Q.Avalanche breakdown is primarily dependent on the phenomenon of :

Avalanche breakdown is a chain-reaction process triggered by high-energy carrier collisions with lattice atoms, distinguishing it from Zener breakdown's direct field-ionisation mechanism.

In a p-n junction diode under large reverse bias, avalanche breakdown occurs in lightly-doped junctions with a relatively wide depletion region. The small reverse saturation current present in the junction consists of minority charge carriers (thermally generated electron-hole pairs). Under the strong reverse electric field, these carriers are accelerated to high velocities and hence high kinetic energies as they cross the depletion region.

When these energetic carriers collide with the semiconductor's lattice atoms, they transfer enough energy to break covalent bonds and knock valence electrons free, creating new electron-hole pairs (impact ionisation). These newly-freed carriers are, in turn, also accelerated by the field and cause further collisions, releasing still more carriers -- a multiplying chain reaction known as avalanche multiplication. The result is a sudden, very large increase in reverse current: avalanche breakdown.

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