Avalanche Effect

With lighter doping, the depletion region is wider and tunnelling is impossible. Instead, the few minority carriers crossing the junction are accelerated by the field. If they gain enough kinetic energy between collisions, they knock electrons out of the crystal lattice — impact ionisation — creating new electron–hole pairs.

These new carriers are accelerated in turn and create even more, so the current multiplies rapidly. The multiplication factor is often approximated as:

$M = \frac{1}{1 - (V_R / V_{BR})^n}$

which shoots up as the reverse voltage $V_R$ approaches the breakdown voltage $V_{BR}$.

Characteristics:

• occurs at higher breakdown voltages (above about 7 V);
• a sharp knee;
• positive temperature coefficient — hotter lattices vibrate more, carriers collide more often before gaining enough energy, so a higher voltage is needed.

Avalanche multiplication is exploited deliberately in avalanche photodiodes to amplify tiny light signals.