Patent ID: 6946691

Claim:
A field effect transistor, comprising: a semi-insulating semiconductor substrate; a semiconductor layer structure provided on the semiconductor substrate, said semiconductor layer structure including a non-doped first compound semiconductor layer, an n-type second compound semiconductor layer having an impurity doping density ranging from 1×10 17 cm −3 to 1×10 18 cm −3 and a layer thickness ranging from 1 nm to 10 nm and having an energy level at a lower end of a conduction band thereof, which is less than or equal to an energy level at a lower end of a conduction band of the first compound semiconductor layer, and an n-type third compound semiconductor layer having an energy level at a lower end of a conduction band thereof, which is less than the energy level at the lower end of the conduction band of the second compound semiconductor layer, said non-doped first compound semiconductor layer, said n-type second compound semiconductor layer and said n-type third compound semiconductor layer being sequentially disposed on the semiconductor substrate from the semiconductor substrate side; and a gate electrode disposed on the semiconductor layer structure, and a source electrode and a drain electrode, said source electrode and said drain electrode being opposite to each other with the gate electrode interposed therebetween and being disposed over the semiconductor layer structure.