Question

How reverse current suddenly increase at the breakdown voltage?

Answer 1

Specially if u observe in the case of zener diode, a sudden increase in reverse current happens.

Here the reverse voltage is applied across the diode, a strong electric field is induces into the narrow depletion region there by this strong field pulls some of the valence electrons into conduction band by breaking their covalent bonds. A large number of free electrons are generated like this. There by a sharp rise in current happens....