The turning on of the device is attained by enhancing the gate voltage VG so that it is greater than the threshold voltage Vth. This results in an inversion layer forming under the gate that provides a channel linking the source to the drift region of the device. Then Electrons are injected from the source into the drift region whereas at the same time junction J3, which is forward biased, injects holes into the n- doped drift region. This injection causes conductivity modulation of the drift region where the electron and hole both densities are several orders of magnitude higher than the original n- doping. It is this conductivity modulation which gives the IGBT its low on-state voltage because of the reduced resistance of the drift region. Some of the injected holes will recombine in the drift region, while others will cross the region via drift and diffusion and will reach the junction with the p-type region where they will be collected. The operation of the IGBT can, therefore, be considered like a wide-base pnp transistor whose base drive current is supplied by the MOSFET current through the channel. A simple equivalent circuit is, therefore, as shown in Figure 6.23(b). Also shown is the lateral resistance of the p-type region. If the current flowing through this resistance is high enough it will produce a voltage drop that will forward bias the junction with the n+ region turning on the parasitic transistor which forms part of a parasitic thyristor. Once this happens there is a high injection of electrons from the n+ region into the p region and all gate control is lost. This is known as latch up and usually leads to device destruction.