Rapid thermal annealing (RTA) has been employed for the electrical activation of shallow n-channel layer by Si+ implantation in the fabrication of GaAs MESFET. To prevent considerable outdiffusion of gallium and arsenic from GaAs substrate during annealing, encapsulating layers such as SiNx and SiNx/SiO2 are deposited. The SiNx/SiO2 double dielectric encapsulant is shown to be more effective to improve the electrical activation. Depending on RTA temperature between 900 and 950°C, the maximum activation efficiency exhibits 77% at the implanted energy of 70 keV and the dose of 5x1012 cm−2. SIMS analyses show the reduction of the hydrogen contained in the silicon nitride and no outdiffusion of Ga and As during RTA. It also shows the sharp Si-profile after RTA at 950°C, 30 sec. The MESFET fabricated using activation with RTA provides better transconductance than that with furnace-annealed activation.