Boundary of power-MOSFET, unclamped inductive-switching (UIS), avalanche-current capability

The avalanche-current capability of a new class of power MOSFETs called MegaFETs is characterized under UIS (unclamped inductive switching) conditions. These devices are shown to be limited in avalanche current only when the junction temperature exceeds the intrinsic temperature of 425°C. Second breakdown is shown to occur independent of the parasitic bipolar transistor. The devices tested exhibit a cubic relationship between the maximum avalanche current and inductance, which represents a nonconstant energy capability. This maximum current decreases linearly with initial junction temperature, indicating a thermally initiated failure mechanism. The maximum junction temperature at failure was found to be 400-450°C. The power-supply voltage reduces the maximum avalanche current by increasing the time in avalanche. The current capability is also proportional to total die area when the avalanche time is constant. The avalanche-current capability of a planar diode was found to verify that parasitic bipolar turn-on is not necessary to cause second breakdown. Second breakdown due to the thermal generation of carriers represents the limiting boundary of power-MOSFET avalanche-current capability