Modulation-doped field-effect transistors and fabrication processes

A method of fabricating a high-power microwave field-effect transistor in Group III nitride compound semiconductors, comprising the steps of:

• providing a layered semiconductor structure which includes:
• a) a first Group III nitride compound semiconductor layer having a first band gap and doped with a charge carrier;
• b) a second Group III nitride compound semiconductor layer having a second band gap that is less than said first band gap and positioned below said first semiconductor layer to generate a two-dimensional electron gas in said second semiconductor layer;
• c) a high-resistivity substrate which supports said first and second semiconductor layers;
• and
• d) source and drain ohmic contacts which communicate with said two-dimensional electron gas;
• with electron-beam lithography processes, forming a metallic gate between said source and drain ohmic contacts;
• after said forming step, dry etching said semiconductor structure to isolate said gate, said source and drain ohmic contacts, said first semiconductor layer and said two-dimensional gas on a mesa with a substantially vertical mesa sidewall;
• tilting a passivation flux sufficiently from a portion of said mesa sidewall to cover an exposed edge of said two-dimensional electron gas with a passivation layer;
• and
• depositing a gate feed which passes over said passivation layer and contacts said gate;
• said gate feed and said exposed edge of said two-dimensional gas thus separated by said passivation layer.