GaAs PCSS's can work under linear and nonlinear modes. When the PCSS's work at the field below 3.5 kV/cm, current pulse string and corresponding light pulse string have the same rise time and pulse width. The resistance of PCSS's recovers, as soon as the light pulse disappears. When the electrical field is larger than 4.3 kV/cm, the light energy is greater than 0.46 mJ, GaAs-PCSS's work at nonlinear mode, which also is called high gain mode or lock-on effect. By calculation, Gunn domains come into being in GaAs in lock-on effect, and the high-gain mechanism is explained by optically activated charge domain model. So the microcosmic conditions of lock-on have been found. The requirement on the triggering laser energy is essential to meet the requirement of Gunn-domain formation by generating enough carriers. The requirement on the electrical field threshold is borne on the requirement of NDR threshold (Gunn threshold), which ranges from 3.2 kV/cm to 4 kV/cm for GaAs. In our experiments, the electric field threshold of high-gain mode is from 4.1 kV/cm to 11 kV/cm, which is higher than NDR threshold of GaAs. We can reduce the electrical field down to Gunn threshold by designing the external circuit. In this paper, two circuits are introduced which is designed by Sandia National Laboratories, and can be used to induce fast recovery from lock-on.