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Back-gating effects in GaAs MESFET's have been studied for small (10-30 µm) and large (~150 µm) back-gate separations and for different geometric positions of the back-gate. It is found that the substrate currents just at the onset of back-gating and when the channel is pinched off by the gate voltage are independent of the proximity of the back-gate. The geometric position of the back-gate is found to have a very strong influence on the back-gating characteristics. Illumination of the devices with white light gives rise to large substrate currents even for small back-gate voltages, and thus hastens the onset of back-gating. A simple qualitative model is presented to explain the observed results. Deep-level traps in the substrate, channel-substrate interface, and the channel have been studied by observing the drain current transients induced by a pulsed back-gate voltage or by a pulsed front-gate voltage. In addition to the EL2 electron trap with activation energy 0.82 eV detected in the substrate, a nondiscrete band of interface traps have been identified.