When a short-channel MOSFET is driven into the avalancheinduced breakdown region, the drain current increases rapidly and usually shows a snapback characteristic. Both the substrate current and the current collected by a nearby reverse biased p-n junction also increases with increasing drain current in this region of operation. AU of these effects are associated with minority-carrier injection from the source junction into the substrate. A model for the drain I-V characteristics is proposed. Aiso presented is a related model incorporating conductivity modulation that predicts !inear relationships between the substrate and the collection currents and the drain current in this region of operation. Experimental results agree weil with the models.