A compound semiconductor device comprising: a GaN based carrier transit layer formed over a semiconductor substrate; a GaN based carrier supply layer formed on said carrier transit layer; a GaN based protective layer formed on said carrier supply layer; a source electrode, a drain electrode and a gate electrode formed on said protective layer; an AlN layer formed on said protective layer, and positioned between said gate electrode and said source electrode, and between said gate electrode and said drain electrode; and an insulator layer formed on said AlN layer; wherein a GaN based compound semiconductor layer is formed between said AlN layer and said insulator layer, and a silicon layer is formed between said AlN layer and said compound semiconductor layer.