In this work, we demonstrate and model the deep-level defect physics of semi-insulating gallium arsenide bulk photoconductive semiconductor switches (PCSS) with gap size of 10 μ and 25 μ in dark-mode operation. Experimental measurements up to biasing field of 10 kV/cm show near-bistable characteristics in the dark-mode current-voltage relations for the PCSS, which cannot be reproduced through commercial Technology Computer-Aided Design simulations. Thus, we model the PCSS by solving for homogeneous non-equilibrium steady-state of the PCSS trap dynamics, where we introduce two semi-Analytical models both involving two deep levels with impact ionization effects. Both models have an excited deep-level that can capture electrons from or emit electrons to the conduction band. The two models differ, however, by the fact that one has a ground state with capture and emission, whereas the other does not include such mechanisms but instead includes electron excitation and relaxation processes directly between the ground state and the excited state without interactions with the conduction band. We find that the former does not fit with experimental near-bistable features while the latter achieves a good match with the same total number of fitting parameters. Further measurements of bias upto 50 kV/cm on one 10 μ PCSS confirms the validity of the second model as well. Finally, a brief discussion of the implications on the illuminated operation of the PCSS is also given to illustrate the importance of including defect interactions and defect avalanche effects.