A simple and efficient way of modeling backgating in GaAs MESFET's is presented through depletion width modulation of Schottky junction and channel-substrate interface. It is shown semi-empirically that such a modulation of depletion widths causes serious troubles in designing precision circuits since backgating drastically reduces threshold voltage of MESFET as well as drain current. Finally, some of the results are compared with reported experimental results. This model may serve as a starting point for rigorous characterization of backgating effect on various device parameters of GaAs MESFET's.