Quantify the process impact ionization is characterized by the coefficients of impact ionization, which is numerically equal to the number of pairs of charge carriers formed by the primary carrier on the unit path. By analogy with the theory of electrical discharge in gases, the coefficients of impact ionization in semiconductors represent . The impact ionization coefficients are highly dependent on the electric field. For practical calculations often use the empirical approximation.
The probability of transition of electrons from the valence band to the conduction band and, conversely, from the conduction band to the valence zone is the same. But the transition of electrons from the valence band dominates, because there is much more than in the conduction band. Therefore, the concentration of charge carriers increases with the tunneling.
The tunnel effect in semiconductors is shown at very large intensities of electric fields in silicon, In/cm, in Germany, At/cm of the electric field, under which appears the effect of tunneling is different for different materials, since the thickness of the potential barrier ( ) depends on the band gap of the semiconductor at a constant electric field, i.e. at a constant slope of the energy bands.
Now consider the effect of a strong electric field on the mobility of the charge carriers.