What is the principle of operation of the IGBT?

The equivalent circuit of an IGBT is shown below. When both the gate-emitter (G-E) and collector-emitter (C-E) paths are positively biased, the N-channel MOSFET conducts, causing drain current to flow. This drain current also flows to the base of Q_PNP and causes the IGBT to turn on. Since the DC current gain (α) of Q_PNP is very small, almost the entire emitter current (I_E(pnp)) flows as base current (I_B(pnp)). However, part of I_E(pnp) flows as collector current (I_C(pnp)). The I_C(pnp) does not turn on Q_NPN because it bypasses the R_BE inserted between the base and emitter of the Q_NPN.

Therefore, almost the entire collector current of the IGBT flows as the drain current of the N-channel MOSFET via the emitter-base paths of Q_PNP. At this time, holes are injected into the high-resistance drift region of the N-channel MOSFET from the emitter of Q_PNP. This causes the resistivity of the drift region  (R_d(MOS)) to decrease considerably, reducing the on-resistance during a conduction period. This phenomenon is called conductivity modulation.

Turning off the gate (G) signal causes the N-channel MOSFET to turn off and therefore causes the IGBT to turn off.