This paper presents the characterization and validation of a time-domain physical model for illuminated high-frequency active devices and shows the possibility of use of the electromagnetic analysis of FDTD not only for electromagnetic interaction and scattering but also for the device simulation as a good candidate for a microwave simulator. The model is based on Boltzmann's Transport Equation, which accurately accounts for carrier transport in microwave and millimeter wave devices with sub-micrometer gate lengths. Illumination effects are accommodated in the model to represent carrier density changes inside the illuminated device. The simulation results are compared to available experimental records for a typical MESFET for validation purposes. Simulation results show that the microscopic as well as the macroscopic characteristics of the active device are altered by the light energy. This fact makes the model an important tool for the active device design method under illumination control.