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.