Abstract: Optoelectronic is one of the thrust areas for the recent research activity. One of the key components of the optoelectronic family is photo detector to be widely used in broadband communication, optical computing, optical transformer, optical control etc. Result that theoretical predicted for device and using MATLAB are quite similar. Present paper includes Id–Vd characteristics and Numerical model (3D)of a GaAs MESFET Photo detector has been presented in this paper. The model takes into account all the major effects that determine the device characteristics in the illuminated condition. It has been found that in a short channel MESFET photo detector, the drain current saturation is caused by the velocity saturation of the carriers rather than the pinch off condition. By considering both the photo conductive effect in the channel and photo voltaic effect at the gate Schottky barrier, the major limitations of the existing model have been overcome.