Current filamentation in bipolar power devices during dynamic avalanche breakdown

High voltage, high power bipolar devices like diodes or GTOs exhibit a limi ted safe operating area (SOA). During turn-off or reverse recovery the maxi mum permissible losses remain well below values which could trigger thermal runaway. Up to now the limiting destruction mechanism is not understood. 2 D-simulations of GTO turn-off at turn-off gain 1 revealed that during dynam ic avalanche multiplication current filamentation may occur. Then current a nd power loss density are locally increased by a factor of about 20 which c ould explain a destruction. To exclude filamentation caused by specific poi nts of the GTO cell-structure or the underlying grid also simulations of re verse recovery of a diode structure at similar operating conditions were ca rried out. In this case too, current filamentation can be observed which oc curs simultaneously with sudden voltage drops during the general voltage ri se. Measurements on diodes confirm the simulated characteristic notches in the voltage curve. A physical model is derived which explains current filam entation to be due to a possible negative differential resistance during dy namic avalanche breakdown. (C) 2000 Elsevier Science Ltd. All rights reserv ed.