Considering that semi-insulating gallium arsenide photoconductive switches can be triggered into the high gain mode and no reliable theories can account for the observed transient characteristics, we propose the monopole charge domain model to explain the peculiar switching phenomena occurring in the high gain mode and we discuss the requirements for the lock-on switching. During operation on this mode, the applied field across the switch and the lock-on field are all larger than the Gunn threshold field. Our developed monopole charge domain is based on the transferred-electron effect, but the domain is only composed of large numbers of electrons piled up due to the negative differential mobility. Using the model and taking the physical mechanism of the avalanche impact ionization and recombination radiation into consideration, we interpret the typical phenomena of the lock-on effect, such as the time delay between the beginning of optical illumination and tuing-on of the switch, and the conduction mechanism of the sustaining phase. Under different conditions of bias field intensity and incident light energy, the time delay of the switching is calculated. The results show that the physical mechanisms of impact ionization and recombination radiation occurring in the monopole charge domain are responsible for the lock-on switching.