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