A comprehensive investigation of the reverse-bias safe operation area (RBSOA) of large area MOS controlled thyristor (MCT) and insulated gate bipolar transistor (IGBT) was performed and results are reported in this paper. Multi-cell devices turn-off failure due to non-uniform gate delay was first investigated. Fundamental device characteristic difference between MCT and IGBT was discussed. It is found that, under isothermal and homogeneity condition, the RBSOAs of both devices are determined by the sustain-mode dynamic avalanche limitation and the maximum controllable current density limitation. If device inhomogeneity exists, the turn-off failure will occur at power densities that are much lower than the RBSOA decided by these two limitations. A new parameter, called dynamic avalanche conductance (g(dynamic)) was defined to describe the characteristic of dynamic avalanche of the two devices. Finally, the RBSOAs of large area MCT and IGBT are summarized and compared. (C) 2002 Elsevier Science Ltd. All rights reserved.