The effect of optical radiation and surface recombination on the RF switching parameters of a GaAs MESFET has been analysed. The study shows that the internal gate source capacitance increases with increasing radiation flux density under enhancement mode, and decreases under depletion mode. However, the surface recombination slightly reduces these effects. Further, the drain source resistance (RdS) reduces with increasing radiation flux density at a particular trap centre density and increases with increasing surface trap density at a fixed flux density. It is also observed that the RC time constant decreases with increasing trap densities at a fixed flux density and absorption coefficient. The variation with doping density is also found to be of a decreasing nature for different flux densities and trap centre densities. At a fixed doping density, RC time constant is higher for higher flux density and lower trap density.