  A theory of transverse electron transport coupled with heat transfer in
semiconductor thin films is developed conceptually modeling structures of
modern electronics. The transverse currents generate Joule heat with positive
feedback through thermally activated conductivity. This can lead to instability
known as thermal runaway, or hot spot, or reversible thermal breakdown. A
theory here is based on the optimum fluctuation method modified to describe
saddle stationary points determining the rate of such instabilities and
conditions under which they evolve. Depending on the material and system
parameters, the instabilities appear in a manner of phase transitions, similar
to either nucleation or spinodal decomposition.
