The periodicity and weakening surge of GaAs Photoconductive Semiconductor Switches is analyzed. We think periodicity and weakening surge of the output current waveform is caused by self-excitation of the circuit. The electric field threshold and maintenance field of Gunn threshold electric field are modulated by the AC electric field; the output waveform became two main modes of trans-electron oscillator, namely delay dipole domain mode and quenched dipole domain mode.