Mechanism of ohmic contact formation in AlGaN/GaN high electron mobility transistors using microwave annealing

The formation of Ti/Al/Ni/Au ohmic contact in AlGaN/GaN high electron mobility transistor (HEMT) by microwave annealing (MWA) has been proposed and studied. In this paper, we investigated the electrical characteristics of this contact structure, as well as its transmission electron microscopy (TEM) images, to analyze the mechanism of MWA for the formation of ohmic contact. Our analysis indicates that the enhanced microwave energy absorption results in enhanced alloy reactions in the contact region. The rapidly changing electromagnetic field generated by the microwave coil is shielded by the thin metal sheet and the eddy current is induced in contact metal pads, which can generate more heat on the metal stacks than other areas. Due to the local heating effect, a direct path for electron flow can be formed between the two-dimensional electron gas (2-DEG) and the metal pad, at a much lower substrate temperature than the conventional thermal annealing process.