An analytical model for the time dependent characteristic of ion implanted buried gate MESFET have been obtained by solving continuity equation with variation of light , turning ON and OFF in the active layer of buried gate through the optical fiber. This paper provides new insight into the cause of channel conductance variation in an ion implanted buried gate MESFET with front side illumination. At time‘t’ is equal to zero, the light through the optical fiber is turning ‘ON’ and ‘OFF’ has been considered. The channel conductance of the buried gate MESFET evaluated with front side illumination. These results indicate very good performance of the device compared with the other devices like MESFET with back and front illumination. Buried gate MESFET highly used in Aerospace System Technology and Microwave communication.