GT Advanced Technologies Inc fka GTAT

[Netsurfer]

Not sure that there is enough bandwidth here for a full explanation. Growing single crystal GaN is a real challenge...high pressures and temperatures. The real world deposits a vapor of GaN onto a substrate or carrier that could be Silicon or Sapphire. The critical parameter is that the orientation of the substrate be such that the GaN will deposit with the correct orientation. If orientation is not maintained the material will not function correctly as a semiconductor.
Silicon is cheap and there is a hugh installed base that can be adapted to GaN deposition. Epitaxy is common with making circuits on Silicon. I believe that Silicon is cut off axis to optimize the crystal lattice. Alumina aka sapphire has a better lattice match that does Silicon but still has its limitations. The match is not perfect and the sapphire and GaN do not move together with changes in temp which results in defects that affect component yield. I do not believe that maximum operating temps can be achieved with Silicon vs. Sapphire so some performance is lost but with big $ savings. The holy grail is single crystal GaN but that is a long ways away for commercially available qtys. There may be prototype material available for military applications.

GT was making 3" and 4" SiC...Silicon Carbide wafers for high temp, high freq applications. Not making active components but selling the growth furnaces to OEM's. Cree is the biggest player in SiC. I will post a Semiconductor road map that I foung on line a year or so ago. You might find it interesting if you follow this arena.