Avalanche vs Zener Effect

	Zener effect	Avalanche effect
Physical cause	Quantum tunnelling across a thin barrier (field emission)	Impact ionisation and carrier multiplication
Doping	Heavy	Light to moderate
Depletion width	Very narrow	Wider
Typical breakdown voltage	Below about 5 V	Above about 7 V
Temperature coefficient	Negative ($V_Z$ falls as T rises)	Positive ($V_{BR}$ rises as T rises)
Knee of the I–V curve	Softer	Sharper
Dominant in	Low-voltage Zener diodes	Higher-voltage "Zener" diodes, rectifiers, APDs

Why the opposite temperature coefficients?

• Zener: higher temperature → smaller band gap → tunnelling easier → lower breakdown voltage.
• Avalanche: higher temperature → more lattice vibration (phonon scattering) → carriers lose energy in collisions more often → higher field needed → higher breakdown voltage.

A practical consequence: around 5–6 V the two effects roughly cancel, so diodes of about 5.6 V have nearly zero temperature coefficient. This is why ~5.6 V Zeners (or a Zener in series with a forward-biased diode) are popular as simple stable references.

Both breakdowns are reversible: the diode is only damaged if the power $V_Z \cdot I_Z$ overheats it.