Zener Effect

Heavy doping makes the depletion region extremely narrow (about 10 nm or less). The electric field across it becomes enormous — around $10^8$ V/m (1 MV/cm) — even at a few volts of reverse bias.

At such fields, electrons in the valence band of the p-side can tunnel quantum-mechanically through the thin energy barrier into empty states in the conduction band of the n-side, without needing to gain energy first. This is called field emission or band-to-band tunnelling.

Characteristics:

• occurs at low breakdown voltages (below about 5 V);
• the knee of the curve is relatively soft (rounded);
• negative temperature coefficient — as temperature rises the band gap shrinks, tunnelling gets easier and $V_Z$ falls.