During reverse biasing, ionization of the atoms takes place. Since, the polarity of the voltage applied is negative the minority charge carries are accelerated. When the doping concentration is small, these minority carriers collide with other atoms releasing new electrons and thus more charge carriers, which causes breakdown as the number of charge carriers increase.

When the doping concentration is large, there are a large number of ions in the depletion region, which creates a strong electric field, which causes breakdown.