Breakdown Mechanism
Two physical effects cause reverse breakdown in a PN junction: Zener (tunnelling) breakdown and avalanche breakdown.
Under strong reverse bias, current suddenly increases. Two different mechanisms can cause this, and which one dominates depends on the doping level, and therefore on the breakdown voltage:
- Zener breakdown — dominant in heavily doped junctions, below about 5 V.
- Avalanche breakdown — dominant in lightly doped junctions, above about 7 V.
- Between roughly 5 and 7 V both contribute.
Despite the name, most "Zener diodes" above about 6 V actually operate by avalanche breakdown.
Sub-topics (3)
Zener Effect
In heavily doped junctions the depletion region is so thin that electrons quantum-tunnel straight from the valence band to the conduction band.
Avalanche Effect
In lightly doped junctions, carriers accelerated by the field knock new electron–hole pairs loose, which multiply like an avalanche.
Avalanche vs Zener Effect
A side-by-side comparison: what causes each breakdown, at what voltages, and why their temperature coefficients have opposite signs.
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