it has been shown that the classical theory of critical charge, Q(cr), developed earlier for silicon (Si) thyristors cannot be applied adequately to silicon carbide (SiC) thyristor structures. The critical charge concept has been generalized to describe the situation typical for SiC thyristors, when rather high gate current is required to switch the thyristor on and intermediate injection level in blocking base of the thyristor is reached. A new mechanism of Q(cr) formation governed by the dependence of the transport factor in low-doped blocking base on the injection level has been analyzed. The obtained analytical results have been checked and confirmed by computer simulation. (C) 2003 Elsevier Science Ltd. All rights reserved.