Turn-off failure of power MOSFETs

Experimental results of the failure of power MOSFETs during inductive turn-off are discussed. The electrical characteristics of these devices during failure are shown to be identical to those of a bipolar transistor undergoing second breakdown. Other comparisons of the power MOSFET failure and bipolar second breakdown are made. A nondestructive measurement system is used which allows repeated measurements of the failure characteristics as a function of various parameters to be made on a single device. It is shown that commercially available power MOSFETs do not fail as a result of dV/dt currents. Drain voltage slew rates up to 22 V/ns were studied. Other measurements show that the drain voltage at which failure occurs increases with temperature, the critical current above which failure occurs decreases with temperature, and the magnitude of the load inductance has no effect on the failure. The results of this study are consistent with the theory that activation of the parasitic bipolar transistor initiates the power MOSFET failure during turn-off.