Concept:Heavily doped diodes have very thin depletion regions (\( < 10\text{ nm} \)), creating strong electric fields at low reverse voltages (\( < 6\text{ V} \)) that enable Zener breakdown via quantum mechanical tunneling.
Formula:$$\text{Zener Breakdown: } V_Z < 6\text{ V} \quad (\text{Negative Temperature Coefficient } \frac{d V_Z}{d T} < 0)$$
$$\text{Avalanche Breakdown: } V_{\text{BR}} > 6\text{ V} \quad (\text{Positive Temperature Coefficient } \frac{d V_{\text{BR}}}{d T} > 0)$$
Solution:- Heavy doping produces an intense electric field across a narrow depletion layer.
- At reverse voltages below 6 V, this field pulls electrons directly from the valence band into the conduction band (Zener tunneling).
- Above 6 V in lighter-doped junctions, avalanche breakdown (impact ionization) is the dominant mechanism.
Why other options are incorrect:- Option A: Avalanche multiplication dominates in lightly doped diodes at breakdown voltages above 6 V.
- Option B: Thermal runaway is an overheating failure mode, not the primary breakdown mechanism.
- Option D: Photoelectric ionization requires incident light, which is not required for Zener breakdown.
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