Low Source/Drain Contact Resistance for AlGaN/GaN HEMTs with High Al Concentration and Si-HP [111] Substrate

An optimized fabrication process of ohmic contacts is proposed to reduce the source/drain access resistance (RC) and enhance DC/RF performance of AlGaN/GaN HEMTs with a high Al concentration. We show that source/drain RC can be considerably lowered by (i) optimally etching into the barrier layer using Ar+ ion beam, and by (ii) forming recessed contact metallization using an optimized Ti/Al/Ni/Au (12 nm/200 nm/40 nm/100 nm) multilayers. We found that a low RC of ∼0.3 Ω.mm can be achieved by etching closer to the 2-Dimensional Electron Gas (2DEG) at an optimum etching depth, 75% of the barrier thickness, followed by a rapid thermal annealing at 850°C. This is due to the very small distance between the alloy and the 2DEG (higher electric field) as shown by 2D drift-diffusion simulations combined with Transmission Line Model (TLM) extractions.