A power electronic device that has a high-impedance input, like a MOSFET, requiring only a small energy to switch the device. The output resembles that of a bipolar junction transistor (BJT) in the on-state: a high current capability with a low voltage drop resulting in low conduction losses. The structure of the IGBT is a variant of a vertical MOSFET, with an additional semiconductor layer to produce the output bipolar transistor (see diagram).

In the ‘off-state’, the gate-source voltage is below threshold for the input MOSFET, so this device is switched off, and the drain-source voltage is dropped across the reverse-biased p-n junction formed by the body and drift regions. Only a small leakage current flows. In the ‘on-state’, the gate-source MOSFET is switched on, providing a current path between the n⁺ source regions and the drift region. The electron current flowing in this path causes substantial hole injection from the p⁺ drain, creating a p-n-p bipolar junction transistor between drain and source. The reverse-biased body-drift region p-n junction is the collector of this transistor. A substantial output current can now flow. Switching times are comparable with power BJTs, approximately one microsecond.

Structure of IGBT showing MOSFET and BJT components