New process for low sheet and ohmic contact resistance of AIN/GaN MOS-HEMTs

This paper reports a novel method for producing low ohmic contact resistance, R C , as well as low sheet resistance, R sh , on AlN/GaN MOS-HEMT structures. The method relies on the protection of the very sensitive AlN epi-layer from exposure to liquid chemicals during processing using evaporated Al, which on thermal oxidation forms Al 2 O 3 . The Al 2 O 3 acts as a surface passivant and as a gate dielectric for transistors that are then fabricated. In contrast to previous approaches, the ohmic contact regions are prepared for metal deposition only using wet etching with 16H 3 PO 4 :HNO 3 :2H 2 O aluminium etch solution and so no damage to the surface associated with dry etching techniques occurs. From the ohmic contact optimisations, low average values of R C and R sh of ~0.49 Ω.mm and ~159 Ω/□, respectively, extracted from transmission line method (TLM) characterisation. Fabricated two-finger AlN/GaN MOS-HEMTs with 3 μm gate length and 100 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 DSmax of ~1000 mA/mm. The peak extrinsic transconductance, G max , of the device is ~230 mS/mm at V DS = 4 V. Current gain cut-off frequency, f T and maximum oscillation frequency, f MAX were 2.8 and 7.9 GHz respectively. This approach provides a simple fabrication process for realising high performance AlN/GaN MOS-HEMT for high power and high frequency applications.