High voltage, high power bipolar devices like diodes or GTOs exhibit a limited safe operating area (SOA). During turn-off or reverse recovery the maximum permissible losses remain well below values which could trigger thermal runaway. Up to now the limiting destruction mechanism is not understood. 2D-simulations of GTO turn-off at turn-off gain 1 revealed that during dynamic avalanche multiplication current filamentation may occur. Then current and power loss density are locally increased by a factor of about 20 which could explain a destruction. To exclude filamentation caused by specific points of the GTO cell-structure or the underlying grid also simulations of reverse recovery of a diode structure at similar operating conditions were carried out. In this case too, current filamentation can be observed which occurs simultaneously with sudden voltage drops during the general voltage rise. Measurements on diodes confirm the simulated characteristic notches in the voltage curve. A physical model is derived which explains current filamentation to be due to a possible negative differential resistance during dynamic avalanche breakdown.