Abstract: Simulation of the DC characteristics of depletion mode AlGaAs/GaAs Modulation Doped Field Effect Transistor (MODFET) under backside optical illumination is presented. A device structure with fiber inserted into the substrate up to the GaAs layer is considered for direct illumination into the GaAs layer. The AlGaAs layer is considered transparent to illumination. The photoconductive effect which increases the two dimentional electron gas (2DEG) channel electron concentration alone is considered. These electrons generated in the GaAs layer is collected in 2DEG, which increases the source to drain current. The photo generated holes in GaAs layer drifts towards the semi-insulating substrate and is capacitively coupled into the grounded source. The I-V characteristics of MODFET under dark and illuminated conditions have been simulated and discussed.