An intermediate layer composed of i-AlN is formed between a channel layer and an electron donor layer, a first opening is formed in an electron donor layer, at a position where a gate electrode will be formed later, while using an intermediate layer as an etching stopper, a second opening is formed in the intermediate layer so as to be positionally aligned with the first opening, by wet etching using a hot phosphoric acid solution, and a gate electrode is formed so that the lower portion thereof fill the first and second openings while placing a gate insulating film in between, and so that the head portion thereof projects above the cap structure.