While IGBTs and other bipolar devices(*1) are on, carrier injection into the high-resistance drift region causes its resistivity to decrease. This is called conductivity modulation.
The N- drift region in IGBTs and other high-voltage switching devices is thick and has low dopant concentration. Therefore, the N- drift region has extremely high resistivity. The IGBT turns on when the gate-emitter and collector-emitter paths are positively biased as shown below. At this time, holes are injected into the N- region from the collector P region via the N region. Consequently, the carrier concentration in the high-resistance N- region increases, causing its resistivity to decrease. As a result, its forward voltage drop decreases, causing the IGBT to act as a switching device with low on-state voltage.
This increase in conductivity (i.e., a reduction in resistivity) during a conduction period is called conductivity modulation.
(*1) A bipolar device is a type of semiconductor device that uses both electrons and holes as charge carriers for current conduction. In contrast, unipolar devices such as MOSFETs use only one type of charge carrier.