Concept:Heavy doping produces an extremely narrow depletion layer (\( < 10\text{ nm} \)), creating high electric fields (\( > 10^6\text{ V/m} \)) at low voltages that allow electrons to tunnel directly across the bandgap.
Formula:$$\mathcal{E} = \frac{V_R}{W} > 10^6\text{ V/m} \quad (\text{Zener field threshold})$$
Solution:- Heavy doping narrows the depletion width \( W \).
- A moderate reverse voltage creates a strong electric field across this thin layer.
- This field aligns the valence band of the P-side with the conduction band of the N-side, allowing valence electrons to tunnel directly through the barrier without collisions.
Why other options are incorrect:- Option A: Thermal runaway is a destructive failure mode, whereas Zener breakdown is reversible and controlled.
- Option B: Impact ionization is the mechanism for Avalanche breakdown at higher voltages (\( > 6\text{ V} \)).
- Option D: Thermionic emission describes carrier transport over barriers, not tunneling through narrow barriers.
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