Collective impact ionization has been used to explain lock-on, an optically-triggered electrical breakdown occurring in some photoconductive semiconductor switches (PCSS's). Lock-on is observed in GaAs and InP but not in Si or GaP. Here, a rate equation implementation of collective impact ionization is discussed, and it leads to new insights both about intrinsic electrical breakdown in insulating materials in general and about lock-on specifically. In this approach, lock-on and electrical breakdown are steady state processes controlled by competition between carrier generation and recombination. This leads to theoretical definitions for both the lock-on field and the breakdown field. Our results show that lock-on is a carrier-density dependent form of electrical breakdown which exists in principle in all semiconductors. Results for GaAs, InP, Si, and GaP are discussed.