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Semiconductor device structures and methods of fabricating semiconductor devices structures are provided that include a semi-insulating or insulating GaN epitaxial layer on a conductive semiconductor substrate and/or a conductive layer. The semi-insulating or insulating GaN epitaxial layer has a thi

Semiconductor device structures and methods of fabricating semiconductor devices structures are provided that include a semi-insulating or insulating GaN epitaxial layer on a conductive semiconductor substrate and/or a conductive layer. The semi-insulating or insulating GaN epitaxial layer has a thickness of at least about 4 μm. GaN semiconductor device structures and methods of fabricating GaN semiconductor device structures are also provided that include an electrically conductive SiC substrate and an insulating or semi-insulating GaN epitaxial layer on the conductive SiC substrate. The GaN epitaxial layer has a thickness of at least about 4 μm. GaN semiconductor device structures and methods of fabricating GaN semiconductor device structures are also provided that include an electrically conductive GaN substrate, an insulating or semi-insulating GaN epitaxial layer on the conductive GaN substrate, a GaN based semiconductor device on the GaN epitaxial layer and a via hole and corresponding via metal in the via hole that extends through layers of the GaN based semiconductor device and the GaN epitaxial layer.

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1. A semiconductor device structure, comprising: an electrically conductive semiconductor substrate;a semi-insulating or insulating GaN epitaxial layer on the semiconductor substrate, the semi-insulating or insulating GaN epitaxial layer having a thickness of at least about 4 μm; anda conductive buf

1. A semiconductor device structure, comprising: an electrically conductive semiconductor substrate;a semi-insulating or insulating GaN epitaxial layer on the semiconductor substrate, the semi-insulating or insulating GaN epitaxial layer having a thickness of at least about 4 μm; anda conductive buffer layer disposed between the substrate and the semi-insulating or insulating GaN epitaxial layer. 2. The semiconductor device structure of claim 1, wherein the GaN epitaxial layer has a thickness of at least about 8 μm. 3. The semiconductor device structure of claim 1, wherein the GaN epitaxial layer has a thickness of at least about 10 μm. 4. The semiconductor device structure of claim 1, wherein the GaN epitaxial layer has a resistivity of at least 105 Ω-cm. 5. The semiconductor device structure of claim 1, wherein the GaN epitaxial layer has an isolation voltage of at least about 50V. 6. The semiconductor device structure of claim 1, wherein the GaN epitaxial layer has an isolation voltage of at least about 100V. 7. The semiconductor device structure of claim 1, wherein the conductive buffer layer comprises: a first conductive layer of a first conductivity type; anda second conductive layer of a second conductivity type, opposite the first conductivity type, on the first conductive layer and disposed between the first conductive layer and the GaN epitaxial layer. 8. The semiconductor device structure of claim 1, further comprising a GaN based semiconductor device on the GaN epitaxial layer. 9. The semiconductor device structure of claim 8, further comprising a via hole and corresponding via metal in the via hole that extends through layers of the GaN based semiconductor device and the GaN epitaxial layer. 10. The semiconductor device structure of claim 9, wherein the via hole and via metal extend to the substrate and wherein the via metal provides an ohmic contact to the substrate. 11. The semiconductor device structure of claim 10, further comprising a region of higher doping concentration in the substrate beneath the via. 12. The semiconductor device structure of claim 9, wherein the via hole and the via metal extend to the conductive buffer layer and wherein the via metal provides an ohmic contact to the conductive buffer layer. 13. The semiconductor device structure of claim 9, further comprising an etch stop layer disposed between the conductive buffer layer and the GaN epitaxial layer. 14. The semiconductor device structure of claim 9, wherein the conductive buffer layer comprises: a first conductive layer of a first conductivity type; anda second conductive layer of a second conductivity type, opposite the first conductivity type, on the first conductive layer and disposed between the first conductive layer and the GaN epitaxial layer; andwherein the via hole and the via metal extend through the second conductive layer to the first conductive layer. 15. The semiconductor device structure of claim 9, further comprising a two dimensional electron gas structure (2DEG) disposed between the substrate and the GaN epitaxial layer. 16. The semiconductor device structure of claim 15, wherein the via hole and via metal extend through the 2DEG structure and to the substrate and wherein the via metal provides an ohmic contact to the substrate. 17. The semiconductor device structure of claim 1, wherein the GaN epitaxial layer is doped with a deep level transition metal dopant. 18. The semiconductor device structure of claim 17, wherein the GaN epitaxial layer is doped with Fe, Co, Mn, Cr, V and/or Ni. 19. The semiconductor device structure of claim 18, wherein the concentration of the deep level transition metal dopant is at least about 1×1016 cm−3. 20. A GaN semiconductor device structure, comprising: a semiconductor substrate;an insulating or semi-insulating GaN epitaxial layer on the semiconductor substrate, the GaN epitaxial layer having a thickness of at least 4 μm; anda conductive semiconductor layer disposed between the semiconductor substrate and the insulating or semi-insulating GaN epitaxial layer. 21. The semiconductor device structure of claim 20, wherein the GaN epitaxial layer has a thickness of at least about 8 μm. 22. The semiconductor device structure of claim 20, wherein the GaN epitaxial layer has a thickness of at least about 10 μm. 23. The semiconductor device structure of claim 20, wherein the semiconductor substrate comprises an insulating or semi-insulating semiconductor substrate. 24. The semiconductor device structure of claim 23, wherein the substrate comprises silicon carbide and/or sapphire. 25. The semiconductor device structure of claim 23, wherein the substrate comprises diamond. 26. The semiconductor device structure of claim 20, wherein the semiconductor substrate comprises an electrically conductive substrate. 27. The semiconductor device structure of claim 26, wherein the electrically conductive substrate comprises silicon carbide and/or diamond. 28. The semiconductor device structure of claim 20, wherein the conductive semiconductor layer comprises conductive SiC, conductive diamond, SiN and/or a conductive GaN based semiconductor material. 29. The semiconductor device structure of claim 20, further comprising a GaN based semiconductor device on the GaN epitaxial layer. 30. The semiconductor device structure of claim 29, further comprising a via hole and corresponding via metal in the via hole that extends through layers of the GaN based semiconductor device and the GaN epitaxial layer. 31. The semiconductor device structure of claim 30, wherein the via hole and via metal extend to the substrate and wherein the via metal provides an ohmic contact to the substrate. 32. The semiconductor device structure of claim 31, wherein the via hole and the via metal extend to the conductive semiconductor layer and wherein the via metal provides an ohmic contact to the conductive semiconductor layer. 33. The semiconductor device structure of claim 32, wherein the conductive semiconductor layer comprises: a first conductive layer of a first conductivity type; anda second conductive layer of a second conductivity type, opposite the first conductivity type, on the first conductive layer and disposed between the first conductive layer and the GaN epitaxial layer; andwherein the via hole and the via metal extend through the second conductive layer to the first conductive layer. 34. The semiconductor device structure of claim 20, wherein the conductive semiconductor layer comprises: a first conductive layer of a first conductivity type; anda second conductive layer of a second conductivity type, opposite the first conductivity type, on the first conductive layer and disposed between the first conductive layer and the GaN epitaxial layer. 35. The semiconductor device structure of claim 20, further comprising an etch stop layer disposed between the conductive semiconductor layer and the GaN epitaxial layer. 36. A GaN semiconductor device structure, comprising: an electrically conductive SiC substrate;an insulating or semi-insulating GaN epitaxial layer on the conductive SiC substrate, the GaN epitaxial layer having a thickness of at least 4 μm; anda conductive buffer layer disposed between the conductive SiC substrate and the GaN epitaxial layer. 37. The semiconductor device structure of claim 36, wherein the GaN based epitaxial layer has a thickness of at least about 8 μm. 38. The semiconductor device structure of claim 36, wherein the GaN based epitaxial layer has a thickness of at least about 10 μm. 39. The semiconductor device structure of claim 36, wherein the GaN epitaxial layer has a resistivity of at least about 105 Ω-cm. 40. The semiconductor device structure of claim 36, wherein the GaN epitaxial layer has an isolation voltage of at least about 50V. 41. The semiconductor device structure of claim 36, wherein the GaN epitaxial layer has an isolation voltage of at least about 100V. 42. The semiconductor device structure of claim 36, wherein the GaN based epitaxial layer is doped with a deep level transition metal dopant. 43. The semiconductor device structure of claim 42, wherein the GaN epitaxial layer is doped with Fe, Co, Mn, Cr, V and/or Ni. 44. The semiconductor device structure of claim 42, wherein the concentration of the deep level transition metal dopant is at least about 1×1016 cm−3. 45. The semiconductor device structure of claim 36, further comprising an etch stop layer disposed between the conductive buffer layer and the GaN epitaxial layer. 46. The semiconductor device structure of claim 36, wherein the conductive buffer layer comprises an epitaxial SiC layer having a higher doping concentration than the SiC substrate. 47. The semiconductor device structure of claim 36, wherein the conductive buffer layer comprises an implanted SiC layer in the SiC substrate, which has a higher doping concentration than the SiC substrate. 48. The semiconductor device structure of claim 36, wherein the conductive buffer layer comprises: a first conductive layer of a first conductivity type; anda second conductive layer of a second conductivity type, opposite the first conductivity type, on the first conductive layer and disposed between the first conductive layer and the GaN epitaxial layer. 49. The semiconductor device structure of claim 36, further comprising a two dimensional electron gas (2DEG) structure disposed between the conductive substrate and the GaN epitaxial layer. 50. The semiconductor device structure of claim 36, further comprising a GaN based semiconductor device on the GaN epitaxial layer. 51. The semiconductor device structure of claim 50, wherein the GaN based semiconductor device comprises a GaN based high electron mobility transistor on the GaN epitaxial layer. 52. The semiconductor device structure of claim 50, further comprising a via hole and corresponding via metal in the via hole that extends through layers of the GaN based semiconductor device and the GaN epitaxial layer. 53. The semiconductor device structure of claim 52, wherein the via hole and via metal extend through the SiC substrate. 54. The semiconductor device structure of claim 52, wherein the via hole and via metal extend to the substrate and wherein the via metal provides an ohmic contact to the substrate. 55. The semiconductor device structure of claim 54, further comprising a region of higher doping concentration in the substrate beneath the via. 56. The semiconductor device structure of claim 55, further comprising an etch stop layer disposed between the substrate and the GaN epitaxial layer. 57. The semiconductor device structure of claim 50, wherein the via hole and the via metal extend to the conductive buffer layer and wherein the via metal provides an ohmic contact to the conductive buffer layer. 58. The semiconductor device structure of claim 57, further comprising an etch stop layer disposed between the conductive buffer layer and the GaN epitaxial layer. 59. The semiconductor device structure of claim 57, wherein the conductive buffer layer comprises: a first conductive layer of a first conductivity type; anda second conductive layer of a second conductivity type, opposite the first conductivity type, on the first conductive layer and disposed between the first conductive layer and the GaN epitaxial layer; andwherein the via hole and the via metal extend through the second conductive layer to the first conductive layer. 60. The semiconductor device structure of claim 50, further comprising a two dimensional electron gas (2DEG) structure disposed between the substrate and the GaN epitaxial layer and wherein the via hole and the via metal extend to the 2DEG structure and wherein the via metal provides an ohmic contact to the 2DEG structure. 61. The semiconductor device structure of claim 60, further comprising an etch stop layer disposed between the 2DEG structure and the GaN epitaxial layer. 62. The semiconductor device structure of claim 50, further comprising a two dimensional electron gas (2DEG) structure disposed between the substrate and the GaN epitaxial layer and wherein the via hole and the via metal extend through the 2DEG structure and wherein the via metal provides an ohmic contact to the substrate. 63. A GaN semiconductor device structure, comprising: an electrically conductive GaN substrate;an insulating or semi-insulating GaN based epitaxial layer on the conductive GaN substrate;a GaN based semiconductor device on the GaN based epitaxial layer;a conductive buffer layer disposed between the conductive GaN substrate and the GaN epitaxial layer; anda via hole and corresponding via metal in the via hole that extends through layers of the GaN based semiconductor device and the GaN based epitaxial layer. 64. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer has a thickness of at about least 5 μm. 65. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer has a thickness of at about least 10 μm. 66. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer has a resistivity of at least 105 Ω-cm. 67. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer has an isolation voltage of at least about 50V. 68. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer has an isolation voltage of at least about 100V. 69. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer is doped with a deep level transition metal dopant. 70. The semiconductor device structure of claim 69, wherein the GaN based epitaxial layer is doped with Fe, Co, Mn, Cr, V and/or Ni. 71. The semiconductor device structure of claim 69, wherein the concentration of the deep level transition metal dopant is at least about 1×1016 cm −3. 72. The semiconductor device structure of claim 63, wherein the via hole and the via metal extend to the conductive buffer layer and wherein the via metal provides an ohmic contact to the conductive buffer layer. 73. The semiconductor device structure of claim 72, further comprising an etch stop layer disposed between the conductive buffer layer and the GaN based epitaxial layer. 74. The semiconductor device structure of claim 72, wherein the conductive buffer layer comprises: a first conductive layer of a first conductivity type; anda second conductive layer of a second conductivity type, opposite the first conductivity type, on the first conductive layer and disposed between the first conductive layer and the GaN epitaxial layer; andwherein the via hole and the via metal extend through the second conductive layer to the first conductive layer. 75. The semiconductor device structure of claim 72, wherein the conductive buffer layer comprises an epitaxial layer having a higher doping concentration than the GaN substrate. 76. The semiconductor device structure of claim 72, wherein the conductive buffer layer comprises an implanted layer in the GaN substrate and having a higher doping concentration than the GaN substrate. 77. The semiconductor device structure of claim 63, further comprising a two dimensional electron gas (2DEG) structure disposed between the conductive substrate and the GaN based epitaxial layer, wherein the via hole and the via metal extend to the 2DEG structure and wherein the via metal provides an ohmic contact to the 2DEG structure. 78. The semiconductor device structure of claim 63, further comprising a two dimensional electron gas (2DEG) structure disposed between the conductive substrate and the GaN based epitaxial layer, wherein the via hole and the via metal extend through the 2DEG structure and wherein the via metal provides an ohmic contact to the substrate. 79. The semiconductor device structure of claim 63, wherein the GaN based semiconductor device comprises a GaN based high electron mobility transistor on the GaN epitaxial layer. 80. The semiconductor device structure of claim 63, wherein the via hole and via metal extend through the GaN substrate. 81. The semiconductor device structure of claim 63, wherein the via hole and via metal extend to the substrate and wherein the via metal provides an ohmic contact to the substrate. 82. The semiconductor device structure of claim 63, further comprising a region of higher doping concentration in the substrate beneath the via. 83. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer comprises GaN. 84. The semiconductor device structure of claim 63, wherein the GaN based epitaxial layer comprises GaN doped with Fe.