Answer 1—An IGBT is a special structure that combines the features of a MOSFET and a bipolar transistor. The symbol and equivalent circuit are shown below.

Note that what’s considered to be an “NPN” IGBT is actually the combination of an N-channel FET and a PNP transistor. As the “base” (gate) voltage crosses the threshold voltage of the FET, base current begins to flow in the PNP output transistor, turning it on. For additional details, refer to www.elec.gla.ac.uk/groups/dev_mod/papers/igbt/igbt.html and www.irf.com/technical-info/appnotes.htm.

Contributor: David Tweed