Patent Publication Number: US-2022223681-A1

Title: Vertical mosfet

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/663,808, filed on Jul. 30, 2017, which claims priority to Japanese Patent Application No. 2016-163995 filed in JP on Aug. 24, 2016, the contents of which is incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a vertical MOSFET and a method of manufacturing the vertical MOSFET. 
     2. Related Art 
     It has been known that a trench gate electrode is provided in a compound semiconductor apparatus (for example, refer to Non-Patent Document 1). Also, it has been known that an insulating film is provided below the trench gate electrode (for example, refer to Patent Document 1). 
     PRIOR ART DOCUMENT 
     Non-Patent Document 
     
         
         [Non-Patent Document 1] Tohru Oka et al., “Vertical GaN-based trench metal oxide semiconductor field-effect transistors on a free-standing GaN substrate with blocking voltage of 1.6 kV”, Applied Physics Express, published 28 Jan. 2014, Volume 7, Number 2, 021002 
       
    
     Patent Document 
     
         
         [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2011-512677 
       
    
     In a bottom portion (particularly, a corner portion) of a trench, if a high voltage is applied between a source and a drain of a gate when the gate is turned off, an electric field is concentrated. Accordingly, there has been a problem that a breakdown or degradation of a gate insulating film is generated. In order to handle this problem, it is thought to provide a relatively thick insulating film to the bottom portion of the trench. However, since the avalanche breakdown generated in the insulating film is an irreversible breakdown phenomenon, once the breakdown of the insulating film is generated, it cannot be used for a second time. 
     SUMMARY 
     In a first aspect of the present invention, a vertical MOSFET having a compound semiconductor layer is provided. The vertical MOSFET may include a gate electrode, a gate insulating film, a drift region, and a high resistance region. A gate insulating film may be provided between the gate electrode and the compound semiconductor layer. The drift region may be provided directly in contact with at least a part of the gate insulating film. The drift region may be a part of the compound semiconductor layer. The high resistance region may be provided at least in the drift region. The high resistance region may be positioned below at least a part of the gate insulating film. A resistance value per unit length of the high resistance region may be higher than that of the drift region. The gate electrode may be a trench type gate electrode. The gate electrode may be embedded in a trench portion provided in the compound semiconductor layer. The high resistance region may be adjacent to a bottom portion of the trench portion. The high resistance region may be spaced from the gate insulating film below at least a part of the gate insulating film. A width of the high resistance region may be wider than a width of the trench portion. The high resistance region may have a resistance value per unit length of equal to or more than 10 Ω·cm. The compound semiconductor layer may be a gallium nitride layer. 
     In a second aspect of the present invention, a vertical MOSFET having a compound semiconductor layer is provided. The vertical MOSFET may include a gate electrode, a gate insulating film, a drift region, and a high resistance region. The gate insulating film may be provided between the gate electrode and the compound semiconductor layer. The drift region may be provided directly in contact with at least a part of the gate insulating film. The drift region may be a part of the compound semiconductor layer. The high resistance region may be provided at least in the drift region. The high resistance region may be positioned below at least a part of the gate insulating film. A resistance value per unit length of the high resistance region may be higher than that of the drift region. The gate electrode may be a planar type gate electrode provided above a front surface of the compound semiconductor layer. The high resistance region may be provided between a pair of base regions which are part of the compound semiconductor layer. The drift region may be remained among the pair of base regions and the high resistance region. A width of the high resistance region may be smaller than a width of the pair of base regions. If the compound semiconductor layer is a gallium nitride layer, a resistance-increasing element in the high resistance region may be nitrogen. If the compound semiconductor layer is a silicon carbide layer, the resistance-increasing element in the high resistance region may be silicon. 
     The resistance value per unit length of the high resistance region may be higher than a resistance value per unit length of the base regions which are part of the compound semiconductor layer. 
     The high resistance region may have a resistance-increasing element with a concentration of equal to or more than 1E+16 cm −3  and equal to or less than 1E+19 cm −3 . 
     The resistance-increasing element may be different from an impurity element which forms majority carriers in the drift region. 
     The resistance-increasing element may be an impurity element which is the same as an impurity element forming majority carriers in the base region that is a part of the compound semiconductor layer. 
     The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a cross section of a vertical MOSFET  100  in a first embodiment. 
         FIG. 2  is a flow diagram showing manufacturing steps of the vertical MOSFET  100 . 
         FIG. 3A  is a schematic view showing a step S 10 . 
         FIG. 3B  is a schematic view showing a step S 20 . 
         FIG. 3C  is a schematic view showing a step S 25 . 
         FIG. 3D  is a schematic view showing a step S 30 . 
         FIG. 3E  is a schematic view showing a step S 40 . 
         FIG. 3F  is a schematic view showing a step S 45 . 
         FIG. 3G  is a schematic view showing a step S 50 . 
         FIG. 3H  is a schematic view showing a step S 60 . 
         FIG. 4  is a schematic view showing a cross section of a vertical MOSFET  120  in a first modification example. 
         FIG. 5  is a schematic view showing a cross section of a vertical MOSFET  140  in a second modification example. 
         FIG. 6  is a schematic view showing a cross section of a vertical MOSFET  160  in a second embodiment. 
         FIG. 7  is a schematic view showing a cross section of a vertical MOSFET  170  in a modification example of the second embodiment. 
         FIG. 8  is a schematic view showing a cross section of a vertical MOSFET  200  in a third embodiment. 
         FIG. 9  is a drawing describing a potential distribution when a gate is turned off. 
         FIG. 10  is a flow diagram showing manufacturing steps of the vertical MOSFET  200 . 
         FIG. 11A  is a schematic view showing a step S 12 . 
         FIG. 11B  is a schematic view showing a step S 14 . 
         FIG. 11C  is a schematic view showing a step S 20 . 
         FIG. 11D  is a schematic view showing a step S 25 . 
         FIG. 11E  is a schematic view showing a step S 40 . 
         FIG. 11F  is a schematic view showing a step S 45 . 
         FIG. 11G  is a schematic view showing a step S 50 . 
         FIG. 11H  is a schematic view showing a step S 60 . 
         FIG. 12  is a schematic view showing a cross section of a vertical semiconductor apparatus  400  in a first experiment example. 
         FIG. 13  is a schematic view showing a cross section of a vertical semiconductor apparatus  500  in a second experiment example. 
         FIG. 14  is a drawing showing a voltage-current characteristic in the vertical semiconductor apparatuses  400  and  500 . 
         FIG. 15  is a schematic view showing a cross section of a vertical MOSFET  260  in a fourth embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, some embodiments of the present invention will be described. The embodiments do not limit the invention according to the claims, and all the combinations of the features described in the embodiments are not necessarily essential to means provided by aspects of the invention. 
       FIG. 1  is a schematic view showing a cross section of a vertical MOSFET (Metal Oxide Semiconductor Field Effect Transistor)  100  in a first embodiment.  FIG. 1  is an X-Z cross-sectional view of the vertical MOSFET  100  in an active region of a semiconductor chip.  FIG. 1  may be a unit configuration of the vertical MOSFET  100 . Note that in the present example, an X direction and a Y direction are directions perpendicular to each other, and a Z direction is a direction perpendicular to an X-Y plane. The X direction, the Y direction, and the Z direction form a so-called right-handed system. 
     The vertical MOSFET  100  of the present example includes an n +  type gallium nitride (hereinafter, abbreviated to GaN) substrate  10 , which is a compound semiconductor substrate, and a GaN layer  20  as a compound semiconductor layer. Note that n means that electrons are majority carriers while p means that holes are majority carriers. Regarding “+” or “−” placed on the upper right side of n or p, “+” means that a carrier concentration is higher than those for which “+” is not placed, and “−” means that the carrier concentration is lower than those for which “−” is not placed. 
     The compound semiconductor of the present example is GaN. However, the compound semiconductor in another example may be silicon carbide (hereinafter, abbreviated to SiC). That is, in the other example, the compound semiconductor substrate may be an n +  type SiC substrate, and the compound semiconductor layer may be a SiC layer. The technical contents of the present example where the compound semiconductor is GaN may be applied to the above-described other example where the compound semiconductor is SiC. Explanation will be given on each occasion when there is particularly any different point between the present example and the above-described other example. 
     Ion species of the n type impurities for GaN may be one or more types of elements selected from among Si (silicon), Ge (germanium), and O (oxygen). In the present example, Si is used as the n type impurities. Also, ion species of the p type impurities for GaN may be one or more types of elements selected from among Mg (magnesium), Ca (calcium), Be (beryllium), and Zn (zinc). On the other hand, the ion species of the n type impurities for SiC may be one or more types of elements selected from among N (nitrogen) and P (phosphorus). Also, the ion species of the p type impurities for SiC may be one or more types of elements selected from among B (boron) and Al (aluminum). 
     The GaN substrate  10  of the present example is a free-standing substrate having a threading dislocation density of less than 1E+7 cm −2 . By setting the GaN substrate  10  to have a low dislocation density, the dislocation density of a GaN layer  20  which is epitaxially formed on the GaN substrate  10  may also be decreased. Note that in the present example, the terms “upper” and “above” mean+Z direction, and the term “below” means −Z direction. ±Z directions do not necessarily mean a vertical direction to the ground. The ±Z directions are merely convenient terms to specify a relative positional relation among the substrate, layer, region, film, and the like. 
     In the present example, a leak current of a power device may be reduced compared to a case where the threading dislocation density is equal to or more than 1E+7 cm −2 . Also, since ion-implanted impurities can be prevented from deeply diffusing following a dislocation during annealing, an impurity implantation region can be set to be closer to a design profile. In this way, the power device can be manufactured in a higher non-defective rate. 
     In the present example, a junction surface between the GaN substrate  10  and the GaN layer  20  is referred to as a boundary surface  12 . The boundary surface  12  may be one of principal surfaces in the GaN substrate  10  and the GaN layer  20 . In the present example, another principal surface of the GaN substrate  10  on the opposite side of the boundary surface  12  is referred to as a back surface  16  of the GaN substrate  10 . Also, another principal surface of the GaN layer  20  on the opposite side of the boundary surface  12  is referred to as a front surface  14  of the GaN layer  20 . In the present example, the boundary surface  12 , the front surface  14 , and the back surface  16  are parallel to the X-Y plane. 
     The GaN layer  20  of the present example has an n type drift region  22 , an n +  type source region  24 , a p type base region  26 , a high resistance region  30 , and a trench portion  50 . In the present example, the drift region  22 , the source region  24 , the base region  26 , and the high resistance region  30  are part of the GaN layer  20 . The trench portion  50  of the present example is a concave portion of the GaN layer  20 . The trench portion  50  has a bottom portion  52  mainly in contact with the drift region  22 , and side portions  54  mainly in contact with the base region  26 . 
     The vertical MOSFET  100  has a gate insulating film  42  which is embedded in the trench portion  50 , and a gate electrode  44 . The gate insulating film  42  is provided between the gate electrode  44  and the GaN layer  20 . The gate insulating film  42  of the present example is provided directly in contact with the bottom portion  52  and the side portions  54  of the trench portion  50 . A part of the gate insulating film  42  may also be provided on the front surface  14  of the GaN layer  20 . Note that the gate insulating film  42  provided on the side portions  54  means that the gate insulating film  42  covers the outermost surface of the side portions  54 , and does not mean that the gate insulating film  42  is positioned in the +Z direction of the side portions  54 . 
     The gate electrode  44  of the present example is a trench type. A part of the gate electrode  44  may be positioned above the front surface  14  of the GaN layer  20 . An interlayer insulating film  48  is provided on a top portion of the gate insulating film  42  and a top portion of the gate electrode  44 . The interlayer insulating film  48  electrically separates the gate electrode  44  from the source electrode  64 . 
     The drift region  22  of the present example has a high resistance region  30 . The high resistance region  30  within the drift region  22  may be adjacent to the bottom portion  52  of the trench portion  50  in the Z direction. That is, the high resistance region  30  of the present example is positioned below at least a part of the gate insulating film  42  and is directly in contact with the gate insulating film  42 . The high resistance region  30  of the present example is directly in contact with the gate insulating film  42  provided on the bottom portion  52 . Note that the drift region  22  may be directly in contact with the gate insulating film  42  provided on the bottom portion  52  in the X direction and the Y direction. 
     The source region  24  is provided in contact with the front surface  14  and the side portions  54  of the trench portion  50 . The source region  24  is provided in a part of the base region  26 . The source region  24  may be provided from the front surface  14  to a predetermined depth. At least a part of the source region  24  may be directly in contact with the source electrode  64  in the Z direction. The source region  24  may be a region with low resistance to electrons. The source region  24  of the present example allows access of electrons between the source electrode  64  and the base region  26 . 
     A part of the base region  26  serves as a channel forming region  25 . The channel forming region  25  may be adjacent to the side portion  54  of the trench portion  50  in the X direction. Also, the channel forming region  25  may be positioned between the drift region  22  and the source region  24  in the Z direction. In a case where a voltage equal to or more than a predetermined threshold is applied to the gate electrode  44 , an inverting layer may be formed in the channel forming region  25 . 
     For example, if the predetermined voltage is applied between the source electrode  64  and a drain electrode  74  and a voltage equal to or more than a threshold voltage is applied from the gate terminal  40  to the gate electrode  44  (the gate is turned on), the inverting layer is formed in the channel forming region  25 . Accordingly, currents flow from a drain terminal  70  to a source terminal  60 . Also, if a voltage lower than the threshold voltage is applied to the gate electrode  44  (the gate is turned off), the inverting layer in the channel forming region  25  is eliminated. Accordingly, the currents are interrupted. In this way, the vertical MOSFET  100  may switch on/off the currents between the source terminal  60  and the drain terminal  70 . 
     Although the high resistance region  30  of the present example is a region of the compound semiconductor, the high resistance region  30  is a region having as few free carriers so as to be able to be regarded as an insulator. The high resistance region  30  may have a function to protect the gate insulating film  42  in the bottom portion  52  (including its corner portion) when the gate is turned off. The high resistance region  30  of the present example is provided between the bottom portion  52  and the drift region  22 . The high resistance region  30  of the present example is a region, into which the resistance-increasing element is ion-implanted, of the drift region  22  below the bottom portion  52 . 
     The high resistance region  30  of the present example is not an insulator such as silicon oxide, silicon nitride, or the like, and is a part of the GaN layer  20  on which the ion implantation has been performed. That is, the high resistance region  30  is a part of the compound semiconductor layer. For this reason, the high resistance region  30  is advantageous in the point that even if the avalanche breakdown is generated, the irreversible breakdown like that in the insulator is not generated in the high resistance region  30 . 
     Note that, different from the present example, in order to form a relatively thick insulating film on the bottom portion  52  of the trench portion  50 , a special process, such as reflow and the like after deposition of the insulating film, is required. On the other hand, in the present example, the high resistance region  30  may be formed according to simple processes of ion implantation of the resistance-increasing element and annealing after the ion implantation. 
     The resistance-increasing element of the present example is the ion species to be ion-implanted into the drift region  22  in order to form the high resistance region  30 . The resistance-increasing element may be an impurity element which is the same as an impurity element that forms majority carriers in the base region  26 . By the resistance-increasing element, a part of the drift region  22  may be counter-doped. Also, crystallinity of a part of the drift region  22  may also be broken. Accordingly, the resistance of the region into which the resistance-increasing element is ion-implanted may be increased. In the present example, since the impurity element forming the majority carriers in the base region  26  is Mg, the resistance-increasing element may be Mg. Note that as for the other example where the compound semiconductor is SiC, since the impurity element forming the majority carriers in the base region  26  is Al, the resistance-increasing element may be Al. 
     The resistance-increasing element may be different from the impurity element forming the majority carriers in the drift region  22 . According to the resistance-increasing element, the crystallinity of a part of the drift region  22  may be broken so as to increase the resistance. In the present example, the impurity element forming the majority carriers in the drift region  22  is Si, and the resistance-increasing element is Mg, Ar (argon), or N. Note that in the other example where the compound semiconductor is SiC, the impurity element forming the majority carriers in the drift region  22  is N or P, and the resistance-increasing element is Al, Ar, or Si. Note that the resistance-increasing element may have one or more of the above-described plurality of elements. 
     The high resistance region  30  may have a resistance-increasing element with a concentration of equal to or more than 1E+16 cm −3  and equal to or less than 1E+19 cm −3 . Although the high resistance region  30  of the present example has an impurity concentration within the above-described range, hole carriers enough to be able to be defined as the p type region do not exist in the high resistance region  30  of the present example. In the present example, after the ion implantation of the resistance-increasing element is performed, the GaN layer  20  is annealed at a temperature within a range that the p type impurities are not activated. For this reason, no p type characteristics are exhibited in the high resistance region  30 . Also, although the high resistance region  30  of the present example has the impurities with a concentration within the above-described range, the high resistance region  30  of the present example is not the n type region, either. Since the high resistance region  30  is a region which does not have any specific conductivity type (p type and n type) (even if it is assumed that the high resistance region  30  has a conductivity type, it is of very low concentration), the high resistance region  30  may have the electrical characteristics like the insulator. 
     The high resistance region  30  of the present example has a higher resistance value per unit length than that of the drift region  22 . In the present example, the resistance value per unit length means a linear resistance. A value of the linear resistance of the drift region  22  depends on the withstand voltage class, and is, for example, 1 Ω·cm. On the other hand, the linear resistance of the high resistance region  30  is, for example, equal to or more than 10 Ω·cm. The linear resistance of the high resistance region  30  may be twice or more of the linear resistance of the drift region  22 , or may be 5 times or more, 10 times or more, or 20 times or more of the linear resistance of the drift region  22 . 
     Also, it is desirable that the resistance value per unit length of the high resistance region  30  of the present example is higher than the resistance value per unit length of the base region  26 . The linear resistance of the base region  26  is 3 Ω·cm, for example. The linear resistance of the high resistance region  30  may be twice or more of the linear resistance of the base region  26 , or may be 5 times or more, 10 times or more, or 20 times or more of the linear resistance of the base region  26 . 
       FIG. 2  is a flow diagram showing manufacturing steps of the vertical MOSFET  100 . The manufacturing steps of the present example are performed in an order of S 10  to S 60 . The manufacturing steps of the present example include a step (S 10 ) of epitaxially forming the drift region  22  and the base region  26 , a step (S 20 ) of ion implanting the n type impurities into the base region  26 , a step (S 25 ) of annealing the GaN layer  20  to form the source region  24 , a step (S 30 ) of forming the trench portion  50 , a step (S 40 ) of ion implanting the resistance-increasing element into the drift region  22  from the bottom portion  52  of the trench, a step (S 45 ) of annealing the GaN layer  20  to form the high resistance region  30 , a step (S 50 ) of forming the gate insulating film  42  and the like, and a step (S 60 ) of forming the interlayer insulating film  48  and the like. 
       FIG. 3A  is a schematic view showing the step S 10 . In the step S 10 , the drift region  22  is epitaxially formed on the GaN substrate  10  according to metal organic chemical vapor deposition (MOCVD), halide vapor phase epitaxy (HYPE), or the like. After this, the base region  26  is epitaxially formed on the drift region  22 . 
     The drift region  22  may have an appropriate thickness in accordance with the withstand voltage. For example, the drift region  22  has the thickness of equal to or more than 1 μm and equal to or less than 50 μm in the Z direction. The drift region  22  of the present example is the n type region having the n type impurities of equal to or more than 1.0E+15 cm −3  and equal to or less than 5.0E+16 cm −3 . Note that E represents a power of 10. For example, 1.0E+16 means 1.0×10 16 . The base region  26  may have the thickness of equal to or more than 0.5 μm and equal to or less than 2 μm in the Z direction. The base region  26  is the p type region having the p type impurities of equal to or more than 1.0E+17 cm −3  and equal to or less than 1.0E+18 cm −3 . 
       FIG. 3B  is a schematic view showing the step S 20 . In the step S 20  of the present example, the source region  24  is formed by ion implantation. In the present example, the n type impurities are ion-implanted into the base region  26  via a mask  80 - 1 . In the present example, Si is ion-implanted into the base region  26  by multi-stage implantation with a dosage of equal to or more than 1E+15 cm −2  and equal to or less than 1E+16 cm −2  at acceleration voltages of 10 keV, 20 keV, 40 keV, 70 keV, 110 keV, and 150 keV. A box profile is formed by reducing the dosage as the acceleration voltage is smaller and increasing the dosage as the acceleration voltage is larger. An ion implantation depth of the present example is set to be equal to or more than 0.1 μm and equal to or less than 0.5 μm from the front surface  14 . The source region  24  of the present example is the n +  type region having a Si concentration of equal to or more than 1.0E+18 cm −3  and equal to or less than 1.0E+19 cm −3 . 
     The mask  80 - 1  of the present example has an opening  82 - 1  in a position into which the n type impurities are ion-implanted. The material of the mask  80  may be silicon dioxide (hereinafter, abbreviated to SiO 2 ) which can be selectively removed for GaN. In another example, the material of the mask  80  may be photoresist. The mask  80 - 1  of the present example is selectively removed from the GaN layer  20  after the ion implantation is performed. 
       FIG. 3C  is a schematic view showing the step S 25 . In the step S 25  of the present example, the GaN layer  20  is annealed by using an annealing apparatus  300  at a temperature of equal to or more than 1000° C. and equal to or less than 1200° C. In the present example, the annealing of GaN layer  20  means that the laminated body of the GaN substrate  10  and the GaN layer  20  is annealed at a treatment chamber of the annealing apparatus  300 . According to the annealing, defects generated due to the ion implantation are repaired to some extent and the n type impurities are activated. Note that in another example, the front surface  14  side of the base region  26  may be partially removed by etching to selectively regrow the n +  type source region  24  in the removed region. In a case where the regrowing is to be performed, the step S 25  of annealing may be omitted. 
       FIG. 3D  is a schematic view showing the step S 30 . In the step S 30  of the present example, the GaN layer  20  is partially removed by etching via a mask  80 - 2  to form the trench portion  50 . In the present example, the base region  26  below an opening  82 - 2  of the mask  80 - 2  is completely removed and an upper portion of the drift region  22  is partially removed. 
       FIG. 3E  is a schematic view showing the step S 40 . In the step S 40  of the present example, the resistance-increasing element is ion-implanted into the drift region  22  by continuously using the mask  80 - 2  of the step S 30 . The resistance-increasing element of the present example is Mg. In the present example, Mg is ion-implanted into the drift region  22  by multi-stage implantation with a dosage of equal to or more than 1E+12 cm −2  and equal to or less than 1E+15 cm −2  at acceleration voltages of 30 keV, 80 keV, and 180 keV. The ion implantation depth of the present example is set to be equal to or more than 0.5 μm and equal to or less than 2.0 μm from the bottom portion  52 . 
     In the present example, the region where the ion implantation is performed has Mg with an impurity concentration of equal to or more than 1.0E+16 cm −3  and equal to or less than 1.0E+19 cm −3 . The region may have a higher p type impurity concentration than that of the base region  26 . The region becomes the high resistance region  30  through the annealing process. 
       FIG. 3F  is a schematic view showing the step S 45 . In the step S 45  of the present example, the GaN layer  20  is annealed by using the annealing apparatus  300 . Accordingly, the high resistance region  30  is formed. Note that nitrogen gas (N 2  gas) may be filled in the treatment chamber of the annealing apparatus  300  during annealing. 
     The GaN layer  20  may be annealed at a temperature of equal to or more than 700° C. and equal to or less than 1300° C., and more preferably, at a temperature of equal to or more than 1000° C. and equal to or less than 1300° C. On the other hand, in a case where the compound semiconductor layer is a SiC layer, the SiC layer may be annealed at a temperature of equal to or more than 1000° C. and equal to or less than 1500° C., and more preferably, at a temperature equal to or more than 1300° C. and equal to or less than 1500° C. By annealing at a temperature within the above-described range, the crystallinity broken due to the ion implantation can be repaired to some extent. That is, the state can be returned from the amorphous state to the crystal state to some extent. 
     If the region where the ion implantation has been performed is left in the amorphous state, there is a risk that the characteristics of the vertical MOSFET  100  may be changed due to the heat generated during the operation of the vertical MOSFET  100 . In the present example, the change of the characteristics during the operation can be prevented by repairing the crystallinity to some extent. Accordingly, an operational reliability of the vertical MOSFET  100  can be secured. Of course, if the temperature is within the above-described range, no p type characteristic is exhibited in the p type impurities as the resistance-increasing element ion-implanted into the GaN layer  20  or the SiC layer. 
     Note that, as a comparison example, in a case where the semiconductor layer is Si, after B (the p type impurities) with the impurity concentration of equal to or more than 1.0E+16 cm −3  and equal to or less than 1.0E+19 cm −3  is ion-implanted, as the semiconductor layer is annealed at the temperature which is for repairing the crystallinity, the region where the ion implantation has been performed becomes the p type region. That is, if the semiconductor layer is Si, the high resistance region  30  like the present example cannot be obtained. Also, if the semiconductor layer is Si, the resistance of the p type region becomes lower in proportion to the p type impurity concentration. 
       FIG. 3G  is a schematic view showing the step S 50 . In the step S 50  of the present example, the gate insulating film  42  and the gate electrode  44  are formed. The gate insulating film  42  may have a thickness of 100 nm. The gate insulating film  42  may be a SiO 2  film or may be an aluminum oxide (Al 2 O 3 ) film. The gate electrode  44  may be polysilicon. The gate insulating film  42  and the gate electrode  44  may be patterned in desired shapes by using a known photolithographic technique. 
       FIG. 3H  is a schematic view showing the step S 60 . In the step S 60  of the present example, the interlayer insulating film  48 , the source electrode  64 , and the drain electrode  74  are formed. The interlayer insulating film  48  may be BPSG 
     (Borophosphosilicate glass). The source electrode  64  may be a laminated electrode having a Ti (titanium) layer and an Al layer on the Ti layer. The drain electrode  74  may be a laminated electrode having a Ti layer below the back surface  16  and an Al layer below the Ti layer. 
       FIG. 4  is a schematic view showing a cross section of a vertical MOSFET  120  in a first modification example. The high resistance region  30  of the present example is provided in the drift region  22  across the entire Z direction from the bottom portion  52  to the boundary surface  12 . The present example is different from the first embodiment in the above point. The present example may also receive the advantageous effect in the first embodiment. 
       FIG. 5  is a schematic view showing a cross section of a vertical MOSFET  140  in a second modification example. The high resistance region  30  of the present example is provided from the bottom portion  52  to the inside of the GaN substrate  10  through the boundary surface  12 . The present example is different from the first embodiment in the above point. The present example may also receive the advantageous effect in the first embodiment. 
       FIG. 6  is a schematic view showing a cross section of a vertical MOSFET  160  in a second embodiment. The high resistance region  30  of the present example is spaced from the gate insulating film  42  below at least a part of the gate insulating film  42 . In the present example having the trench portion  50 , the high resistance region  30  is provided separating from the bottom portion  52  below the bottom portion  52 . Note that the position where the high resistance region  30  is to be formed may be controlled by controlling the acceleration voltage during the forming of the high resistance region  30 . The present example is different from the first embodiment in the above point, but is the same as the first embodiment in the other points. 
     In a case where the gate insulating film  42  and the high resistance region  30  are directly in contact with each other, the MOS interface characteristics are vulnerable to damages on the compound semiconductor layer when the resistance-increasing element is ion-implanted into the compound semiconductor layer. For example, the increased accumulation of electric charges on the gate insulating film  42  may result in the degradation and breakdown of the gate insulating film  42  in some cases. For this reason, like the present example, the influence of the damages during the ion implantation can be reduced by providing the high resistance region  30  spaced from the bottom portion  52 . Note that like the first modification example, the high resistance region  30  of the present example may be provided in the drift region  22  to the boundary surface  12  across the entire Z direction. Instead of this, like the second modification example, the high resistance region  30  of the present example may be provided in the drift region  22  across the entire Z direction and to the inside of the GaN substrate  10  through the boundary surface  12 . 
     Note that a width of the high resistance region  30  of the present example is the same as a width of the trench portion  50 . In the present example, the width of the high resistance region  30  is a length in the x direction of the high resistance region  30 . Also, in the present example, the width of the trench portion  50  is a length in the x direction between the side portions  54  of the trench portion  50 . 
       FIG. 7  is a schematic view showing a cross section of a vertical MOSFET  170  in a modification example of the second embodiment. The width of the high resistance region  30  of the present example is wider than the width of the trench portion  50 . Note that the width of the high resistance region  30  may be made to be wider than the width of the trench portion  50  by inclining the direction of the ion implantation relative to the front surface  14 . The present example is different from the second embodiment in the above point, but is the same as the second embodiment in the other points. 
       FIG. 7  is a unit structure of the vertical MOSFET  170 , and the same unit structures are provided in the x direction. If the high resistance region  30  is not connected to the adjacent high resistance region  30  of the unit structure, the high resistance region  30  may be further extended in the x direction. In this way, by making the width of the high resistance region  30  be wider than the width of the trench portion  50 , the withstand voltage of the semiconductor apparatus can be improved compared to a case where the width of the high resistance region  30  is the same as that of the trench portion  50 . 
     Note that in another modification example, the width of the high resistance region  30  may be made narrower than the width of the trench portion  50 . Since the ON resistance can be reduced as the width of the high resistance region  30  is narrower than the width of the trench portion  50 , a turn on loss can be reduced. 
       FIG. 8  is a schematic view showing a cross section of a vertical MOSFET  200  in a third embodiment. Aiming at a brief description, the descriptions for matters in common with the first embodiment are omitted. The vertical MOSFET  200  of the present example does not have the trench portion  50 . The gate electrode  44  of the present example is a planar type and is provided above the front surface  14 . Also, the gate insulating film  42  of the present example is provided between the gate electrode  44  and the front surface  14 . The high resistance region  30  of the present example is positioned between the pair of base regions  26  and is provided adjacent to the front surface  14 . Note that although the pair of base regions  26  are shown in the cross-sectional view of  FIG. 6 , the base regions  26  may be annular regions extending in the Y direction. The channel forming region  25  is positioned within the base region  26  which is just below the gate electrode  44 . 
       FIG. 9  is a drawing describing a potential distribution when the gate is turned off. When the gate is turned off, the bottom portions and the side portions of the base regions  26  and the front surface  14  between the pair of base regions  26  become equipotential (the equipotential lines thereof are shown in bold lines). However, in the vicinity of the front surface  14  below the central portion in the X direction of the gate electrode  44  (in the vicinity of the front surface  14  of the central portion in the X direction of the pair of base regions  26 ), the equipotential lines easily protrude downward due to the influence of the voltage of the drain electrode  74 . Since intervals of the equipotential lines are dense in a region from the equipotential lines protruding downward to the front surface  14 , the region becomes a region (shown as a region A) where the electric field is relatively strong. That is, the breakdown or degradation of the gate insulating film  42  of the present example is easily generated in the region A. 
     Here, in the present example, the high resistance region  30  is provided below the central portion in the X direction of the gate electrode  44 . The high resistance region  30  of the present example is also provided within the drift region  22  by ion implantation. The high resistance region  30  of the present example is provided directly in contact with at least a lower surface of a part of the gate insulating film  42 . The high resistance region  30  may have a prescribed width in the X direction. The high resistance region  30  may have a width of L X /2 in the ±X directions with a center which is the central portion in the X direction of the gate electrode  44 . 
     In order to secure a conduction path of a current I between the source terminal  60  and the drain terminal  70 , it is desirable that the width L X  in the X direction of the high resistance region  30  is smaller than a width L in the X direction between the pair of base regions  26 . In the present example, the width L X  of the high resistance region  30  may be set to a width of equal to or more than 20% of and equal to or less than 80% of the width L between the pair of base regions  26 , or may be set to be a width of equal to or more than 30% of and equal to or less than 70% of the width L between the pair of base regions  26 . 
     If L X  is set wider, since the drift region  22  remained between the base regions  26  becomes narrower, the ON resistance increases according to JFET effect. Note that JFET effect means that a depletion layer extends from adjacent channel forming regions  25  according to the electric field between the source and the drain when the MOSFET is operated, and the path of the current is narrowed. On the other hand, if L X  is set wider, the gate insulating film  42  is further protected. For this reason, L X  may be determined according to the necessity of protection of the gate insulating film  42 . 
       FIG. 10  is a flow diagram showing manufacturing steps of the vertical MOSFET  200 . The manufacturing steps of the present example are performed in an order of S 12  to S 60 . The manufacturing steps of the present example include a step (S 12 ) of epitaxially forming the drift region  22  and removing a part of the drift region  22 , a step (S 14 ) of selectively regrowing the base region  26 , a step (S 20 ) of ion implanting the n type impurities into the base region  26 , a step (S 25 ) of annealing the GaN layer  20  to form the source region  24 , a step (S 40 ) of ion implanting the resistance-increasing element into the drift region  22 , a step (S 45 ) of annealing the GaN layer  20  to form the high resistance region  30 , a step (S 50 ) of forming the gate insulating film  42  and the like, and a step (S 60 ) of forming the interlayer insulating film  48  and the like. 
       FIG. 11A  is a schematic view showing the step S 12 . In the step S 12  of the present example, the n type drift region  22  is epitaxially formed directly in contact with an upper surface of the GaN substrate  10 . After that, in the etching apparatus, a part of the upper portion of the drift region  22  is removed by etching. Accordingly, concave portions  27  are formed in the drift region  22 . 
       FIG. 11B  is a schematic view showing the step S 14 . In the step S 14  of the present example, the p type base region  26  is selectively regrown on the concave portions  27  of the drift region  22 . During the selective regrowing, the base region  26  is epitaxially formed. 
       FIG. 11C  is a schematic view showing the step S 20 . In the step S 20 , similar to the first embodiment, the n type impurities are ion-implanted into the base region  26  by using a mask  80 - 3 . 
       FIG. 11D  is a schematic view showing the step S 25 . In the step S 20 , similar to the first embodiment, the GaN layer  20  is annealed. Accordingly, the n +  type source region  24  is formed. 
       FIG. 11E  is a schematic view showing the step S 40 . In the step S 40 , similar to the first embodiment, the resistance-increasing element is ion-implanted into the base region  26  by using a mask  80 - 4 . In the present example, the ion implantation of the resistance-increasing element is performed so that a box profile is formed from the front surface  14  between the pair of base regions  26  to a depth within a prescribed range. 
       FIG. 11F  is a schematic view showing the step S 45 . In the step S 45 , similar to the first embodiment, the GaN layer  20  is annealed. 
       FIG. 11G  is a schematic view showing the step S 50 . In the step S 50 , the gate insulating film  42  and the gate electrode  44  are formed. 
       FIG. 11H  is a schematic view showing the step S 60 . In the step S 60 , the interlayer insulating film  48  is formed so as to cover the side portion and the upper portion of the gate electrode  44 . Also, similar to the first embodiment, the source electrode  64  and the drain electrode  74  are formed. 
       FIG. 12  is a schematic view showing a cross section of a vertical semiconductor apparatus  400  in a first experiment example. The first experiment example is an experiment for ensuring that the high resistance region  430  serves as an insulator. The vertical semiconductor apparatus  400  has a GaN substrate  410 , a GaN layer  420 , an insulating film  448 , a contact metal  462 , a low-voltage-side electrode  464 , and a high-voltage-side electrode  474 . The GaN layer  420  has a boundary surface  412 , which is between the GaN substrate  410  and the GaN layer  420 , and a front surface  414 . The GaN layer  420  has the drift region  422  and a high resistance region  430 . 
     The drift region  422  is a region of n type GaN, epitaxially formed on the GaN substrate  10 . The thickness in the Z direction of the drift region  422  was set to 11 μm. Also, the n type impurity concentration of the drift region  22  was set to 1.5E+16 cm −3 . The high resistance region  430  was formed by ion implanting Mg into the drift region  422  as the resistance-increasing element followed by the annealing of the GaN layer  420 . The annealing temperature was set to 1300° C. The depth of the high resistance region  430  was set to 0.5 μm and the impurity concentration of Mg was set to 1E+18 cm −3 . 
     An insulating film  448  was provided on the front surface  414  of the GaN layer  420 . The insulating film  448  was set as a SiO 2  film. An opening  449  for contact was provided to the insulating film  448 . The high resistance region  430  was exposed to the entire opening  449  in the X-Y plane. 
     A contact metal  462  was provided on the insulating film  448 . A lower surface of the contact metal  462  was contacted to the high resistance region  430  only through the opening  449 . Ni (nickel) was used in the contact metal  462 . Note that it may be considered that the same result as the first experiment example can be obtained even if the contact metal  462  is made of Pd (palladium) or Pt (platinum). The low-voltage-side electrode  464  was provided on an upper surface of the contact metal  462 . Al was used in the low-voltage-side electrode  464  of the present experiment. Note that by extending the low-voltage-side electrode  464  in the ±X directions than the high resistance region  430 , the extended part of the low-voltage-side electrode  464  was used as a field plate. 
     The high-voltage-side electrode  474  was provided below the back surface  416 , directly in contact with the GaN substrate  410 . Grounding the low-voltage-side electrode  464 , the voltage of the high-voltage-side electrode  474  was increased from 0V to above 1300V. At this moment, the current flowing between the low-voltage-side electrode  464  and the high-voltage-side electrode  474  was measured. 
       FIG. 13  is a schematic view showing a cross section of a vertical semiconductor apparatus  500  in a second experiment example. The vertical semiconductor apparatus  500  does not have the high resistance region  430 . That is, in the vertical semiconductor apparatus  500 , the contact metal  462  and the drift region  422  were Schottky-joined with each other. Also, grounding the low-voltage-side electrode  464 , the voltage of the high-voltage-side electrode  474  was increased from 0V to 1200V. The present example is different from the first experiment example in the above point. 
       FIG. 14  is a drawing showing a voltage-current characteristic in the vertical semiconductor apparatuses  400  and  500 . The horizontal axis indicates the voltage (V) applied to the high-voltage-side electrode  474 . The vertical axis indicates the current density (A/cm −2 ). The current density was calculated by dividing the currents flowing between the low-voltage-side electrode  464  and the high-voltage-side electrode  474  by the area of the opening  449  in the X-Y plane. 
     The result of the first experiment example is shown by solid lines. In the first experiment example, the current density was less than 1.0E-7 when the voltage was within a range of equal to or more than 0V and equal to or less than 100V. The current density showed an increasing tendency as the voltage increased. Even if the voltage was equal to 1000V, the current density was less than 1.0E-6. When the voltage was close to 1200V, the current density rapidly increased. When the voltage exceeded 1300V, the avalanche breakdown was generated, and the current density further increased rapidly. Note that the current density of about 1.0E-7 when the voltage was 0V is larger than an ideally expected current density. It is thought that the reason is because of a point defect and a dislocation defect in the epitaxially formed GaN layer  420  and because that the damage due to the ion implantation when the high resistance region  430  was formed cannot be completely eliminated. 
     The result of the second experiment example is shown by a plurality of plots shown in a diamond shape and dotted lines connecting them. In the second experiment example, the current density was less than 1.0E-6 when the voltage was within a range of equal to or more than 0V and equal to or less than 100V. The current density showed an increasing tendency as the voltage increased. The current density was equal to or more than 1.0E-5 when the voltage was 1000V. The current density of the second experiment example when the voltage was equal to or more than 0V and equal to or less than 1200V was larger than that of the first experiment example by about one or more digits. As expected, since the metal electrode and the n type drift region  422  are Schottky-joined with each other in the vertical semiconductor apparatus  500  of the second experiment example, the current density was higher than that of the vertical semiconductor apparatus  400  of the first experiment example. 
       FIG. 15  is a schematic view showing a cross section of a vertical MOSFET  260  in a fourth embodiment. Similar to the second embodiment, the high resistance region  30  of the present example is spaced from the gate insulating film  42 , below at least a part of the gate insulating film  42 . The present embodiment is different from the third embodiment in the above point, but is the same as the third embodiment in the other points. The present example is also advantageous in the point that the influence of the damage during the ion implantation can be reduced. 
     While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. 
     The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order. 
     EXPLANATION OF REFERENCES 
       10  . . . GaN substrate,  12  . . . boundary surface,  14  . . . front surface,  16  . . . back surface,  20  . . . GaN layer,  22  . . . drift region,  24  . . . source region,  25  . . . channel forming region,  26  . . . base region,  27  . . . concave portion,  30  . . . high resistance region,  40  . . . gate terminal,  42  . . . gate insulating film,  44  . . . gate electrode,  48  . . . interlayer insulating film,  50  . . . trench portion,  52  . . . bottom portion,  54  . . . side portion,  60  . . . source terminal,  64  . . . source electrode,  70  . . . drain terminal,  74  . . . drain electrode,  80  . . . mask,  82  . . . opening,  100 ,  120 ,  140 ,  160 ,  170 ,  200 ,  260  . . . vertical MOSFET,  300  . . . annealing apparatus,  400  . . . vertical semiconductor apparatus,  410  . . . GaN substrate,  412  . . . boundary surface,  414  . . . front surface,  416  . . . back surface,  420  . . . GaN layer,  422  . . . drift region,  430  . . . high resistance region,  448  . . . insulating film,  449  . . . opening,  462  . . . contact metal,  464  . . . low-voltage-side electrode,  474  . . . high-voltage-side electrode,  500  . . . vertical semiconductor apparatus