---
title: "Reverse breakdown effect"
subject: "Basic Electrical and Electronics Engineering"
chapter: 3
key: "semester_1/basic_electrical_and_electronics_engineering:reverse-breakdown-effect"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/reverse-breakdown-effect
site: BCT Engineering
---

# Reverse breakdown effect

Basic Electrical and Electronics Engineering · Chapter 3: Introduction to Electronics Engineering

- Asked in: 2081 Kartik (new course), 3 marks
- 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_1/basic_electrical_and_electronics_engineering/reverse-breakdown-effect
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_and_electronics_engineering/chapter-3

## Question

Define and explain reverse breakdown effect.

## Short answer

Reverse breakdown is the sharp increase in reverse current when the reverse voltage exceeds a critical value. It occurs via avalanche breakdown (carrier multiplication in wide depletion region) or Zener breakdown (field emission in thin depletion region).

## Answer

When a p-n junction diode is reverse biased, only a tiny reverse saturation current (due to minority carriers) flows. If the reverse voltage is increased beyond a certain value, the current suddenly rises sharply. This effect is called **reverse breakdown**, and the voltage at which it occurs is the breakdown voltage. If the current is not limited by a resistor, the heat can destroy the diode; if it is limited, the junction is not harmed.

There are two mechanisms:

1. **Avalanche breakdown** (usually above about 5-6 V, in lightly doped junctions). The wide depletion region lets minority carriers gain high kinetic energy in the strong field. They collide with atoms and release new electron-hole pairs, which are accelerated and collide again, so the current multiplies like an avalanche.
2. **Zener breakdown** (below about 5 V, in heavily doped junctions). The depletion layer is very thin, so even a small reverse voltage creates a very strong field ($>10^7$ V/m) that pulls electrons directly out of the covalent bonds (field emission), giving a large current.

```text
                     I (forward)
                     |        /
                     |       /  forward
                     |      /   conduction
  -V ------+---------+-----'------------> +V
         -Vbr        |
           |  tiny reverse saturation current
           |
           |  breakdown: current rises sharply
           v  while the voltage stays near Vbr
```

The nearly constant voltage during breakdown is used in Zener diodes for voltage regulation.

## Check yourself

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

1. What current flows in a reverse-biased diode before breakdown?
   A. A large current of majority carriers
   B. No current at all
   C. A forward diffusion current
   D. A tiny reverse saturation current of minority carriers
   Answer: D. Only minority carriers cross the junction, giving a very small saturation current. (Key point: Before breakdown)

2. How does avalanche breakdown occur?
   A. Field pulls electrons directly out of bonds in a very thin depletion layer
   B. Carriers gain energy in a wide depletion region, collide and release more electron-hole pairs, multiplying the current
   C. The diode melts
   D. Majority carriers recombine
   Answer: B. Impact ionisation multiplies carriers, typical in lightly doped junctions above about 5 to 6 V. (Key point: Avalanche breakdown)

3. Which describes Zener breakdown?
   A. Occurs above 6 V in lightly doped junctions
   B. Caused by collisions with atoms
   C. Heavily doped junction, very thin depletion layer, strong field causes field emission below about 5 V
   D. Only occurs in forward bias
   Answer: C. The very strong field (more than 10^7 V/m) pulls electrons from covalent bonds. (Key point: Zener breakdown)

## Related in other subjects

- [Reverse breakdown effect in diodes](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Areverse-breakdown-diodes) — Electronics Devices and Circuit, chapter 6, asked 2×
- [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