Gated turn-on of four layer switch

The n-p-n-p switch (silicon controlled rectifier--SCR) reacts to a gate electrode signal by first going into the high conductivity state in the neighborhood of the gate. Following this initial event the ON state spreads laterally, the "gate" current to the regions which turn on later being supplied by those regions already ON. The current density in the gate region just prior to turn-on is of the order of 1 amp/cm2. The rate of spread or of contraction of the operative area is crudely computed. Hysteresis between increasing and decreasing current characteristics is explained.