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A photoconductive semiconductor switch (PCSS) with a gap of 0.018 m was fabricated from semi-insulating (SI) GaAs and tested under different bias voltages by using a laser at a wavelength of 1064 nm for triggering. The peculiar photoconductivity in high-power SI GaAs PCSS is reported. The PCSS operates in linear mode at bias voltages of 4 and 6 kV, because the waveform of photocurrent is similar to that of laser pulse. As the bias voltage increases, the full-width at half-maximum of photocurrents also increases, and the PCSS undergoes transition from linear to nonlinear mode. The switch behaved nonlinearly at a bias voltage of 10 kV, with obvious lock-on in photocurrent waveform. However, with bias voltages above 10 kV, the photocurrents increase instead of staying on a constant value, and a two-stage rise of photocurrents is observed. In both linear and nonlinear modes, photocurrents have the same full pulse duration at different bias voltages.