrates a simulation result.  FIG.  3    illustrates the simplification of the simulation result. In  FIG.  3   , for example, BV 80% indicates the value of 80% of a breakdown voltage BV.  FIG.  4    illustrates two levels for the ratio C 1 /C 2 , and illustrates a switching waveform at the time of turning off for each level. In  FIG.  4   , the value of the gate resistance is adjusted such that the surge voltages ΔV are equal. 
     In the present embodiment, the MOSFET  13 , in other words, a SiC-MOSFET of the semiconductor element  10  has a feature in which the capacitance Cgd changes according to the voltage Vds, as illustrated in  FIG.  2   . The value of the capacitance C 1  is larger than the value of the capacitance C 2 . The capacitance Cgd changes suddenly at a time where the voltage Vds reaches a predetermined voltage Vip. For example, when the voltage Vds rises and reaches the voltage Vip, the capacitance Cgd suddenly increases. The voltage Vip is sometimes referred to as an inflection point voltage. According to the above feature, it is possible to reduce the surge voltage generated at the time of the turn-off. 
     In  FIG.  3   , a solid line indicates a switching waveform according to the present embodiment. A broken line indicates a switching waveform according to a reference example. In the reference example, the MOSFET of the semiconductor element includes a substantially feature in which the capacitance Cgd does not change according to the voltage Vds. In other words, the MOSFET of the semiconductor element includes a feature in which the respective values of the capacitances C 1  and C 2  are equal. 
     At the time of the turn-off, the surge voltage ΔV is superimposed on voltage Vds. A voltage Vds increased by the surge voltage ΔV is applied to the semiconductor element, in other words, the MOSFET. The surge voltage ΔV is the product of the switching speed di/dt and the parasitic inductance Ls of the main circuit. The switching speed di/dt is the amount of change in the current Id per unit time, and is also referred to as a current slope. In the structure according to the reference example indicated by a broken line, when the switching speed di/dt is enlarged to reduce the switching loss, in other words, the turn-off loss, the surge voltage ΔV also increases. 
     As indicated by the solid line, in the semiconductor element  10 , in other words, the MOSFET  13  according to the present embodiment, when the voltage Vds reaches the voltage Vip at a time t 1 , the capacitance Cgd suddenly increases according to the above feature. Accordingly, the switching speed Di/dt is smaller than the reference example. Since the current Id changes gradually, the surge voltage can be made smaller than the reference example. Therefore, the voltage Vds on which the surge voltage ΔV is superimposed can be prevented from exceeding the element&#39;s breakdown voltage. 
     The rising waveform of the voltage Vds makes a large contribution to the turn-off loss. When the rise of the voltage Vds slows down, the turn-off loss increases. As shown in  FIG.  3   , the rising waveform of the voltage Vds in the present embodiment is equivalent to the reference example in which the capacitance Cgd does not change. It is possible to reduce the turn-off loss as compared with the structure in which the voltage Vds rises slowly. Therefore, it is possible to reduce the turn-off loss while reducing the surge voltage ΔV by reducing the switching speed di/dt. 
     The following describes the voltage Vip and the voltage Vds corresponding to the value of capacitances C 1 , C 2 . 
     It may be preferable that the voltage Vip is set to be smaller than or equal to 80% of a voltage BV. When the device characteristics such as a gate threshold voltage and a gate input capacitance affecting the switching speed di/dt vary due to process variation, in other words, manufacturing variation, the surge voltage ΔV also varies. When the voltage Vip exceeds 80% of the voltage BV, the voltage Vds on which the surge voltage ΔV is superimposed may exceed the voltage BV due to the variation in the above-mentioned surge voltage ΔV. 
     When the voltage Vip is set to be smaller than or equal to 80% of the voltage BV, even though the surge voltage ΔV varies, it is possible to prevent the voltage Vds on which the surge voltage ΔV is superimposed from reaching the voltage BV. This effect has been confirmed by simulation. 
     In the present embodiment, the capacitance Cgd suddenly changes when the value of the voltage Vds is smaller than or equal to 80% of the voltage BV. The capacitance value C 1  is a value of the capacitance Cgd at a time when the voltage Vds is equal to a value of 80% of the voltage BV, and is larger than the capacitance value C 2 . The capacitance value C 1  corresponds to a first capacitance value. 
     It may be preferable that the region of the voltage Vds in which the capacitance Cgd is relatively small is an inclusive range of 20% to 40% of the voltage BV. When the capacitance Cgd has a relatively small value in the inclusive range of 20% to 40% of the voltage BV, the voltage Vds rises suddenly and the turn-off loss can be reduced. This effect has been confirmed by simulation. The capacitance value C 2  is set to be an arbitrary value taken by the capacitance Cgd in a range where the voltage Vds is 20% to 40% of the voltage BV. In the voltage region, the capacitance Cgd does not change significantly, and exhibits a substantially constant value. The maximum value of the capacitance Cgd in an inclusive range of 20% to 40% of the voltage BV is a sufficiently small value compared to the capacitance value C 1 . The capacitance value C 2  corresponds to a second capacitance value. 
     The following describes the ratio C 1 /C 2  of the capacitance value C 1  to the capacitance value C 2 . 
       FIG.  4    illustrates that the ratio C 1 /C 2  for the solid line is larger than the ratio C 1 /C 2  for the broken line. In a structure in which the surge voltages ΔV are equal, the voltage Vds rises suddenly at the time of the turn-off as the ratio C 1 /C 2  increases. That is, the turn-off loss can be reduced as the ratio C 1 /C 2  increases. This effect is remarkable based on the simulation result illustrated in  FIG.  5   . It is possible to effectively reduce the turn-off loss Eoff, when the ratio C 1 /C 2  is two or larger in particular. A change in the turn-off loss with respect to a change in the ratio of C 1 /C 2  is smaller in a situation where the value of the ratio C 1 /C 2  is 2 or larger than in a situation where the value of the ratio C 1 /C 2  is smaller than 2. Accordingly, it may be preferable to set the ratio C 1 /C 2  to 2 or larger. 
     (Structure of Semiconductor Device) 
     The following describes the structure of the semiconductor device  11 , in other words, the semiconductor element  10  having the above characteristics with reference to  FIGS.  6  to  8   .  FIG.  6    is a cross-sectional view of a semiconductor device  11  according to the present embodiment.  FIG.  6    illustrates a portion of a cell region.  FIG.  7    illustrates a turn-off operation and a change in the capacitance Cgd.  FIG.  8    illustrates the relationship between the voltage BV and a width of a JFET portion.  FIG.  8    illustrates the results of a test element group (TEG), in other words, the results of actual measurements. 
     Hereinafter, a thickness direction of the semiconductor element  10 , in other words, a semiconductor substrate is defined as a Z direction. An extending direction of a trench that is orthogonal to the Z direction is referred to as a Y direction. A direction orthogonal to both the Z direction and the Y direction is defined as an X direction. A plan view in the Z direction is simply referred to as a plan view unless otherwise specified. The length of the JFET portion in the Z direction and a direction orthogonal to the extending direction of the JFET portion is referred to as a width. In the present embodiment, an n-type corresponds to a first conductivity type, and a p-type corresponds to a second conductivity type. 
     As shown in  FIG.  6   , the semiconductor device  11  includes the semiconductor element  10 . The semiconductor element  10  is formed with the MOSFET  13  having a vertical structure on a semiconductor substrate  20  made of SiC as the material. The vertical structure is a structure in which the main current flows in the Z direction as the thickness direction of the semiconductor substrate  20 . The MOSFET  13  is formed at the cell region of the semiconductor substrate  20 . The cell region is also referred to as a main region or an active region. The semiconductor substrate  20  is also referred to as a semiconductor chip. 
     The semiconductor substrate  20  includes a SiC substrate  21 , a low-concentration layer  22 , a junction-gate field-effect transistor (JFET) portion  23 , a deep layer  24 , a current dispersion layer  25 , a coupling layer  26 , a base region  27 , a source region  28 , and a base contact region  29 . The semiconductor substrate  20  is formed by forming semiconductor layers and semiconductor regions made of SiC on the SiC substrate  21  by, for example, epitaxial growth or ion implantation of impurity. The semiconductor layer and the semiconductor region are sometimes referred to as a diffusion layer. The semiconductor substrate  20  includes a main surface  20   a  and a rear surface  20   b  on a side opposite from the main surface  20   a  in the Z direction. 
     The SiC substrate  21  forms the main surface  20   a  of the semiconductor substrate  20 . The SiC substrate  21  is an n-type (n+) substrate made of SiC as the material, and has a higher impurity concentration than the low-concentration layer  22 , the JFET portion  23  and the current dispersion layer  25 . The SiC substrate  21  functions as the drain region of the MOSFET  13 . 
     The low-concentration layer  22  is formed on a surface on a side opposite from the main surface  20   a  at the SiC substrate  21 . The low-concentration layer  22  is an n-type (n−) semiconductor layer made of SiC as the material, and has a lower impurity concentration than the SiC substrate  21 , the JFET portion  23  and the current dispersion layer  25 . 
     The JFET portion  23  is formed on a surface of the low-concentration layer  22  on a side opposite from another surface of the low-concentration layer  22  near the SiC substrate  21 . The JFET portion  23  is an n-type (n) semiconductor layer that is made of SiC as the material, and that has a lower impurity concentration than the SiC substrate  21  and has a higher impurity concentration than the low-concentration layer  22 . The JFET portion  23  extends in a direction orthogonal to the Z direction, and has a stripe shape, in other words, a linear shape in the plan view. 
     In the present embodiment, the JFET portion  23  extends in a direction identical to the extending direction of a trench  30 , in other words, the Y direction. The JFET portion has a width narrower than the low-concentration layer  22  and the current dispersion layer  25  in a direction (the X direction) orthogonal to the extending direction. Therefore, the Y direction corresponds to a first direction. The X direction corresponds to a second direction. 
     The deep layer  24 , as similar to the JFET portion  23 , is formed on a surface of the low-concentration layer  22  on a side opposite from another surface of the low-concentration layer  22  near the SiC substrate  21 . The deep layer  24  is a p-type (p) semiconductor layer made of SiC as the material. The deep layer  24  extends in the same direction as the JFET portion  23 . The deep layer  24  is formed on both sides of the JFET portion  23  in a direction orthogonal to the extending direction of the JFET portion  23  and the Z direction. The deep layer  24  is formed around the JFET portion  23 , and has a stripe shape. 
     In the present embodiment, the deep layer  24  extends in the Y direction. The deep layer  24  sandwiches the JFET portion  23  in the X direction. The respective widths of the sections of the deep layer  24  on both sides of the JFET portion  23  are larger than the width of the JFET portion  23 . 
     The current dispersion layer  25  is a layer that causes the current flowing through the channel region as described later to be distributed in the X direction, in other words, a width direction. The current dispersion layer  25  is formed on a surface of each of the JFET portion  23  and the deep layer  24  on a side opposite from another surface of each of the JFET portion  23  and the deep layer  24  near the low-concentration layer  22 . The current dispersion layer  25  is an n-type (n) semiconductor layer that is made of SiC as the material, and that has a lower impurity concentration than the SiC substrate  21  and has a higher impurity concentration than the low-concentration layer  22 . 
     The current dispersion layer  25  is formed directly above a section of the deep layer  24  located on both sides of the JFET portion  23  to a location directly above the other section of the deep layer  24  across the JFET portion  23 . The width of the current dispersion layer  25  is larger than the width of the JFET portion  23 . The current dispersion layer  25  continues to the SiC substrate  21  through the JFET portion  23  and the low-concentration layer  22 . The current dispersion layer  25  functions as a drift layer together with the low-concentration layer  22  and the JFET portion  23 . 
     The coupling layer  26  is formed on a surface of the deep layer  24  on a side opposite from another surface of the deep layer  24  near the low-concentration layer  22 . The coupling layer  26  is a p-type (p) semiconductor layer made of SiC as the material. The coupling layer  26  is formed on both sides of the current dispersion layer  25  in the X direction. The width of the coupling layer  26  is smaller than the width of the deep layer  24 . 
     The base region  27  is formed on a surface of each of the current dispersion layer  25  and the coupling layer  26  on a side opposite from another surface of each of the current dispersion layer  25  and the coupling layer  26  near the deep layer  24 . The base region  27  is a p-type semiconductor layer made of SiC as the material. The base region  27  is continuous with the deep layer  24  through the coupling layer  26 . 
     The source region  28  is formed on a surface of the base region  27  on a side opposite from another surface of the baser region  27  near the current dispersion layer  25 . The source region  28  is an n-type (n+) semiconductor region made of SiC as the material, and has a higher impurity concentration than the low-concentration layer  22 , the JFET portion  23  and the current dispersion layer  25 . The source region  28  is formed at a position overlapping the current dispersion layer  25  in the plan view. 
     A base contact region  29  is formed on a surface of the base region  27  on a side opposite to another surface of the base region  27  near the current dispersion layer  25 . The base contact region  29  is a p-type (p+) semiconductor region made of SiC as the material, and has a higher impurity concentration than the base region  27 . The base contact region  29  is formed at a position overlapping the coupling layer  26  in the plan view. Each of the base contact region  29 , the base region  27 , the coupling layer  26  and the deep layer  24  corresponds to a second-conductivity-type region. 
     A trench  30  is formed at the semiconductor substrate  20  constructed as described above. The trench  30  is formed with a predetermined depth from the rear surface  20   b  side. The trench  30  penetrates the source region  28  and the base region  27  and reaches the current dispersion layer  25 . Each of the base region  27  and the source region  28  is formed to be in contact with the side surface of the trench  30 . The trench  30  extends in the Y direction. Although not shown, multiple trenches  30  are formed at the semiconductor substrate  20 . Multiple trenches  30  are arranged at regular intervals in the X direction, and form a stripe shape in the plan view. 
     A gate insulating film  31  is formed on the wall surface of the trench  30 . A gate electrode  32  is formed at the surface of the gate insulating film  31  to embed the trench  30 . The gate electrode  32  penetrates the source region  28  and the base region  27  and reaches the current dispersion layer  25 . 
     A source electrode  33  is formed on the rear surface  20   b  of the semiconductor substrate  20 . The source electrode  33  is formed at the cell region. The source electrode  33  is electrically connected to the source region  28  and the base contact region  29 . The source electrode  33  is electrically separated from the gate electrode  32  through an interlayer insulating film  34 . A pad (not shown) being a signal electrode is also formed on the rear surface  20   b  of the semiconductor substrate  20 . The pad includes, for example, a pad for the gate electrode  32 . 
     A drain electrode  35  is formed on the main surface  20   a  of the semiconductor substrate  20 . The drain electrode  35  is formed on almost the entire surface of the main surface  20   a . The drain electrode  35  is electrically connected to the SiC substrate  21 . 
     An n-channel MOSFET with a vertical structure is formed at the semiconductor substrate  20 . A cell region is formed on the semiconductor substrate  20  by placing the MOSFET cells. An outer peripheral breakdown-voltage portion (not shown) such as a guard ring is formed at the semiconductor substrate  20  to surround the cell region in the plan view. 
     The semiconductor element  10 , in other words, the semiconductor device  11  according to the present embodiment executes a turn-on operation by applying a gate voltage of, for example, 20 V in a state where a drain voltage is set to, for example, 1 to 1.5 V. By applying the gate voltage, a channel region is formed at a base region  27  in contact with the gate insulating film  31  and the current flows between the drain electrode  35  and the source electrode  33  in the MOSFET. 
     The semiconductor element  10  executes a turn-off operation by applying the gate voltage smaller than or equal to a threshold voltage to the gate electrode  32 . In the present embodiment, the JFET portion  23  has a structure with a narrow width sandwiched by the deep layer  24 , the JFET portion  23  functions as a depletion adjustment layer, and the deep layer  24  functions as a potential adjustment layer at the time of the turn-off. 
     The following describes the turn-off operation and a change in the capacitance Cgd.  FIG.  7    illustrates that the voltage Vds is divided into three sections. A second voltage section VS 2  has a higher voltage than a first voltage section VS 1 , and a third voltage section VS 3  has a higher voltage than a second voltage section VS 2 . 
     In the first voltage section VS 1 , the depletion layer does not spread to the diffusion layer in a state of Vds=0V, in other words, a full-on state. Therefore, the capacitance Cgd has a capacitance value identical to the capacitance value of the gate insulating film  31 , and has a large capacitance value. 
     When the turn-off begins and then the voltage Vds rises, the depletion layer spreads between the current dispersion layer  25  and the base region  27 . Since the capacitance of the depletion layer is added in series to the capacitance of the gate insulating film  31 , the value of the capacitance Cgd suddenly decreases. 
     In the second voltage section VS 2 , the voltage Vds further rises with respect to the first voltage section VS 1 . When the voltage Vds rises to a predetermined value being 20% or less of the voltage BV, for example, a value slightly fewer than 20% of the voltage BV, the depletion layer extending to the JFET portion  23  from the deep layer  24  extends to be longer than the width of the JFET portion  23 , and the JFET portion  23  is pinched off. The voltage Vds applied to the gate insulating film  31  is shielded by the depletion layer extending to the JFET portion  23 . Since the JFET portion  23  with a narrow width is sandwiched by the deep layer  24 , it is possible to enhance the shield effect. With the shield effect, even when the voltage Vds rises in the second voltage section VS 2 , the value of the capacitance Cgd remains low. 
     In the third voltage section VS 3 , the voltage Vds further rises with respect to the second voltage section VS 2 . When the voltage Vds rises to the inflection point voltage Vip, the deep layer  24  is completely depleted and the shield effect is lost. As a result, the potential of the gate insulating film  31  rises, and the capacitance Cgd suddenly increases. By setting the inflection point voltage Vip to be smaller than or equal to 80% of the voltage BV, it is possible to effectively inhibit the surge voltage ΔV at the time of the turn-off as described above. 
     The following describes the width of the JFET portion  23 . In the following mathematical expression, a variable x denotes the voltage BV (V), and y denotes the width (μm) of the JFET portion  23 . 
     As a result of actual measurement, when the inflection point voltage Vip is 80% of the voltage BV (V), it is obvious that the relationship between the voltage BV and the width of the JFET portion  23  can be approximated by the mathematical expression of y=591.53x −0.997 . 
     By setting the width of the JFET portion  23  to be larger than or equal to the relation of 591.53×BV −0.997,  it is possible to set the inflection point voltage Vip to be smaller than or equal to 80% of the voltage BV. In other words, the capacitance Cgd at the time of the voltage Vds being 80% of the voltage BV can be set to a large value, in other words, the capacitance value C 1 . 
     In addition, when the inflection point voltage Vip is 40% of the voltage BV (V), it is obvious that the relationship between the voltage BV and the width of the JFET portion  23  can be approximated by the mathematical expression of y=278.52x −0.767 . 
     By setting the width of the JFET portion  23  to be smaller than or equal to the relation of 278.52×BV −0.767 , it is possible to set the inflection point voltage Vip to be smaller than or equal to 40% of the voltage BV. In other words, the capacitance Cgd at the time of the voltage Vds being 20% to 40% of the voltage BV can be set to a small value, in other words, the capacitance value C 2 . 
     The semiconductor device  11 , in other words, the semiconductor element  10  according to the present embodiment can reduce the turn-off loss, in other words, the switching loss while reducing the surge voltage ΔV with the above element configuration with a simple structure. 
     Summary of First Embodiment 
     According to the semiconductor device  11  in the present embodiment, the semiconductor element  10 , in other words, the MOSFET  13  has a feature in which the value of the capacitance Cgd changes according to the voltage Vds. In particular, the value C 1  of the capacitance Cgd at the time of the voltage Vds being 80% of the voltage BV is larger than an arbitrary value C 2  of the capacitance Cgd at the time of the voltage Vds being in a range of 20% to 40% of the voltage BV. In other words, the inflection point voltage Vip at which the capacitance Cgd suddenly changes is in an inclusive range of 40% to 80% of the voltage BV. 
     In a region where the voltage Vds is large, the value of the capacitance Cgd becomes larger. Therefore, the switching speed di/dt becomes smaller. Therefore, the surge voltage ΔV can be reduced. In a region where the voltage Vds is small, the value of the capacitance Cgd becomes smaller. Therefore, the rising waveform of the voltage Vds becomes steeper. Therefore, the turn-off loss can be reduced. Even though a capacitance element or a switching element for capacitance adjustment is not externally provided for the semiconductor element  10 , the value of the capacitance Cgd at the time of turn-off changes due to the element characteristics. As described above, it is possible to reduce the turn-off loss while reducing the surge voltage with a simple structure. 
     The capacitance value C 1  may be any value as long as it is greater than the capacitance value C 2 . In other words, the ratio C 1 /C 2  should be at least greater than one. In the present embodiment, the ratio C 1 /C 2  is two or larger. It is possible to effectively reduce the turn-off loss as described above, when the ratio C 1 /C 2  is two or larger in particular. 
     In the present embodiment, the semiconductor element  10  has the JFET portion  23  with a narrow width sandwiched by the deep layer  24 , in other words, the second-conductivity-type region. The width of the JFET portion  23  is set to be larger than or equal to 591.53×BV −0.997 . As a result, the deep layer  24  is completely depleted and the capacitance Cgd suddenly increases in a situation where the value of the voltage Vds is smaller than or equal to 80% of the voltage BV. Accordingly, the value C 1  of the capacitance Cgd at the time of the voltage Vds being 80% of the voltage BV can be set to a large value. In other words, it is possible to reduce the surge voltage ΔV at the time of the turn-off. 
     The inflection point voltage Vip decreases as the width of the JFET portion  23  increases. The width of the JFET portion  23  is set to be smaller than or equal to 278.52×BV −0.767 . As a result, the deep layer  24  is completely depleted and the capacitance Cgd suddenly increases in a situation where the value of the voltage Vds is larger than or equal to 40% of the voltage BV. Accordingly, with the shield effect, the arbitrary value C 2  of the capacitance Cgd can be set to a value smaller than the capacitance value C 1 , in a situation where the voltage Vds is 20% to 40% of the voltage BV. In other words, the turn-off loss can be reduced. 
     The material for the semiconductor substrate  20  is not particularly limited. For example, Si may be used. As described above, SiC is adopted in the present embodiment. A wide bandgap semiconductor such as SiC has a dielectric breakdown electric field strength about an extra digit higher than a silicon semiconductor. As a result, the breakdown voltage and the amount of carriers can be increased by about an extra digit compared to the silicon semiconductor. Since an increase in the amount of the carriers lowers a resistance (on-resistance) value during conduction, it is possible to acquire the semiconductor device with smaller conduction loss and higher breakdown voltage. 
     In a high voltage region where the silicon semiconductor cannot be adopted, it is possible to adopt the wide bandgap semiconductor having a unipolar device structure with high-speed switching. Although the high-speed switching reduces the switching loss, the surge voltage is easily generated. Therefore, by applying the structure described in the present embodiment, it is possible to further enhance the effect. 
     (Modification) 
     Although the above describes the JFET portion  23  extends in the same direction as the trench  30 , in other words, the gate electrode  32 , it is not limited to this example. As in the modification illustrated in  FIG.  9   , the JFET portion  23  extends in a direction orthogonal to the extending direction of the trench  30 .  FIG.  9    illustrates that the trench  30  extends in the Y direction, and the JFET portion  23  extends in the X direction. Even with such a structure, it is possible to reduce the turn-off loss while reducing the surge voltage with a simple structure to acquire the effect identical to the above. 
     Second Embodiment 
     A second embodiment is a modification of the preceding embodiment as a basic configuration and may incorporate description of the preceding embodiment. In the preceding embodiment, the power converter  4  includes the inverter  6  as the power conversion device. Alternatively, the converter with a function of raising voltage may also be further added. 
       FIG.  10    illustrates a circuitry structure of the power converter  4  according to the present embodiment.  FIG.  10    illustrates the motor generator  3  in a simplified form for simplicity. The power converter  4  further includes a converter  15  as a power conversion device and a filter capacitor  17 , in addition to the smoothing capacitor  5  and the inverter  6 . 
     The converter  15  is a DC-DC converter that converts a direct current voltage into another direct current voltage with a different value. The converter  15  is disposed between the direct current power supply  2  and the smoothing capacitor  5 . The converter  15  is a booster circuit having at least a function of raising the direct current voltage supplied from at least the direct current power supply  2 . Further, the converter  15  may have a drop function of charging the direct current power supply  2  with use of the electric charges of the smoothing capacitor  5 . 
     In the present embodiment, the inverter  6  has a structure identical to the preceding embodiment. Each of the upper-lower arm circuits  9  for the corresponding phase included in the inverter  6  has two semiconductor devices  11 , in other words, two semiconductor elements  10 . The P line  7  includes a VH line  7 H and a VL line  7 L. The upper-lower arm circuit  9  included in the inverter  6  is connected between the VH line  7 H and the N line  8 . The converter  15  includes a reactor  16  and the upper-lower arm circuit  9  in which two semiconductor devices  11 , in other words, two semiconductor elements  10  are connected in series, as similar to the inverter  6 . 
     The converter  15  as described above is capable of increasing or decreasing a voltage. The upper-lower arm circuit  9  in the converter  15  is connected between the VH line  7 H and the N line  8 . An end of the reactor  16  is connected to the VL line  7 L, and the other end of the reactor  16  is connected to a connection node between two semiconductor elements  10  in the upper-lower arm circuit  9 . 
     The filter capacitor  17  is connected between the VL line  7 L and the N line  8 . The filter capacitor  17  is connected to the direct current power supply  2  in parallel. For example, the filter capacitor  17  removes, for example, a power supply noise from the direct current power supply  2 . Since the filter capacitor  17  is arranged on the lower voltage side as compared with the smoothing capacitor  5 , the filter capacitor  17  is also referred to as a lower voltage capacitor. 
     Summary of Second Embodiment 
     The power converter  4  according to the present embodiment can enhance an input voltage of the inverter  6  through a boosting operation of the converter  15 , when the motor generator  3  is required to generate a large output. The power converter  4  stops the boosting operation of the converter  15 , when the motor generator  3  is required to generate a small output. By stopping the operation of the converter at the time of having frequent low output, the system as a whole can enhance fuel efficiency while coping with large output. 
     In the present embodiment, the semiconductor device  11 , in other words, the semiconductor element  10  with the structure described in the preceding embodiment is adopted for the upper-lower arm circuit  9  in the inverter  6 . For example, when the input voltage of the inverter  6  is 600 V, as illustrated in  FIG.  11   , the voltage Vds rises at the time of turn-off. When the voltage Vds reaches the inflection point voltage Vip, the capacitance Cgd increases to the capacitance value C 1  from the capacitance value C 2 . This capacitance change reduces the switching speed di/dt. Therefore, the surge voltage ΔV can be reduced, as similar to the preceding embodiment. 
     For example, when the input voltage of the inverter  6  is 300 V, as illustrated in  FIG.  12   , even though the voltage Vds rises at the time of turn-off, the capacitance Cgd does not suddenly increases since the voltage Vds does not reach the inflection point voltage Vip. As a result, the switching speed di/dt can be kept high. Accordingly, it is possible to reduce the turn-off loss in a state where a low input voltage occurs frequently during driving. 
     As described above, by applying the semiconductor element  10 , in other words, the semiconductor device  11  having the characteristics described in the preceding embodiment to the inverter  6  of the power converter  4  having the function of raising voltage, it is possible to further enhance fuel efficiency while coping with large output.  FIG.  11    illustrates 600 V as a high input voltage. However, it is not limited to this example.  FIG.  12    illustrates 300 V as a low input voltage. However, it is not limited to this example. The output voltage of the converter  15  is not limited to 300 V and 600 V. The output voltage of the converter  15  is switchable in multiple levels. 
     The structure of the converter  15  is not limited to the above example. For example, in a situation of having only the function of raising voltage, the diode element may be adopted at the upper arm side of the upper-lower arm circuit  9 . Although the above describes a single-phase converter, a multi-phase converter may be adopted. 
     The upper-lower arm circuit  9  of the converter  15  may include a semiconductor element, in other words, a switching element different from the semiconductor element  10 , in other words, the MOSFET  13  described in the preceding embodiment. In the present embodiment, the upper-lower arm circuit  9  of the converter  15  has the semiconductor element  10 , in other words, the semiconductor device  11  described in the preceding embodiment. Also in the semiconductor element  10  of the converter  15 , the capacitance Cgd changes according to the voltage Vds. In addition to the above effect, even with the converter  15 , it is possible to reduce the turn-off loss while reducing the surge voltage with a simple structure. 
     Other Embodiments 
     The disclosure in this specification, the drawings, and the like is not limited to the exemplified embodiments. The disclosure encompasses the illustrated embodiments and modifications by those skilled in the art based thereon. For example, the disclosure is not limited to the combinations of components and/or elements shown in the embodiments. The disclosure may be implemented in various combinations. The disclosure may have additional portions that may be added to the embodiments. The disclosure encompasses omission of components and/or elements of the embodiments. The disclosure encompasses the replacement or combination of components and/or elements between one embodiment and another. The disclosed technical scopes are not limited to the description of the embodiments. It should be understood that a part of disclosed technical scopes are indicated by claims, and the present disclosure further includes modifications within an equivalent scope of the present disclosure. 
     The disclosure in the specification, drawings and the like is not limited by the description of the present disclosure. The disclosures in the specification, the drawings, and the like encompass the technical ideas described in the claims, and further extend to a wider variety of technical ideas than those in the present disclosure. Therefore, various technical ideas can be extracted from the disclosure of the specification, the drawings and the like without being limited to the description of the present disclosure. 
     Spatial relative terms “inside”, “outside”, “rear”, “bottom”, “low”, “top”, “high”, and the like are used herein to facilitate the description that describes relationships between one element or feature and another element or feature. Spatial relative terms can be intended to include different orientations of a device in use or operation, in addition to the orientations illustrated in the drawings. For example, when a device in a drawing is turned over, elements described as “below” or “directly below” other elements or features are oriented “above” the other elements or features. Therefore, the term “below” can include both above and below. The device may be oriented in another direction (rotated 90 degrees or in any other direction) and the spatially relative terms used herein are interpreted accordingly. 
     The vehicle drive system  1  is not limited to the above structure described in the embodiments. Although the present disclosure describes that one motor generator  3  is provided, it is not limited to the example described in the present disclosure. Multiple motor generators may be provided. Although the above description mentions that the power converter  4  includes the inverter  6  as the power conversion device. For example, a structure with multiple inverters may also be adopted. Only the converter  15  may also be provided. 
     Although the above describes that the semiconductor device  11 , in other words, the semiconductor element  10  is adopted in the inverter  6  or the converter  15 , it is not limited to this example. It can be applied to at least one arm of the vertical circuit included in the power conversion device, in other words, a power conversion circuit. 
     The above describes that the semiconductor device  11  includes only one semiconductor element  10 . However, it is not limited to this example. Multiple semiconductor elements  10  in the vertical circuit for one phase may be provided. Multiple semiconductor elements  10  in the vertical circuit for multiple phases may be provided.