The distinct advantages of optical transmission systems and the increasing use of microwave frequencies within general communication systems, coupled with the ability to integrate microwave and optical components onto a single slice of GaAs have stimulated considerable interest in the development of microwave optoelectronic systems. The optical circuits are advantageous because they can be integrated into the microwave circuits without interfering with them, they have low losses and small dimensions, short reaction time and wide band. This papers present the DC characteristics of a MESFET under different conditions of illumination and different resistances in the gate terminal.