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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 QPNP and causes the IGBT to turn on. Since the DC current gain (α) of QPNP is very small, almost the entire emitter current (IE(pnp)) flows as base current (IB(pnp)). However, part of IE(pnp) flows as collector current (IC(pnp)). The IC(pnp) does not turn on QNPN because it bypasses the RBE inserted between the base and emitter of the QNPN.
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 QPNP. At this time, holes are injected into the high-resistance drift region of the N-channel MOSFET from the emitter of QPNP. This causes the resistivity of the drift region (Rd(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.