Patent ID: 8466495

Claim:
A field effect transistor comprising: a channel layer composed of In y Ga 1-y N (0≦y≦1) in which two-dimensional electron gas is to be formed; a carrier supply layer composed of Al x Ga 1-x N (0≦x≦1), said carrier supply layer being provided over said channel layer and including at least one p-type layer; and a source electrode, a drain electrode and a gate electrode which are disposed facing said channel layer through said p-type layer, and provided over said carrier supply layer, wherein: the following expression is satisfied: x a <x 1 , where x a denotes an Al compositional ratio of said carrier supply layer at an interface with the gate electrode, and x 1 denotes an Al compositional ratio at an interface with said channel layer; and the following expression is satisfied: [Mathematical Formula 3] 5.6×10 11 x 1 <N A ×η×t [cm −2 ]+5.6×10 13 ( x 1 −x a )<5.6×10 13 x 1 , where x a denotes the Al compositional ratio, t denotes a thickness of said p-type layer, N A denotes an impurity concentration, and η denotes an activation ratio.