Abstract
It is shown that already at fairly low electronic densities (of the order 1014 cm-3) an electron exchanges energy with other electrons quicker than with lattice vibrations, mainly because collisions with the latter are nearly elastic. The notation of an electronic temperature T, which in an external field is higher than the lattice temperature, is then introduced and it is shown that a stationary state becomes impossible if the field is above a critical strength Fc the breakdown strength. For nonpolar and for polar substances Fc can be expressed in terms of the electronic mobility in weak fields, and of other measurable quantities.