Patent ID: 7468524

Claim:
A field-effect transistor comprising: a first nitride semiconductor layer having a lattice constant a 1 and a band gap Eg 1 ; a second nitride semiconductor layer, stacked on said first nitride semiconductor layer, having a lattice constant a 2 and a band gap Eg 2 ; a source electrode and a drain electrode formed on said second nitride semiconductor layer; a piezoelectric effect film having a thickness of more than 500 nm formed on said second nitride semiconductor layer in at least a partial region between said source electrode and said drain electrode; and a gate electrode formed on at least a partial region of said piezoelectric effect film, wherein said lattice constants a 1 and a 2 are in the relation a 1 >a 2 , said band gaps Eg 1 and Eg 2 are in the relation Eg 1 <Eg 2 , and said piezoelectric film exerts compressive stress on said second nitride semiconductor layer so as not to allow current flow between said source electrode and said drain electrode when no gate voltage is applied, and cancels said compressive stress so as to allow current flow between said source electrode and said drain electrode when the gate voltage is applied.