The IGBT is a type of power transistor that operates in bipolar mode because of the P layer formed on the drain side of a MOSFET. The IGBT uses a phenomenon called conductivity modulation that exhibits a reduction in the resistivity of the high-resistance N- drift region at turn-on when holes are injected from this P region.
On-state voltage can be reduced because of conductivity modulation, but the IGBT needs to remove minority carriers from the N- drift region when it turns off.
When the IGBT begins to turn off, minority carriers are swept out to external circuitry. When the collector-emitter voltage (VCE) of the IGBT has risen to a certain level (i.e., after the depletion region has expanded), minority carriers contribute to internal recombination current. This current is called tail current. Because tail current is the collector current with a high VCE voltage being applied, it is one of the significant contributors to switching loss.
To reduce tail current and thereby switching loss, IGBTs are principally designed so as to reduce 1) the lifetime of minority carriers and 2) the amount of holes injected from the collector. However, both these techniques cause a rise in on-state voltage. Therefore, IGBTs are designed with optimum trade-offs among these characteristics according to their intended applications.