ELSEVIER Microelctronics Journal 27 (1996) 217-229 Copyright ~) 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0026-2692/96/$15.00 0026-2692(95)00091-7 Simulation of transient thermal effects during GTO turn off P.A. Mawby 1, M. Evans 2 and M.S. "Towers1 I Department of Electricaland ElectronicEngineering, University of Wales, Swansea, Singleton Park, Swansea SA2 8PP, UK. Tel: +(44)(0) 1792 295595. Fax: +(44)(0) 1792 295686. E-mail: p.a.mawby@Swansea.ac.uk ~Westcode SemiconductorsLimited, PO Box 57, Chippenham, Wiltshire SN15 1JL, UK A rigorous two-dimensiorLalphysical model of the GTO thyristor is presented. The :modelincludes the fully coupled effects of self-heating on the device during the turn-off phase of operation. The effects of spatially-dependent minority carrier lifetime, Auger recombination and carrier- cartier scattering are included in the model. Also, the effect of the temperature gradient on the internal current has been included. The simulation has been carried out within a realistic external circuit environment. 1. Introduction T he influence of self-heating behaviour in power devices, especially during switching, becomes critical as switching repetition rates increase. In the case of the GTO, this is particularly true during turn-off where the charge stored during the on-state must be fully extracted before the terminal currents reduce to zero. In the initial phase of turn-off, the gate is reverse biased so as to extract holes from the p- base, which gradually reduces the area of the conducting plasma to a small filament just below the centre of the cathode electrode. During this phase, known as the storage phase, the main anode current continues to flow essentially unchanged, and afterwards the anode current rapidly falls off during the decay phase. Following this, a characteristic tail current is observed and consists of the remaining charge stored in the n-base region being extracted as the depletion region expands. It is during the tail current phase that the device is known to be at its most vulnerable since the anode voltage is rising rapidly, giving a high instantaneous value of Joule heating in the device, which results in a peak rise in temperature. Usually, thermal analysis of semiconductor problems is limited to the steady state [1-4]. However, as the rate of heat diffusion is much slower than the movement of electrical charge [5] it is clearly unacceptable to assume this, especially as switching times become progressively faster. Previous authors have presented modelling results for the transient switching process [6]; however, in [6] the local 217