4. Silicon GTO

As discussed in the previous chapter, the thyristor structure contains a set of coupled transistors that provide a regenerative action during the conduction of current in the on-state. These devices are designed for operation in AC circuits where the anode voltage cycles between positive and negative values. The regenerative action is disrupted whenever the anode voltage reverses from positive to negative. The turn-off of the device then occurs with a reverse recovery process to establish blocking voltage capability. Such device structures are not suitable for applications in DC circuits unless expensive commutation circuits [1] are added to reverse the anode voltage polarity. The development of a thyristor structure that can be designed to turn on and turn off current flow under control by a gate signal in a DC circuit was motivated by this need. Such thyristors have been named gate turn-off (GTO) thyristors. The GTO is turned on in the same manner as the thyristor structures described in the previous chapter, while the turn-off for the GTO is accomplished by the application of a large reverse gate current. The gate current must be sufficient to remove stored charge from the P-base region and disrupt the regenerative action of the internal coupled transistors.