In a zener diode, break down occurs in reverse bias due to

Options

A. thin depletion region
B. Internal field emission
C. High doping concentration
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding the Zener Diode Structure**:
- A Zener diode is a type of p-n junction diode that is heavily doped. This heavy doping leads to a very thin depletion region at the junction.

2. **Reverse Bias Condition**:
- When a Zener diode is reverse-biased, the p-type side is connected to the negative terminal of the battery and the n-type side to the positive terminal. This causes the depletion region to widen initially.

3. **Formation of Electric Field**:
- Due to the high doping concentration, a strong electric field is established in the depletion region when reverse bias is applied. This electric field is crucial for the breakdown process.

4. **Breakdown Mechanism**:
- The strong electric field exerts forces on the charge carriers (electrons and holes). When the electric field becomes sufficiently strong, it can pull electrons from their valence bands, creating additional charge carriers. This process is known as **Zener breakdown**.

5. **Internal Field Emission**:
- As the electric field increases, it can lead to a phenomenon called **internal field emission**, where electrons are emitted from the valence band into the conduction band, contributing to a sudden increase in current.

6. **Conclusion**:
- The breakdown in a Zener diode during reverse bias occurs due to a combination of factors: the thin depletion region, the strong electric field established due to high doping concentration, and the internal field emission of charge carriers.

### Final Answer:
In a Zener diode, breakdown occurs in reverse bias due to the establishment of a strong electric field in the thin depletion region caused by high doping concentration, leading to internal field emission.

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