SCR turn-off transient

Summary

When a reverse bias is applied to the SCR anode in order to turn off the device, it is possible to distinguish four separate phases of the ensuing transient: two storage times and two fall times. The device recovers first at the n-emitter junction because there are only very few electrons injected from the wide n base into the narrow p base. The opposite is true for the n base, since the p base injects heavily into it and replenishes holes as fast as they are collected by the reverse-biased anode. The n-emitter junction breaks down as soon as the junction recovers its depletion region if the applied external potential is high enough. The first storage time and fall time are comparatively short. The second storage time is required to remove the excess charge from the long n base where the holes are continuously replenished by injection by the p base. Finally, all the charge disappears almost at the end of the second fall time.

Fast turn offs can be achieved for devices which have low minority-carrier lifetimes (gold doping) and are turned off with high reverse currents. The reverse biasing of the device gate may speed up primarily the first phase of the turn-off transient.