Patent ID: 7425721

Claim:
A field-effect transistor comprising: a first nitride semiconductor layer having a lattice constant a 1 and a bandgap Eg 1 ; a second nitride semiconductor layer stacked on said first nitride semiconductor layer and having a lattice constant a 2 and a bandgap Eg 2 ; a source electrode and a drain electrode formed on said second nitride semiconductor layer; a piezo-effect film formed on said second nitride semiconductor layer in at least a part of a region between said source electrode and said drain electrode; and a gate electrode formed on a region of at least a part of said piezo-effect film, wherein a relation between said lattice constants a 1 and a 2 is a 1 >a 2 , a relation between said bandgaps Eg 1 and Eg 2 is Eg 1 <Eg 2 , and a residual polarization density at that surface of said piezo-effect film which faces said second nitride semiconductor layer is equal to or higher than a density of electric charges of a two-dimensional electron gas layer at an interface between said first nitride semiconductor layer and said second nitride semiconductor layer, and negative charges are polarized at that surface of said piezo-effect film which faces said second nitride semiconductor layer.