Patent ID: 8618578

Claim:
A field effect transistor comprising: a nitride-based semiconductor multi-layer structure; a source electrode; a drain electrode; a protective film; and a gate electrode, wherein the nitride-based semiconductor multi-layer structure includes at least a base layer made of Al X Ga 1-X N (0â‰¦xâ‰¦1), a channel layer made of GaN or InGaN, a first electron supply layer, which is an undoped or n-type Al y Ga 1-y N layer, a threshold value control layer, which is an undoped Al Z Ga 1-Z N layer, and a second electron supply layer, which is an undoped or n-type Al w Ga 1-w N layer, epitaxially grown in this order over a substrate with a buffer layer interposed therebetween, the source electrode, the drain electrode, and the protective film are provided over a surface of the nitride-based semiconductor multi-layer structure, the gate electrode is provided in a recess structure, which is formed by etching the entire protective film, the entire second electron supply layer, and a portion of the threshold value control layer in a portion of a space between the source electrode and the drain electrode, directly or with a gate insulating film interposed therebetween, and the Al composition of each layer in the nitride-based semiconductor multi-layer structure satisfies 0â‰¦Xâ‰¦Y<1 and 0<Zâ‰¦Yâ‰¦1.