The simulation of coplanar electrodes GaAs photoconductive semiconductor switch（PCSS） exposed in air is done using ANSYS.Based on the electric field results cross-section in X and Y directions

the most breakdown region is the interface among GaAs surface

electrode and air space

especially at the corner.The current distribution of GaAs PCSS covered with of air and Si3Ni4 in X direction is given

it shows that the Si3Ni4 protection can improve the breakdown voltage almost 2 magnitude.Withstand voltage experiments are done with fabricated 3 mm gap coplanar GaAs PCSS.Different insulation techniques for surface protection of high-voltage are tested and different protection materials are coated on the surface of the GaAs switch.Combined with medium insulation and the surface breakdown theory

the experiment datum are analyzed.The simulation and experiment results show that approriate insulation technique can improve the voltage resistance of GaAs PCSS efficiently

and the hold-off field strength reaches 4 kV/mm.