High energy Si implantation into GaAs is of interest for the fabrication of fully implanted, monolithic microwave integrated circuits. Atomic concentration profiles of 8, 12, 16, and 20 MeV Si have been measured using SIMS. The range and shape parameters have been determined for each energy. The theoretical atomic concentration profile for 12 MeV Si calculated using TRIM-88 corresponded to the SIMS experimental profile. No redistribution of the Si was observed for either furnace anneal, 825° C, 15 min, or rapid thermal anneal, 1000°C, 10 s. The activation of the Si improved when co-implanted with S. The co-implanted carrier concentration profiles did not show dopant diffusion. Peak carrier concentration of 2×1018/cm3 was obtained with a Si and S dose of 1.5×104/cm2, each.