Patent Publication Number: US-10784338-B2

Title: Field effect transistor devices with buried well protection regions

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 15/372,713, filed Dec. 8, 2016, entitled “FIELD EFFECT TRANSISTOR DEVICES WITH BURIED WELL PROTECTION REGIONS,” which is a divisional of U.S. patent application Ser. No. 14/857,306, filed Sep. 17, 2015, entitled “FIELD EFFECT TRANSISTOR DEVICES WITH BURIED WELL PROTECTION REGIONS,” which is a divisional of U.S. patent application Ser. No. 13/799,142, filed Mar. 13, 2013, entitled “FIELD EFFECT TRANSISTOR DEVICES WITH BURIED WELL PROTECTION REGIONS,” which are assigned to the assignee of the present application. The present application is also related to U.S. application Ser. No. 13/798,919, filed Mar. 13, 2013, entitled “FIELD EFFECT TRANSISTOR DEVICES WITH REGROWN LAYERS,” U.S. application Ser. No. 13/799,049, filed Mar. 13, 2013, entitled “FIELD EFFECT TRANSISTOR DEVICES WITH PROTECTIVE REGIONS,” and U.S. application Ser. No. 13/799,316, filed Mar. 13, 2013, entitled “FIELD EFFECT TRANSISTOR DEVICES WITH BURIED WELL REGIONS AND EPITAXIAL LAYERS.” The disclosures of each of the foregoing applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electronic devices and fabrication methods. More particularly, the present invention relates to high power insulated gate field effect and bipolar transistors and fabrication methods. 
     BACKGROUND 
     Power semiconductor devices are widely used to regulate large current, high voltage, and/or high frequency signals. Modern power electronic devices are generally fabricated from monocrystalline silicon semiconductor material. One widely used power device is the power Metal-Oxide Semiconductor (MOS) Field Effect Transistor (MOSFET). In a power MOSFET, a control signal is supplied to a gate electrode that is separated from the semiconductor surface by an intervening silicon dioxide insulator. Current conduction occurs via transport of majority carriers, without the presence of minority carrier injection that is used in bipolar transistor operation. 
     MOSFETs can be formed on a silicon carbide (SiC) layer. Silicon carbide (SiC) has a combination of electrical and physical properties that make it attractive as a semiconductor material for high temperature, high voltage, high frequency and/or high power electronic circuits. These properties include a 3.2 eV energy gap, about a 2.4 MV/cm electric breakdownfield, a 4.9 W/cm-K thermal conductivity, and a 2.0×10 7  cm/s electron drift velocity. 
     Consequently, these properties may allow silicon carbide-based power electronic devices to operate at higher junction temperatures, higher power density levels, higher frequencies (e.g., radio, S band, X band), and/or with lower specific on-resistance and/or higher blocking voltages than silicon-based power electronic devices. A power MOSFET fabricated in silicon carbide is described in U.S. Pat. No. 5,506,421 to Palmour entitled “Power MOSFET in Silicon Carbide” and assigned to the assignee of the present invention. 
     Although silicon carbide itself is theoretically capable of sustaining high reverse voltages, it may be desirable to shield certain portions or features of a silicon carbide device, such as the gate insulator, the device edge, etc., from high electric fields, as breakdown may be more likely to occur at these locations. 
     SUMMARY 
     A method of forming a transistor device according to some embodiments includes providing a drift layer having a first conductivity type, forming a first region in the drift layer, the first region having a second conductivity type that is opposite the first conductivity type, forming a body layer having the second conductivity type on the drift layer including the first region, forming a source layer on the body layer, the source layer having the first conductivity type, forming a trench in the source layer and the body layer above the first region and extending into the first region, forming a gate insulator on the inner sidewall of the trench, and forming a gate contact on the gate insulator. 
     Forming the first region may include selectively implanting dopants into the drift layer. 
     Selectively implanting dopant atoms into the drift layer may include selectively implanting dopants at an implant energy of less than about 1000 keV. 
     The body layer and the source layer may be formed by epitaxial regrowth. 
     The body layer may be formed by epitaxial regrowth and the source layer may be formed by ion implantation. 
     The method may further include doping an upper portion of the drift layer adjacent the trench with first conductivity type dopants more heavily than a lower portion of the drift layer to form a current spreading region in the upper portion of the drift layer. 
     The first region may be shallower than the current spreading region. 
     The method may further include forming a second region in the drift layer, the second region having the second conductivity type and being spaced apart from the first region, and forming a body contact region having the second conductivity type, the body contact region extending through the source layer and the body layer and into the second region. 
     A distance between the second region and a bottom corner of the trench may be about 0.1 microns to about 2 microns. 
     A method of forming a transistor device according to further embodiments includes providing a drift layer having a first conductivity type, forming a body layer on the drift layer, the body layer having a second conductivity type that is opposite the first conductivity type, forming a source layer on the body layer, the source layer having the first conductivity type, forming a trench in the source layer and the body layer, the trench extending into the drift layer and having an inner sidewall and a floor, sequentially forming a channel layer and a second layer on the inner sidewall and floor of the trench, the channel layer having the first conductivity type and the second layer having the second conductivity type, thermally oxidizing portions of the second layer on the inner sidewall of the trench, removing the oxidized portions of the second layer to leave an un-oxidized portion of the second layer on the floor of the trench, forming a gate insulator on the sidewall of the trench over the channel layer and on the floor of the trench over the un-oxidized portion of the second layer, and forming a gate contact on the gate insulator. 
     Forming the second layer may include forming the second layer to have a thickness of about 0.5 to 5 microns and a doping concentration of about 1×10 17  cm −3  to about 5×10 18  cm −3 . 
     The method may further include doping an upper portion of the drift layer adjacent the trench with first conductivity type dopants more heavily than a lower portion of the drift layer to form a current spreading region in the upper portion of the drift layer. 
     The trench may extend into but not through the current spreading region. 
     A transistor device according to some embodiments includes a drift layer having a first conductivity type, a body layer on the drift layer, the body layer having a second conductivity type opposite the first conductivity type, a source region on the body layer, the source region having the first conductivity type, a trench extending through the source region and the body layer and into the drift layer, the trench having an inner sidewall and a floor, a channel layer having the first conductivity type on the inner sidewall and floor of the trench, a second conductivity type region on the floor of the trench, the channel layer is between the second conductivity type region and the drift layer, a gate insulator on the channel layer and the second conductivity type region at the bottom of the trench, and a gate contact on the gate insulator. 
     An upper portion of the drift layer adjacent the trench may be doped with first conductivity type dopants more heavily than a lower portion of the drift layer to form a current spreading region in the upper portion of the drift layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate certain embodiment(s) of the invention. In the drawings: 
         FIG. 1  is a cross sectional illustration of a cell of a conventional power MOSFET device. 
         FIG. 2  is a cross sectional illustration of a cell of a power MOSFET device according to some embodiments. 
         FIGS. 3A to 3D  are cross sectional views illustrating the fabrication of a power MOSFET device according to some embodiments. 
         FIG. 4  is a cross sectional illustration of a cell of a power MOSFET device according to further embodiments. 
         FIGS. 5A to 5D  are cross sectional views illustrating the fabrication of a power MOSFET device according to further embodiments. 
         FIG. 6  is a cross sectional illustration of a cell of a power MOSFET device according to further embodiments. 
         FIGS. 7A to 7D  are cross sectional views illustrating the fabrication of a power MOSFET device according to further embodiments. 
         FIGS. 8 and 9  are cross sectional views of insulated gate bipolar transistor devices according to some embodiments. 
         FIG. 10  is a cross sectional illustration of a cell of a power MOSFET according to further embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     Some embodiments of the invention provide silicon carbide (SiC) insulated gate devices that are suitable for high power and/or high temperature applications. 
     Embodiments of the present invention provide trench UMOS structures that include highly doped well regions in the drift layer that protect the bottom corners of the UMOS trench from high electric fields in reverse blocking conditions. Some embodiments further include current spreading layers at an upper surface of the drift layer that may reduce spreading resistance caused by the presence of the highly doped well regions. 
     In conventional n-channel vertical-trench MOSFET structures, when the device is operated in a reverse blocking condition, the lower corners of the well may be subjected to high electric fields due to electric field crowding at the corners of the trench. These electric fields may result in breakdown of the gate insulator at the lower corners of the trench, which degrades device performance and may affect long term reliability of the devices. 
     In contrast, some embodiments of the present invention provide structures that protect the lower corners of the trench against high electric fields while maintaining low on-resistance. 
     A unit cell  10  of a conventional MOSFET structure is shown in  FIG. 1 . The device  10  includes an n-type epitaxial drift layer  14  on an n+ 4H—SiC substrate  12 . 
     The structure further includes a p-type body region  16  on the drift layer  14  and an n+ source region  24  on the p-body region  16 . The structure  10  further includes p+ body contact regions  15  that extend through the n+ source region  24  and into the p-well region  18 . 
     A trench  70  extends through the n+ source region  24  and the p-body region  16  and into the n-type drift layer  14 . A gate insulator  32  is formed on sidewall surfaces and bottom surfaces of the trench  70 . A doped polysilicon gate conductor  34  is on the gate insulator  32 . 
     Source ohmic contacts  43  are formed on the body contact regions  15  as well as on the source region  24 . A drain contact  40  is on the substrate  12  opposite the drift layer  14 . 
     As noted above, in a conventional MOSFET structure, when the device is reversed biased, electric field crowding may lead to high electric fields at the lower corners  70 A,  70 B of the trench  70 . 
     A typical device mesa where both n+ and p+ metal contacts are made to the source region  24  and the body contact regions  15  is in the range of several microns wide. Conventionally, the body contact regions  15  may be formed to extend into the drift layer  14 , and may provide some protection to the lower corners of the trench  70 . However, due to the thickness of the source region  24  and the p-body region  16 , it is difficult to control the distance between the body contact regions  15  and the bottom corners of the trench  70  so that the distance is small enough to provide electrical shielding but not so small as to undesirably increase the on-resistance of the device. Moreover, because high energy ion implantation is required to form the body contact regions  15  deep enough to protect the corners of the trench  70 , lateral implantation straggle may cause the body contact regions  15  to extend close enough to the trench  70  to affect the MOS threshold voltage. 
     A unit cell  100  of a MOSFET structure according to some embodiments is shown in  FIG. 2 . 
     Referring to  FIG. 2 , the device  100  includes an epitaxial drift layer  14  having a first conductivity type on a substrate  12 . The substrate  12  may be a 2° to 8° off-axis 4H—SiC substrate, although other substrates or materials may be used. The drift layer  14  may have a thickness of about 5 μm to about 200 μm, and may be doped with n-type dopants at a doping concentration of about 5×10 13  cm −3  to about 12×10 16  cm −3 . Other doping concentrations/voltage blocking ranges are also possible. The epitaxial layers of the structure may be silicon carbide or other materials. 
     In the embodiments illustrated in  FIG. 2 , the drift layer  14  of the device  100  includes a first sub-layer  14 A at the lower part of the drift layer  14  adjacent the substrate  12  and a second sub-layer  14 B at the upper part of the drift layer  14  opposite the substrate  12 . 
     The second sub-layer  14 B has a higher doping concentration than the first sub-layer  14 A. In particular, the first sub-layer  14 A may have a doping concentration of about 1×10 16  cm −3  to 1×10 17  cm −3 , while the second sub-layer  14 A may have a doping concentration of about 5×10 13  cm −3  to 2×10 16  cm −3 . 
     The second sub-layer  14 B may have a thickness that is greater than the depth of the buried well region  80  described below. That is, the buried well region  80  may extend into the second sub-layer  14 B but may not extend all the way to the first sub-layer  14 A. In some embodiments, the second sub-layer  14 B may have a thickness that is greater than about 1 μm. 
     The second sub-layer  14 B may help to reduce the spreading resistance of the device. 
     The device  100  further includes a second conductivity type body region  16  and a first conductivity type source region  24  that may be formed by selective implantation of opposite conductivity type dopants, respectively, into the drift layer  14 . Alternatively, one or both of the body region  16  and the source region  24  may be formed by epitaxial growth on the drift layer  14 . The source region  24  has the same conductivity type as the drift layer (i.e., the first conductivity type), while the body region  16  has the second conductivity type. 
     The body region  16  may have a doping concentration from about 1×10 15  cm −3  to about 5×10 18  cm −3  and may have a thickness of about 0.1 μm to 10 μm. In particular embodiments, the body region  16  may have a doping concentration of about 2×10 18  cm −3  and may have a thickness of about 0.5 μm to 2 μm 
     A trench  70  extends through the source region  24  and the body region  16  and into the drift layer  14 . 
     The device  100  further includes body contact regions  18  that extend through the source region  24  and the body region  16 . The body contact regions  18  have the second conductivity type. 
     The heavily doped source layer  24  can be epitaxially grown or formed by ion implantation into the body layer  16 . The source layer  24  may have a thickness of 0.1 μm to 1 μm and may have a doping concentration in the range of 1×10 18  cm −3  to 1×10 21  cm −3 . 
     Source ohmic contacts  43  are formed on exposed portions of the first body contact regions  18  as well as the source region  24 . The source ohmic contacts  43  may include, for example, Ni, Al, Ti, Si, etc. 
     A gate insulator  32  is formed on sidewall surfaces and bottom surfaces of the trench  70 . The gate insulator  32  may, for example, be silicon oxide. 
     A gate conductor  34  is on the gate insulator  32 . The gate conductor  34  may, for example, include doped polysilicon. The gate conductor  34  may also extend up over the gate insulator  32  and over portions of the source region  24 . 
     The device  100  further includes a highly doped buried well region  80  having the second conductivity type at an upper surface of the drift layer  14  beneath the trench  70 . The buried well region  80  may be a floating region in the drift layer, and may contact a bottom surface of the gate insulator  32 . 
     In some embodiments, the buried well region  80  may be formed by selective ion implantation into the drift layer  14  before formation of the body region  16  by epitaxial regrowth. Accordingly, the buried well region  80  may be formed using low energy implantation, which may reduce implant straggle and may provide more precise control over the location of the buried well region  80  and, more particularly, more precise control over the distance between the buried well regions  20  and the lower corners  70 A,  70 B of the trench  70 , so that the buried well region  80  may not substantially increase the on-resistance of the device. 
       FIGS. 3A to 3D  are cross sectional views illustrating the fabrication of a power MOSFET device according to some embodiments. 
     Referring to  FIG. 3A , a substrate  12  is provided. The substrate  12  may be an off-axis SiC substrate having the 2H, 4H, 6H, 3C or 15R polytype. In particular embodiments, the substrate  12  may be an n+40 off-axis 4H SiC substrate. 
     A drift layer  14  is formed on the substrate  12 . The drift layer  14  may have a thickness of about 5 μm to 200 μm, and may be doped with n-type dopants, such as nitrogen or phosphorus, at a doping concentration of about 5×10 13  cm −3  to about 2×10 16  cm −3 . Other doping concentrations/voltage blocking ranges are also possible. In particular embodiments, the substrate may include a 4° off-axis 4H—SiC substrate and the drift layer may have a thickness of about 10 μm to 15 μm and may be doped with dopants at a doping concentration of about 6×10 15  cm −3  to about 1×10 16  cm −3 . 
     The drift layer  14  includes a first sub-layer  14 A at the lower part of the drift layer  14  adjacent the substrate  12  and a second sub-layer  14 B at the upper part of the drift layer  14  opposite the substrate  12 . 
     The second sub-layer  14 B has a higher doping concentration than the first sub-layer  14 A. In particular, the second sub-layer  14 B may have a doping concentration of about 1×10 16  cm −3  to 1×10 17  cm −3 , while the first sub-layer  14 A may have a doping concentration of about 5×10 13  cm −3  to 2×10 16  cm −3 . 
     Next, a buried well region  80  may be formed in the drift layer  14  by selective implantation of p-type dopant ions  13 , such as aluminum ions, into the drift layer  14 . The p-type dopant ions may be implanted to have a uniform or non-uniform doping profile, such as a graded doping profile. The p-type dopant ions may be implanted at an implant energy of 10 to 500 keV and a dose of 1×10 14  to 1×10 16  cm −2 , such that the buried well region  80  may have a doping concentration of about 1E18 cm −3  to about 1E21 cm −3  and may extend a depth of about 0.1 μm to 1 μm into the drift layer  14 . 
     Referring to  FIG. 3B , a body region  16  is formed on the drift layer  14 . The body region  16  may be doped with p-type dopants at a doping concentration of about 2×10 18  cm −3 , and may have a thickness of about 0.5 μm to about 2 μm. The body region  16  may be formed by ion implantation and/or epitaxial growth, on the drift layer  14 . 
     A source region  24  is formed on the body region  16 . The source region  24  may be doped with n-type dopants at a doping concentration of about 1×10 18  cm −3  to about 1×10 21  cm −3 , and may have a thickness of about 0.1 μm to about 1 μm. The source region  24  may be formed by ion implantation and/or epitaxial growth. The doping profile in the source region  24  may be uniform in some embodiments. In other embodiments, the doping profile in the source region  24  may have a gradient and/or a delta doping profile. 
     Referring to  FIG. 3C , body contact regions  18  are formed by ion implantation into the structure on a side of the epitaxial region opposite the substrate  12 . The body contact regions  18  may be formed to extend through the source region  24  and into the body region  16 . The body contact regions  18  may be formed using a multiple implant profile including implants having a dose of 1×10 15  to 5×10 15  cm −2  and an implant energy of from 10 to 1000 keV, resulting in a doping concentration of about 1×10 17  to about 5×10 18  cm −3 . The body contact regions  18  may have a width of about 0.5 μm to 5 μm. 
     The implanted dopants may be activated by annealing the structure at a temperature of about 1600° C. with a silicon over pressure and/or covered by an encapsulation layer such as a graphite film. A high temperature anneal may damage the surface of the silicon carbide epitaxy without these conditions. The silicon overpressure may be provided by the presence of silane, or the close proximity of silicon carbide coated objects that provide a certain amount of silicon overpressure. Alternatively or in combination with silicon overpressure, a graphite coating may be formed on the surface of the device. Prior to annealing the device to activate the implanted ions, a graphite coating may be applied to the top/front side of the structure in order to protect the surface of the structure during the anneal. The graphite coating may be applied by a conventional resist coating method and may have a thickness of about 1 μm. The graphite coating may be heated to form a crystalline coating on the drift layer  14 . The implanted ions may be activated by a thermal anneal that may be performed, for example, in an inert gas at a temperature of about 1600° C. or greater. In particular the thermal anneal may be performed at a temperature of about 1600° C. in argon for 5 minutes. The graphite coating may help to protect the surface of the drift layer  14  during the high temperature anneal. 
     The graphite coating may then be removed, for example, by ashing and thermal oxidation. 
     Referring to  FIG. 3C , a trench  70  is formed in the structure between the body contact regions  18  and above the buried well region  80 . The trench may be sized so that lower corners  70 A,  70 B of the trench are outside the edges of the respective buried well regions  20 . The trench  70  extends through the source layer  24  and the body layer  16  to the drift layer  14 . A gate insulator  32  is formed in the trench  70  by a gate oxidation process, with a final gate oxide thickness of 400-600 Å. 
     In particular, the gate insulator  32  may be formed using an annealed high temperature or a PECVD deposition process. 
     In some embodiments, an oxide layer may be grown by a dry-wet oxidation process that includes a growth of bulk oxide in dry O 2  followed by an anneal of the bulk oxide in wet O 2  as described, for example, in U.S. Pat. No. 5,972,801, the disclosure of which is incorporated herein by reference in its entirety. As used herein, anneal of oxide in wet O 2  refers to anneal of an oxide in an ambient containing both O 2  and vaporized H 2 O. An anneal may be performed in between the dry oxide growth and the wet oxide growth. The dry O 2  oxide growth may be performed, for example, in a quartz tube at a temperature of up to about 1200° C. in dry O 2  for a time of at least about 2.5 hours. Dry oxide growth is performed to grow the bulk oxide layer to a desired thickness. The temperature of the dry oxide growth may affect the oxide growth rate. For example, higher process temperatures may produce higher oxide growth rates. The maximum growth temperature may be dependent on the system used. 
     In some embodiments, the dry O 2  oxide growth may be performed at a temperature of about 1175° C. in dry O 2  for about 3.5 hours. The resulting oxide layer may be annealed at a temperature of up to about 1200° C. in an inert atmosphere. In particular, the resulting oxide layer may be annealed at a temperature of about 1175° C. in Ar for about 1 hour. The wet O 2  oxide anneal may be performed at a temperature of about 950° C. or less for a time of at least about 1 hour. The temperature of the wet O 2  anneal may be limited to discourage further thermal oxide growth at the SiC/SiO 2  interface, which may introduce additional interface states. In particular, the wet O 2  anneal may be performed in wet O 2  at a temperature of about 950° C. for about 3 hours. The resulting gate oxide layer may have a thickness of about 500 Å. 
     In some embodiments, the dry O 2  oxide growth may be performed at a temperature of about 1175° C. in dry O 2  for about 4 hours. The resulting oxide layer may be annealed at a temperature of up to about 1175° C. in an inert atmosphere. In particular, the resulting oxide layer may be annealed at a temperature of about 1175° C. in Ar for about a time duration ranging from 30 min to 2 hours. Then the oxide layer receives an anneal in NO ambient at a temperature ranging from 1175° C. to 1300 C, for a duration ranging from 30 minutes to 3 hours. The resulting gate oxide layer may have a thickness of about 500 Å. 
     Referring to  FIG. 3D , after formation of the gate insulator  32 , a polysilicon gate  34  may be deposited in the trench  70  and doped, for example, with boron. 
     Ni may be deposited as the n-type source ohmic contact  43  and the drain ohmic contact  40 . The contacts may be sintered in a Rapid Thermal Annealer (RTA). 
       FIG. 4  is a cross sectional illustration of a unit cell  200  of a MOSFET structure according to some further embodiments. The structure of the device  200  is similar to the structure of the device  100 , except that highly doped buried well regions  20  having the second conductivity type are formed at an upper surface of the drift layer  14 . The buried well regions  20  contact the body contact regions  18  and extend towards respective lower corners  70 A,  70 B of the trench  70 . A distance d from a buried well region  20  to a corresponding lower corner  70 A,  70 B of the trench may be about 0.1 μm to about 2 μm. 
     In some embodiments, the buried well regions  20  may be formed by selective ion implantation into the drift layer  14  before formation of the body region  16  by epitaxial regrowth. Accordingly, the buried well regions  20  may be formed using low energy implantation, which may reduce implant straggle and may provide more precise control over the locations of the buried well regions  20  and, more particularly, more precise control over the distance between the buried well regions  20  and the lower corners  70 A,  70 B of the trench  70 . 
     In other embodiments, the body contact regions  18  and the buried well regions  20  may be formed by implanting a first species of dopant ion through into the source layer  24  and the body layer  16  to form the body contact regions  18  and implanting a second species of dopant ion through into the source layer  24  and the body layer  16  and into the drift layer  14  to form the buried well regions  20 , where the second species of dopant has a greater diffusivity in the semiconductor material than the first species of dopant. The structure is then annealed to cause the second species of dopant to diffuse closer to the trench than the first species of dopant. In some embodiments, the semiconductor material may include silicon carbide, the first species of dopant may include aluminum, and the second species of dopant may include boron, which has a higher diffusivity in silicon carbide than aluminum. 
     The buried well regions  20  may have a doping concentration from about 1×10 18  cm −3  to about 1×10 21  cm −3  and may extend a depth of about 0.1 μm to 3 μm into the drift layer  14 . 
       FIGS. 5A to 5D  are cross sectional views illustrating the fabrication of a power MOSFET device according to further embodiments. 
     Referring to  FIG. 5A , a substrate  12  is provided. The substrate  12  may be an off-axis SiC substrate having the 2H, 4H, 6H, 3C or 15R polytype. In particular embodiments, the substrate  12  may be an n+8° off-axis 4H SiC substrate. 
     A drift layer  14  including a first sub-layer  14 A and a second sub-layer  14 B is formed on the substrate  12  in the manner described above. 
     Next, buried well regions  20 ,  80  are formed in the drift layer  14  by selective implantation of p-type dopant ions  13 , such as aluminum ions, into the drift layer  14 . The p-type dopant ions may be implanted to have a uniform or non-uniform doping profile, such as a graded doping profile. The p-type dopant ions may be implanted at an implant energy of 10 to 500 keV and a dose of 1×10 14  cm −2  to 1×10 16  cm −2 , such that the buried well regions may have a doping concentration of about 1×10 18  cm −3  to about 1×10 20  cm −3  and may extend a depth of about 0.1 μm to 1 μm into the drift layer  14 . 
     Referring to  FIG. 5B , a body region  16  is formed on the drift layer  14 . The body region  16  may be doped with p-type dopants at a doping concentration of about 2×10 18  cm −3 , and may have a thickness of about 0.5 μm to about 2 μm. The body region  16  may be formed by ion implantation and/or epitaxial growth, on the drift layer  14 . 
     A source region  24  is formed on the body region  16 . The source region  24  may be doped with n-type dopants at a doping concentration of about 1×10 18  to about 1×10 21  cm −3 , and may have a thickness of about 0.1 μm to about 1 μm. The source region  24  may be formed by ion implantation and/or epitaxial growth. The doping profile in the source region  24  may be uniform in some embodiments. In other embodiments, the doping profile in the source region  24  may have a gradient and/or a delta doping profile. 
     Still referring to  FIG. 5B , body contact regions  18  are formed by ion implantation into the structure on a side of the epitaxial region opposite the substrate  12 . The body contact regions  18  may be formed to extend through the source region  24  and the body region  16  and into the buried well regions  20 . The body contact regions  18  may be formed using a multiple implant profile including implants having a dose of 1×10 15  cm −2  to 1×10 16  cm −2  and an implant energy of from 10 to 1000 keV, resulting in a doping concentration of about 1×10 18  cm −2  to about 5×10 20  cm −3 . The body contact region  18  may have a width of about 0.5 to 5 μm. 
     The implanted dopants may be activated as described above. 
     Referring to  FIG. 5C , a trench  70  is formed in the structure between the body contact regions  18 . The trench may be sized so that lower corners  70 A,  70 B of the trench are spaced about 0.1 μm to 1 μm from the edges of the respective buried well regions  20 . The trench  70  extends through the source layer  24  and the body layer  16  to the drift layer  14 . A gate insulator  32  is formed in the trench  70 . 
     Referring to  FIG. 5D , after formation of the gate insulator  32 , a polysilicon gate  34  may be deposited in the trench  70  and doped, for example, with boron. 
     Ni may be deposited as the n-type source ohmic contact  43  and the drain ohmic contact  40 . The contacts may be sintered by rapid thermal annealing. 
       FIG. 6  illustrates a MOSFET device  300  according to further embodiments. As shown in  FIG. 6 , a doped well region  92  is provided between the gate insulator  32  and a current spreading layer  90  that is formed on sidewalls and the bottom surface of the trench  70 . The current spreading layer  90  may have the first conductivity type, i.e., the conductivity type of the drift layer  14  and the source region  24 . 
       FIGS. 7A to 7D  illustrate fabrication of the device  300  of  FIG. 6 . Referring to  FIG. 7A , a trench  70  is formed through a source region  24 , a body region  16  and into a drift layer  14 . A current spreading layer  90  is formed on the sidewalls and floor of the trench  70  by epitaxial regrowth. The current spreading layer may be doped to a doping concentration of about 1×10 16  cm −3  to 1×10 17  cm −3  with, for example, n-type dopants, and may be grown to a thickness of about 1 μm to 5 μm. 
     A heavily doped p-type layer  86  is then grown by epitaxial regrowth on the current spreading layer  90 . The p-type layer  86  may have a doping concentration of about 1×10 17  cm −3  to 5×10 18  cm −3 , and may be grown to a thickness of about 0.5 μm to 5 μm. 
     Referring to  FIG. 7B , the heavily doped p-type layer  86  is then thermally oxidized at a temperature of 1100° C. to 1300° C. When the trench sidewalls are a-face 4H silicon carbide, the sidewalls have a higher rate of oxidation than the floor of the trench. Accordingly, the heavily doped p-type layer  86  is thermally oxidized until the p-type material on the sidewalls of the trench is completely consumed by the oxide  88 , leaving the current spreading layer  90  intact and a heavily doped p-type region  92  on the bottom of the trench  70 . 
     The oxide is then removed, as shown in  FIG. 7C . 
     Then, as shown in  FIG. 7D , a gate insulator  32  may be formed in the trench so that the p-type region  92  is between the gate insulator  32  and the bottom of the trench  70 . 
     According to these embodiments, a simultaneous (and unwanted) secondary ion implantation into the trench sidewall during p+ implantation at the bottom of the trench  70  can be completely avoided, which may result in a desirable threshold voltage (e.g., 2-3 V) that allows the device to be turned on at a lower gate bias. 
     Insulated gate bipolar transistor (IGBT) device structures  400 ,  500  according to some embodiments are illustrated in  FIGS. 8 and 9 . As shown therein, the IGBT devices  400 ,  500  include n− drift epitaxial layers  214  on p-type epitaxial layers  212 . The p-type epitaxial layers  212  are formed on heavily doped p-type, 4° or 8° off-axis 4H—SiC substrates or layers  210 . The n− drift layers  214  may have a thickness of about 100 m to about 120 μm, and may be doped with n-type dopants at a doping concentration of about 2×10 14  cm −3  to about 6×10 14  cm −3  for a blocking capability of about 10 kV. 
     The devices  400 ,  500  include collector ohmic contacts  243  on collector regions  224  and emitter contacts  245  on the substrate  210 . The remainder of the structures are similar to the structures shown in  FIGS. 4 and 6 , respectively. 
     Some embodiments of the present invention provide trench UMOS structures that include epitaxially regrown channel layers on the sidewalls of the trench, wherein the channel layers have the same conductivity type as the body layers. For example, an n-channel UMOS device according to some embodiments may include a p-type epitaxial channel layer on a sidewall of the UMOS trench. The epitaxially regrown channel layers may improve inversion channel mobility as well as maintain a high threshold voltage. 
     Conventional n-channel vertical-trench MOSFET structures typically include a p-type trench sidewall that is formed by a selective plasma dry etch. However, due to the damage caused to the trench sidewalls by the dry etch process, the crystal structure of the etched surface may be very poor, which can degrade the inversion channel mobility and lead to poor MOS channel conductivity. 
     A conventional approach to overcome this problem in an n-channel device is to form a thin, lightly doped n-type channel layer by epitaxial regrowth on a vertical sidewall of the trench. The n-type channel layer acts as an accumulation layer and provides a current path that connects the n-type source and drain regions of the device. As a result, forward conduction of the device may be greatly improved. However, providing an n-type channel layer on a p-type body region may lower the threshold voltage of the device, which affects the off-state blocking capability and reliability of the device, especially at high temperatures. 
     In contrast, some embodiments of the present invention provide an epitaxial channel layer on a sidewall of a UMOS trench that has the opposite conductivity type from the source/drain regions (i.e. for an n-channel device, the epitaxial channel layer is p-type). The gate insulator and gate are arranged so that both horizontal and vertical inversion layers are formed in the epitaxial channel layer upon application of a gate voltage, which provides a current path between the source and drain regions of the device during forward operation. 
     For example, a unit cell of a MOSFET structure  600  according to some further embodiments is shown in  FIG. 10 . As shown therein, the device  600  includes a channel layer  30  formed on a sidewall of the trench  70 . The channel layer  30  may have the second conductivity type. That is, when the body region  16  is p-type, the channel layer  30  may also be p-type. The channel layer  30  may be formed by epitaxial regrowth, which may allow the channel layer  30  to have a high crystal quality and/or a tightly controlled doping level. 
     A gate insulator  32  is on sidewall surfaces and bottom surfaces of the trench  70 . The gate insulator  32  may extend up over the channel layer  30  and onto the source region  24 . 
     A gate electrode  34  is on the gate insulator  32 . The gate electrode  34  may, for example, include doped polysilicon. The gate conductor  34  may also extend up over the channel layer  34  and onto the source region  24 . 
     When a sufficient voltage is applied to the gate electrode  34 , an inversion channel is formed at both a side surface and an upper surface of the channel layer  30 , allowing charge carriers to flow from the source region  24  through the channel layer  30  and to the drift layer  14  along the path  60 . Forming the channel layer to have the same conductivity type as the body region  16  may increase the threshold voltage by about 1 volt, which can significantly decrease leakage current in the device. 
     The channel layer  30  formed on the sidewall of the trench  30  may provide a SiC layer with a high crystal quality, which may improve the quality of the interface between the channel layer  30  and the gate insulator  32 . This may improve the inversion channel mobility, and may also maintain the threshold voltage high enough for robust off-state performance as well as improving long term reliability and high temperature stability. 
     The channel layer  30  may be formed by epitaxially growing a semiconductor layer having the same conductivity type as the body region  16  on the source layer and on the sidewall and floor of the trench  70 , then anisotropically etching the semiconductor layer to remove the semiconductor layer from the source layer and from the floor of the trench to thereby form the channel layer  30  on the sidewall of the trench adjacent the source layer and the body layer. Formation of an epitaxial channel layer  30  is described in more detail in U.S. application Ser. No. 13/798,919, entitled “FIELD EFFECT TRANSISTOR DEVICES WITH REGROWN LAYERS,” filed concurrently herewith, the disclosure of which is incorporated herein by reference. 
     It will be appreciated that although some embodiments of the invention have been described in connection with silicon carbide IGBT and MOSFET devices having n-type drift layers, the present invention is not limited thereto, and may be embodied in devices having p-type substrates and/or drift layers. Furthermore, the invention may be used in many different types of devices, including but not limited to insulated gate bipolar transistors (IGBTs), MOS controlled thyristors (MCTs), insulated gate commutated thyristors (IGCTs), junction field effect transistors (JFETs), high electron mobility transistors (HEMTs), etc. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “lateral” or “vertical” may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. 
     Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. The thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a discrete change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention. 
     Some embodiments of the invention are described with reference to semiconductor layers and/or regions which are characterized as having a conductivity type such as n-type or p-type, which refers to the majority carrier concentration in the layer and/or region. Thus, n-type material has a majority equilibrium concentration of negatively charged electrons, while p-type material has a majority equilibrium concentration of positively charged holes. Some material may be designated with a “+” or “−” (as in n+, n−, p+, p−, n++, n−−, p++, p−−, or the like), to indicate a relatively larger (“+”) or smaller (“−”) concentration of majority carriers compared to another layer or region. However, such notation does not imply the existence of a particular concentration of majority or minority carriers in a layer or region. 
     In the drawings and specification, there have been disclosed typical embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.