Abstract: GaAs PCSS can work on avalanche mode under high electric field , so a vertical GaAs PCSS with bulk structure is fabricated to enhance the switch field. The designed switch has 2mm thickness and 3mm gap, and is triggered by semiconductor laser diode. When charge voltage exceed 8kV, the output voltage impulse increase rapidly and faster than the laser impulse which shows PCSS turn into avalanche mode, and along with switch electric field enhance, the output voltage increase linearly. Triggering by different laser diode energy, there are no changing on the output voltage and output waveform, but the switch jitter is lower at higher bias electric field and higher trigger energy, the lowest switch jitter of 500ps is obtained at this experiment.