A simple analytical model for the optical response and sensitivity of an ion-implanted MESFET has been developed with the presence of interface states and interfacial layer at the gate–channel contact. It is assumed that the applied drain field is very high and the device follows velocity saturation model. The mathematical expressions for drain current and threshold voltage of the device under optical irradiation have been derived. The dependence of drain current and threshold voltage as functions of drain bias and interface state density is studied for dark and illuminated conditions. The analytical expressions of optical sensitivities of drain current and threshold voltage are derived, and their dependence on different material and device parameters is discussed. It is observed that the device offers better optical sensitivity if excess carrier lifetime is high and for higher value of absorption coefficient optical sensitivity gradually degrades.