Carbon (C), based on its high carrier density and low diffusivity, has been applied extensively in GaAs devices, where p-active layers are required. In this study, we used C as an implant source to form p-type active layers, rather than using the conventional epitaxial approach. The dopant activation increased dramatically if an additional argon (Ar) implantation was used. p-channel GaAs metal-semiconductor field-effect transistors were fabricated with this C plus Ar co-implantation technique and demonstrated a promising improvement on device performance.