There is huge argument about MOSFET thermal runaway:-
BJTs suffer from a property known as "thermal runaway." Thermal runaway happens because the conductivity of a BJT increases with temperature. Because transistors tend to heat up in proportion to current flowing through them this means that the conductivity and temperature of BJTs can increase exponentially. This can damage the BJT and makes designing circuits for BJTs more difficult. MOSFETs do not suffer from thermal runaway.
Power MOSFETs typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop.
Here we need an expert to show us a light.
@OP, FET is normally referring as JFET. Your title "Burned FET" means "Burned Power JFET" to me and make me click it rightway. 8)