Surface passivation of AlN/GaN MOS-HEMTs using ultra-thin Al2O3 formed by thermal oxidation of evaporated aluminium

A simple method is reported for the fabrication of AlN/GaN MOS-HEMTs. Ultra-thin Al^sub 2^O^sub 3^, which is formed using thermal oxidation of evaporated Al, was used for surface passivation and as a gate dielectric. Prior to the formation of Al^sub 2^O^sub 3^, Al protects the very sensitive AlN epilayer from exposure to processing liquid chemicals. Fabricated two-finger AlN/GaN MOS-HEMTs with 3 μm gate length and 200 μm gate width showed good gate control of drain currents up to a gate bias of 3 V and achieved a maximum drain current, I^sub DSmax^, of ~900 mA/mm. The peak extrinsic transconductance, G^sub max^, of the device is ~100 mS/mm at V^sub DS^ = 8 V. Capacitance-voltage characteristics of Al^sub 2^O^sub 3^/AlN/GaN circular test MOS structures were observed and measured. They exhibited no hysteresis, indicating good quality of thermally grown Al^sub 2^O^sub 3^ for realising AlN/GaN MOS-HEMTs in high power and high frequency applications.