An enhanced optical response of the depletion mode AlGaAs / GaAs MODFET under backside optical illumination with DC light 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 photoconductive effect in the GaAs layer which increases the 2DEG channel electron concentration and the photovoltaic effect which forward bias the AlGaAs – gate junction are considered. In addition the reflection effect of the contact metals which also increases the 2DEG channel electron concentration is also considered.  The electrons generated in the GaAs layer and AlGaAs layer are collected in 2DEG, there by increasing the source to drain current and the photo generated holes are drift towards the semi insulating substrate and are capacitively coupled in to the grounded source. The I-V characteristics, transconductance, photovoltage and transfer characteristics have been evaluated and discussed . The I-V characteristics is compared with available experimental data at a particular gate source voltage with and without illumination showing good agreement .

Key Words: MODFET, photovoltage, internal reflection, transconductance.