Low-Temperature Ohmic Contact Formation in AlN/GaN HEMT Using Microwave Annealing

In this brief, a low-temperature microwave annealing (MWA) method is demonstrated for the formation of ohmic contact to AlN/GaN high electron mobility transistors (HEMTs) for the first time. Compared with the traditional rapid thermal annealing (RTA) technique, MWA-HEMT can achieve a comparable low ohmic contact resistance with much smoother surface of ohmic contacts. Transmission electron microscopy results show that no direct current path connecting the 2-D electron gas and the metal is formed. Temperature-dependent contact resistance measurement indicates that field emission tunneling dominates the current transport mechanism in ohmic contact formation. The maximum dc output current density of 1.4 A/mm and the peak extrinsic transconductance ( ${G}_{\textsf {m}}$ ) of 270 mS/mm are measured on ${2} \times {8} \,\,{\sf \mu } $ $m^2$ gate MWA-HEMTs. Besides, MWA-HEMTs have higher ${I}_{ \mathrm{ ON}}/{I}_{ \mathrm{ OFF}}$ ratio due to the lower gate leakage current than that of RTA-HEMTs.