An ensemble Monte Carlo simulation method has been employed to study the operating modes of semi-insulated GaAs (SI-GaAs) photoconductive semiconductor switches (PCSS) excited by a femtosecond laser pulse and biased in an electrical field. The results show that, under a bias field lower than the Gunn threshold field (which is 4.2kV/cm for GaAs), or the optical pulse energy is below the optical threshold, the time-resolved current in the switch operates in the linear mode. When the bias field and triggering optical energy are respectively greater than the electrical and optical thresholds, the PCSS operates in the nonlinear mode. The simulation shows that when triggered by laser pulse with higher photon energy, the switch has lower optical threshold in the nonlinear mode. The mechanism of the nonlinear mode for the switch is concluded to be due to the inter-valley scattering of the photo-generated carriers, leading to local high field in the switch.