It is the voltage between the drain and the source at which the FET begins to conduct with no gate voltage applied. The gate to source voltage is zero and the FET is supposed to be off but you have applied too much voltage (exceeded the drain source breakdown voltage) so avalanche current flows and your FET blows up.
hero the question was breakdown voltage not conduction of 10,20 amps. if it breakdown at 400v and the leakage is .001amps that is 4 watts and because it gets hot at 4w it will beging to heat up more LEAKING MORE current and eventualy blow up.
ok here is my question so that you will finaly understand. on a square wave where is the maximun heat dissipation rise time, saturation, or fall time, or off. asuming a transistor switch.
finaly if I LIMIT THE CURRENT DURING BREAKDOWN of any device it can sustain that indefenetly. is true or false?