The subject MOSFET is a C2M0280120D, a serious 1.2kV SiC TO-247 MOSFET. The scope pic shows driving it with a -3.6 to +16.5V gate pulse, 100ns long in this case, and it's switching a 50-ohm resistor from +200V, or 4A (part of my fast HV pulser project.) I previously had it making a burst of five 15ns-long 250V 5A pulses, and everything was fine, perfect.
But after fooling around a bit, it stopped making 15ns pulses. Yes, it still turned on in under 10ns, as programmed, but it turned off with a 50us time constant. Scope traces:
Carefully replacing the MOSFET with a new one (from a different batch, as it happened), I started the signalling pulse generator, and observed the exact same failure mode on the very first pulse.
There are two issues, A. how did I damage the part, and B. what is its failure mode: able to turn on in