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 /spl Aring/) and lightly doped (4/spl times/10/sup 17/ cm/sup -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/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ was achieved. The deviation of contact resistance and surface roughness were improved to 1.5% and 84 /spl Aring/, respectively. Using the improved ohmic contacts, high-performance GaAs MOSFETs (2 /spl mu/m/spl times/100 /spl mu/m) with a large drain current density (350 mA/mm) and a high transconductance (90 mS/mm) were fabricated.