6 Critical Charge

The critical charge in a thyristor determines the maximum voltage ramp (dV/dt) of the thyristor ^[38], spread velocity of the "on" state ^[52], minimum gate control current density ^[90], holding current, and parameters of current filaments in gate-controlled thyristors ^[91]. The concept of the critical charge was put forward and developed in Refs. ^[37], ^[38]. This concept unifies the numerous methods known to turn a thyristor on. These methods can be divided into two groups: pulse methods and dc methods.

The pulse methods to turn a thyristor on include dV/dt ramp, short gate pulse, short light pulses etc.(The term "short pulse" implies that the duration of the actuating pulse t _p is much less than the minority carrier lifetime ? _p in the thinner and higher doped base of the thyristor). The minimum critical charge Q _cr, required to turn on the thyristor is an intrinsic parameter describing the "sensitivity" of the thyristor. It is noteworthy that the current flowing through the thyristor cannot be used as a measure of this sensitivity for the pulse methods of the thyristors turn-on. For example, a thyristor evidently could not be turned on in the limiting case of ?-function current pulse, when the maximum current achieves an infinite value independent of the charge generated in the thyristor. By other words, even very high current will not turn the thyristor on if the charge generated in the bases...