Low-Damage Atomic Layer Etching for Contact Resistance Reduction in Millimeter Wave AlGaN/GaN HEMTs on Si

High-performance AlGaN/GaN MOSHEMTs on Si were demonstrated using atomic layer etching (ALE) for ohmic recess. The ALE technique enabled atomic-scale precision and effectively mitigated plasma-induced damage, leading to a smoother surface morphology ( R RMS ) and a notable reduction in contact resistance ( R c ) to 0.15 Ω·mm. The fabricated device exhibited enhanced maximum drain current ( I DS, MAX ), transconductance ( g m ), and a more positive threshold voltage ( V th ), along with suppressed gate and drain lag. Owing to the reduction in access resistance ( R S and R D ) and parasitic capacitance ( C GS and C GD ) extracted from small-signal modeling, the current-gain cut-off frequency ( f T ) and maximum oscillation frequency ( f MAX ) were enhanced to 58.7/121.5 GHz. At 28 GHz, the device achieved 3.02 W mm −1 output power ( P out ), 17.6% power-added efficiency ( PAE ), and 7.9 dB gain. In addition, power performance evaluated at both 6 GHz and 28 GHz showed clear improvements at 5 V and 10 V, with significantly greater enhancement observed at 20 V. These results demonstrate the effectiveness of ALE ohmic recess for high-power and high-performance mm-wave GaN-on-Si applications.