A physics-based analytical model for an optically biased GaAs metal-semiconductor field effect transistor, suitable for simulating optoelectronic integrated circuits, is presented. The main objective of the model is to show the effect of optical radiation and temperature on important parameters such as the bandgap energy, mobility, and carrier lifetime. It is found that optical radiation has negligible effect on these parameters, but the temperature affects them significantly. The effect of different scattering mechanisms on the drain-source current is also studied. Using this analysis, the model extends to describe optical radiation and temperature-dependent device characteristics such as the threshold voltage, transconductance, cutoff frequency, transit time, and responsiv- ity. Under illumination, a significant enhancement in device transconductance leading to an increase in cutoff frequency is observed.