The Insulated Gate Bipolar Transistor is a three-terminal Power semiconductor device that combines the simple gate drive characteristics of the MOSFETs with the high current and low saturation voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device.

 

shows the structure of a typical n-channel IGBT. All discussion here will be concerned with the n-channel type but p-channel IGBT's can be considered in just the same way.