---
title: "Reverse breakdown effect in diodes"
subject: "Electronics Devices and Circuit"
chapter: 6
key: "semester_3/electronics_devices_and_circuit:reverse-breakdown-diodes"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/reverse-breakdown-diodes
site: BCT Engineering
---

# Reverse breakdown effect in diodes

Electronics Devices and Circuit · Chapter 6: Power Supplies, Breakdown Diodes, and Voltage Regulators

- Asked in: 2068 Chaitra, 4 marks; 2068 Baisakh, 3 marks (asked 2 times)
- Video: [Avalanche Breakdown and Zener Breakdown Effect Explained](https://www.youtube.com/watch?v=EzlSafjMltc) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/reverse-breakdown-diodes
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6

## Question

Define and explain the reverse breakdown effect in diodes.

## Short answer

Reverse breakdown is a sharp rise in reverse current at a critical voltage VBR. Avalanche breakdown (VBR>6V) occurs via impact ionization in lightly doped junctions; Zener breakdown (VBR<5V) occurs via field emission in heavily doped junctions. Diodes can safely operate in this region with current limiting for voltage regulation.

## Answer

**Reverse breakdown** is the sudden large increase in reverse current when the reverse bias voltage across a p-n junction diode reaches a critical value, the breakdown voltage VBR. Below VBR a reverse-biased diode carries only a very small leakage current (reverse saturation current). At VBR the current rises sharply while the voltage stays nearly constant. If the current is not limited by an external resistor, the heat produced destroys the diode.

```text
              I ^
                |           /
                |          /  forward
      -VBR      |         /
  ------+-------+--------/-------> V
        |   small leakage  0.7 V
        |   (reverse saturation current)
        |
        |   steep rise in reverse
        v   current at VBR
```

There are two mechanisms.

#### 1. Avalanche breakdown
- Occurs in lightly doped junctions with a wide depletion region, usually for VBR above about 6 V.
- The minority carriers are accelerated by the strong field and gain enough kinetic energy to knock electrons out of the atoms by collision (impact ionisation). The new carriers cause further collisions, a multiplying chain like an avalanche.
- Has a positive temperature coefficient (VBR increases with temperature).

#### 2. Zener breakdown
- Occurs in heavily doped junctions with a very thin depletion region, for VBR below about 5 V.
- The electric field (about 3 × 10⁷ V/m) is strong enough to pull electrons directly out of the covalent bonds (field ionisation or tunnelling).
- Has a negative temperature coefficient. Around 5 to 6 V the two effects cancel, giving a diode with almost zero temperature coefficient.

#### Uses
Breakdown is not harmful if the current is limited. Zener diodes are designed to operate in this region for voltage regulation, voltage reference and clipping circuits. An ordinary rectifier diode must never be operated beyond its peak inverse voltage (PIV).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is reverse breakdown?
   A. A drop of reverse current to zero at VBR
   B. A sudden large rise in reverse current at the breakdown voltage VBR with the voltage nearly constant
   C. Forward current above 0.7 V
   D. Gradual linear rise of reverse current from 0 V
   Answer: B. Below VBR only small leakage flows; at VBR the current rises sharply. (Key point: Definition)

2. Which describes avalanche breakdown?
   A. Lightly doped wide junction; carriers gain energy and cause impact ionisation; VBR above about 6 V, positive temperature coefficient
   B. Heavily doped thin junction; field emission; VBR below 5 V
   C. Forward-bias effect with negative temperature coefficient
   D. Occurs only in rectifier diodes below 1 V
   Answer: A. Avalanche multiplication needs a wide depletion region and high voltage. (Key point: Avalanche)

3. Which describes Zener breakdown?
   A. Lightly doped junction with impact ionisation; positive temperature coefficient
   B. Occurs only above 10 V
   C. Caused by heating the junction to melting
   D. Heavily doped thin junction; strong field pulls electrons from covalent bonds; VBR below about 5 V; negative temperature coefficient
   Answer: D. Field ionisation in a narrow depletion region dominates at low voltages. (Key point: Zener)

4. Is breakdown always destructive?
   A. Yes, it always destroys the diode instantly
   B. No, the diode never heats in breakdown
   C. No, it is safe if an external resistor limits the current; Zener diodes are designed for it
   D. Only for Zener diodes, never for rectifiers
   Answer: C. Damage comes from excess current and heat; limiting current allows use in regulation and reference. (Key point: Safety)

## Related in Electronics Devices and Circuit

- [Reverse breakdown; Zener as voltage regulator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Areverse-breakdown-zener-regulator) — Electronics Devices and Circuit, chapter 6
- [Zener reverse breakdown as voltage reference](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Azener-reference-justify) — Electronics Devices and Circuit, chapter 6

## Related in other subjects

- [Reverse breakdown effect](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Areverse-breakdown-effect) — Basic Electrical and Electronics Engineering, chapter 3
- [Thyristor protection and diode reverse recovery numerical](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Advdt-didt-protection-reverse-recovery) — Power Electronics, chapter 1