Voltage shifts in the forward V-I characteristics of p-n-p-n power switches

The forward V-I characteristics of p-n-p-n power switches usually consist of one or more voltage shifts having negative resistance characteristics at small forward current. This phenomenon is attributed to local turning on of the switch due to shorted emitter and/or shorting dots used to enhance the dv/dt capability of the device, since lateral current flow plays a significant role in determining the V-I characteristics at small current, It is shown that the threshold current responsible for the voltage shift phenomenon has an empirical temperature dependence of the form
I_{T} \sim \exp [- a(T - T_{0})]
over the temperature range 20-125°C. The hysteresis loop associated with each negative resistance region can be explained in terms of the difference in turn-on current and holding current of a localized area in the vicinity of shorted emitter or shorting dots.