About one forgotten mechanism of nonlinearity in the theory of hot   electrons
  It is shown that in general electron gas heating inevitably results in the change of the carrier concentration in the conduction band. It is proved, that this change, as a rule, leads to the kinetic coefficient nonlinearity of the same order, as the change of mobility does. The conditions are determined, when this change can be neglected. 
Yu. G. Gurevich, I. N. Volovichev