It is shown that a phenomenon of second breakdown similar to that in bipolar transistors can occur in vertical power MOSFET's. A model for the phenomenon of second breakdown involving the avalanche multiplication of the channel current, the parasitic bipolar transistor, and base resistance is proposed. After presenting the theory, this model is compared with experiments on four-terminal V-groove test devices in which the substrate can be accessed independently. Good agreement is achieved between calculated and measured boundaries of the safe operating area. The model should be applicable to DMOS devices as well.