Surface passivation of AlN/GaN MOS-HEMTs using ultra-thin Al ₂ O ₃ formed by thermal oxidation of evaporated aluminium

A simple method is reported for fabrication of AlN/GaN MOS-HEMTs. Ultra-thin Al2O3, which is formed using thermal oxidation of evaporated Al, was used for surface passivation and as a gate dielectric. Prior to formation of Al2O3, the 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, IDSmax, of ∼900 mA/mm. The peak extrinsic transconductance, Gmax, of the device is ∼100 mS/mm at VDS = 8 V. Capacitance-voltage (C-V) characteristics of Al2O3/AlN/GaN circular test MOS structures were observed and measured. They exhibited no hysteresis, indicating the good quality of the thermally grown Al2O3 for realising AlN/GaN MOS-HEMTs for high power and high frequency applications.