Temperature dependences of the (apparent) mobility µas-grown of an as-grown GaN/Al2O3 layer, about 6-µm thick, and the (apparent) mobility µetched of the same layer etched to a thickness of 0.75 µm. Also shown are µD-H, which is the average mobility of the top 5 µm, as determined by differential-Hall measurements, and µB-I, which is the average mobility of the "bulk" region (also approximately the top 5 µm), calculated by correcting µas-grown for the degenerate interface layer.