An Eaton Nova Rapid Optical Annealer (ROA 400) has been used to activate n and n+ Si implants for use in power and low noise FET structures for GaAs MMIC's. PECVD SiN capped 3" SI GaAs wafers were annealed at temperatures ranging from 800 to 970 °C for times ranging from 0 (transient light pulse) to 20 sec. Doping profiles were determined using a Polaron concentration profiler; FATFET's were used for measuring drift mobility; and short gate (l.0μm gate length) FET's were fabricated to establish activation uniformity and to determine d.c. and r.f. performance. Results have indicated peak implant activation as high as 90% and electron mobilities of up to 4700cm2/V-sec for carrier concentrations of 1.3×1017/cm3 – results comparable to conventional furnace annealing. The most significant improvement of optical annealing comes in device uniformity. Saturated current uniformities of < ±3% have been achieved over 3" wafers with excellent reproducibility from wafer to wafer. Power FET structures with zero bias gm of 120mS/mm with uniformities of <±5mS/mm have been measured. R.f. measurements on these devices yielded output powers of >500mW/mm with power added efficiencies as high as 35%.