Assessments of WSix reactive-ion etching in terms of the different CF4 to O-2 flow rate ratio were characterized. Based upon the evaluations from etching rates, side-wall profiles, surface roughness, and damages, we observed that the optimum etching condition was at a ratio of 10:1. The recovery of reactive-ion-etching-treated GaAs damaged layers through the thermal treatment was also investigated as a function of the annealing temperatures and duration times. These parameter evaluations were for the purpose of achieving a high performance GaAs metal-semiconductor field-effect transistor.