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Operation of the IGBT connected as in Fig. 3-14(a) is shown below.
(1) Inversion layer is made in P layer under gate by applying positive voltage to gate. The Nch MOSFET in Fig. 3-14(b) turns on like a normal Nch MOSFET.
(2) When the Nch MOSFET is ON status, collector’s potential is positive. So, holes are injected from P+ through N+ to N-, and this injection accelerates injection of electrons from emitter.
(3) As a result, increment of carriers (electrons and holes) decreases resistance of N- layer that normally has high resistance (conductivity modulation effect).
Thus, ON resistance of the Nch MOSFET varies to lower as shown in Fig. 3-14(b).
Fig. 3-14(a) Operation of IGBT
Fig. 3-14(b) Equivalent circuit of IGBT and image of actual operation