A field effect transistor (FET) includes a III-Nitride channel layer, a III-Nitride barrier layer on the channel layer, wherein the barrier layer has an energy bandgap greater than the channel layer, a source electrode electrically coupled to one of the III-Nitride layers, a drain electrode electrically coupled to one of the III-Nitride layers, a gate insulator layer stack for electrically insulating a gate electrode from the barrier layer and the channel layer, the gate insulator layer stack including an insulator layer, such as SiN, and an AlN layer, the gate electrode in a region between the source electrode and the drain electrode and in contact with the insulator layer, and wherein the AlN layer is in contact with one of the III-Nitride layers.