Patent ID: 7160766

Claim:
A method for making a field-effect semiconductor device comprising the steps of: forming an AlGaN barrier layer over an insulating substrate; forming a GaN layer over the AlGaN barrier layer; forming an AlGaN spacer layer over the GaN layer; forming a Si-doped AlGaN layer over the ALGaN spacer layer; forming an ALGaN cap layer over the Si-containing carrier-supplying layer; wherein the AlGaN barrier layer, the GaN layer, the AlGaN spacer layer, the Si-doped AlGaN layer, and the AlGaN cap layer comprise a gallium nitride-based compound semiconductor layer represented by the formula Al x In y Ga 1−x−y N, wherein x+y=1,0≦x≦1, and 0≦y≦1; forming a multi-layer gate electrode on the gallium nitride-based compound semiconductor layer, the multi-layer gate electrode being formed to include at least one high-melting-point metal layer having a thickness greater than 200 nm; and forming a source electrode and a drain electrode by self-alignment using the gate electrode as a mask.