Compound semiconductor device having a mesfet that raises the maximum mutual conductance and changes the mutual conductance

A compound semiconductor device comprising:

• a substrate formed of a first compound semiconductor;
• a buffer layer formed on the substrate;
• a graded channel layer formed on the buffer layer, said graded channel layer composed of a second compound semiconductor layer doped with an impurity of which one constituent element of said second compound semiconductor layer has a peak distribution in the inside of said graded channel layer in a thickness direction, thereby an energy band gap of the graded channel layer is made narrower in the inside than at both ends in the thickness direction;
• a barrier layer formed on the graded channel layer;
• a gate electrode formed on the barrier layer to come into Schottky-contact with the barrier layer;
• and
• a source electrode and a drain electrode formed on both sides of the gate electrode to flow a current into the graded channel layer via the barrier layer, wherein
• the first compound semiconductor constituting the substrate is InP, and the second compound semiconductor layer constituting the graded channel layer is an InAsP or a GaAsSb or an InPSb, and one constituent element contained in the second compound semiconductor layer is indium or antimony.