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The suitability of MBE-grown GaAs layers on Si substrates has been studied for ion-implanted GaAs MESFET technology. The undoped as-grown GaAs layers had a carrier concentration below 1014cm-3. Uniform Si ion implants into 4-µm-thick GaAs layers on Si were annealed at 900°C for 10 s, using a rapid-thermal-annealing (RTA) system. Both the activation and the doping profile were similar to those obtained in bulk semi-insulating GaAs under similar conditions. The SIMS profiles of Si and As atoms near the GaAs/Si heterointerface were identical before and after the RTA process, indicating negigible interdiffusion during the implant activation. Dual implants of a shallow n+ layer and an n-channel layer were used to fabricate GaAs MESFET's with a recess-gate technology. Selective oxygen ion implantation was used for device isolation. The maximum transconductance obtained was 135 mS/ mm compared to typical values of 150-180 mS/mm obtained in our laboratory on GaAs substrates in similar device structures.