Field effect transistor

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 AlXGa1-XN (0≦x≦1), a channel layer made of GaN or InGaN, a first electron supply layer, which is an undoped or n-type AlyGa1-yN layer, a threshold value control layer, which is an undoped AlZGa1-ZN layer, and a second electron supply layer, which is an undoped or n-type AlwGa1-wN 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 0≦X≦Y<1 and 0<Z≦Y≦1.