Patent ID: 8872233

Claim:
A semiconductor structure comprising: a barrier layer comprising AIGaInN, AIInN or AIGaN; a spacer structure comprising a first aluminum nitride layer, an intermediate layer and a second aluminum nitride layer, wherein the intermediate layer comprises a group III nitride and is arranged between the first aluminum nitride layer and the second aluminum nitride layer and wherein the intermediate layer comprises a first free charge carrier density at an interface with the second aluminum nitride layer; and a channel layer comprising a group III nitride, wherein the spacer structure is arranged between the barrier layer and the channel layer, wherein the spacer structure is grown on the channel layer and the AIGaInN barrier layer, the AIInN barrier layer or the AIGaN barrier layer is grown on the spacer structure, and wherein the channel layer comprises a second free charge carrier density at an interface with the first aluminum nitride layer of the spacer structure, wherein the first aluminum nitride layer, the intermediate layer and the second aluminum nitride layer comprise layer thicknesses such that the first free charge carrier density is less than 10% of the second free charge carrier density.