Optimization of AuGe-Ni-Au ohmic contacts for GaAs MOSFETs

GaAs-based metal-oxide-semiconductor field-effect transistors (MOSFETs) are promising devices for high-speed and high-power applications. One important factor influencing the performance of a GaAs MOSFET is the characteristics of ohmic contacts at the drain and source terminals. In this paper, AuGe-Ni-Au metal contacts fabricated on a thin (930 Å) and lightly doped (4×10¹⁷ cm^-3) n-type GaAs MOSFET channel layer were studied. The effects of controllable processing factors such as the AuGe thickness, the Ni/AuGe thickness ratio, alloy temperature, and alloy time to the characteristics of the ohmic contacts were analyzed. Contact qualities including specific contact resistance, contact uniformity, and surface morphology were optimized by controlling these processing factors. Using the optimized process conditions, a specific contact resistance of 5.6×10^-6 Ω·cm² was achieved. The deviation of contact resistance and surface roughness were improved to 1.5% and 84 Å, respectively. Using the improved ohmic contacts, high-performance GaAs MOSFETs (2 μm×100 μm) with a large drain current density (350 mA/mm) and a high transconductance (90 mS/mm) were fabricated.