High electron mobility transistor (HEMT) structure with refractory gate metal

An indium phosphide (InP) high electron mobility transistor (HEMT) structure having improved long-term performance, the HEMT structure comprising:

• a semi-insulating substrate of indium phosphide (InP);
• an epitaxial structure formed on the substrate and comprising a semiconductor channel layer of indium gallium arsenide (InGaAs) and an overlying semiconductor barrier layer of indium aluminum arsenide (InAlAs);
• a metal gate structure extending into the barrier layer and forming a Schottky junction with the barrier layer, wherein an electrical voltage applied to the metal gate structure controls current flow in the channel layer, and wherein the metal gate structure comprises:
• a first layer of metal in contact with the barrier layer, the first layer of metal being a refractory metal,
• a second layer of metal overlying the first layer, the second layer of metal being titanium (Ti),
• a third layer of metal overlying the second layer, the third layer of metal being platinum (Pt), and
• a fourth layer of metal overlying the third layer, the fourth layer of metal being gold (Au);
• and wherein the first layer of the metal gate structure mitigates long-term degradation of the Schottky junction and improves long-term reliability of the HEMT structure.