Operating current level of IGBT is generally lower than the rated current. VCE during low current turn-off transient shows lower slope and different shape from that during the rated current switching. With higher VCE slope at early stage of turn-off transient, turn-off loss could be decreased. In this work, turn-off transient is divided into several stages and the mechanism which determines VCE slope during the turn-off transient at rated current and low current is investigated by numerical simulations and experiments. Moreover, factors which control charge distribution and injection level were investigated. Carrier concentration under high level injection in n-drift layer, the amount of hole injected from the collector, the concentration of the stored charge and the speed of depletion around the channel affect VCE slope.
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