On the Impact Ionization in the Space-Charge Region of p–n Junctions †

The Townsend equation for impact ionization in semiconductors applies only to the case where field current and total current are equal. This condition is not fulfilled at low voltages in the space-charge region of p–n junctions and a more exact treatment of the impact ionization in space-charge regions is given. It is shown that a “‘ soft ’ breakdown must precede the Townsend breakdown in p–n junctions. This soft breakdown may often have only a negligible effect on the characteristic, especially when it is masked by surface effects.