Avalanche Breakdown:

This type of breakdown takes place when both sides of junction are lightly doped and consequently the depletion layer is large. In this case, the electric field across the depletion layer is not so strong to produce Zener breakdown. Here the minority carries accelerated by the field collide with the semiconductor atoms in the depletion region. Due to the collision with the valence electrons, covalent bonds are broken and electron hole pairs are generated. So produced acquire energy from applied potential in turn produces additional carriers. This forms a cumulative process called as avalanche multiplication. The breakdown is called avalanche breakdown. This break down occurs at higher reverse voltages.