Concept:Zener breakdown occurs in heavily doped diodes with narrow depletion regions (\( < 10\text{ nm} \)), where a reverse voltage produces a very strong electric field (\( > 10^6\text{ V/m} \)) that pulls electrons directly out of covalent bonds.
Formula:$$E = \frac{V_R}{W} \ge 10^6\text{ V/m} \quad (\text{Field Emission / Direct Band Tunneling})$$
Solution:- Heavy doping produces an extremely thin depletion layer.
- Applying a modest reverse voltage (typically \( < 6\text{ V} \)) creates an electric field strong enough to pull valence electrons into the conduction band via quantum tunneling.
- This produces a sharp, non-destructive increase in reverse current.
Why other options are incorrect:- Option A: Thermal runaway is an overheating failure mode, not the quantum mechanism of Zener breakdown.
- Option B: Impact ionization by high-velocity carriers in lightly doped junctions defines Avalanche breakdown (typically \( > 6\text{ V} \)).
- Option D: Recombination decreases during reverse bias because carriers are separated rather than combined.
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