An experimental investigation has been carried out for clarifying the external mechanical stress effect on GaAs MESFET performance. The stress was induced by bending wafers. It was found that threshold voltage varied linearly with the applied stress. In the case of refractory-gate n+ self-aligned FET's (LG= 1 µm), the magnitude was about 10 mV for 2 × 108-dyn/cm2stress change. The threshold voltage shift direction was opposite for [011]- and [011]-oriented FET's. These results were found to be caused by a change in refractory-gate electrode stress which produces piezoelectrical charge densities in the GaAs substrate.