---
title: "Avalanche vs Zener breakdown; Zener V-I curve"
subject: "Electronics Devices and Circuit"
chapter: 6
key: "semester_3/electronics_devices_and_circuit:avalanche-vs-zener-breakdown"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/avalanche-vs-zener-breakdown
site: BCT Engineering
---

# Avalanche vs Zener breakdown; Zener V-I curve

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

- Asked in: 2076 Ashwin, 3+2 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_3/electronics_devices_and_circuit/avalanche-vs-zener-breakdown
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6

## Question

Differentiate between avalanche and zener breakdown. Draw V-I characteristic curve of zener diode and briefly explain about it.

## Short answer

Avalanche breakdown occurs in lightly doped junctions above ~6 V by impact ionization with a positive temperature coefficient. Zener breakdown occurs in heavily doped narrow junctions below ~5 V by quantum tunneling with a negative temperature coefficient. The Zener V-I curve shows forward conduction (~0.7 V), low reverse leakage, then sharp breakdown at Vz with nearly constant voltage.

## Answer

#### Difference between avalanche and Zener breakdown
| Point | Avalanche breakdown | Zener breakdown |
|---|---|---|
| Cause | Impact ionisation by collision of fast carriers | Direct breaking of covalent bonds by a strong electric field |
| Doping | Lightly doped junction | Heavily doped junction |
| Depletion region | Wide | Very narrow (below 10 nm) |
| Breakdown voltage | Above about 6 V (high) | Below about 5 V (low) |
| Temperature coefficient | Positive (VBR rises with temperature) | Negative (VBR falls with temperature) |
| Characteristic | Sharp knee, noisy | Smooth, rounded knee |
| Mechanism | Carrier multiplication | Field emission (tunnelling) |

#### V-I characteristic of a Zener diode
```text
                I ^
                  |          /
                  |         /  forward
       -Vz        |        /
  ------+---------+-------/-------> V
        |  leakage (µA)  0.7 V
        +-- Izk (knee)
        |
        |   reverse current rises sharply
        |   at almost constant V = Vz
        v   Iz(max)
```

- **Forward region:** it acts as a normal silicon diode, conducting above about 0.7 V.
- **Reverse region before Vz:** only a tiny leakage current (µA) flows.
- **Breakdown (Zener) region:** at the Zener voltage Vz, the reverse current rises very sharply at nearly constant voltage. The knee current Izk is the minimum current needed to stay in this region, and Iz(max) = Pz(max)/Vz is the upper safe limit.
- In this region the voltage is nearly constant (small dynamic resistance rz), so the Zener is used as a voltage regulator and voltage reference. A series resistor limits the current.

## Check yourself

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

1. What is the cause of each breakdown?
   A. Avalanche: field emission; Zener: impact ionisation
   B. Avalanche: impact ionisation by fast carriers; Zener: field breaking covalent bonds directly
   C. Both: thermal heating
   D. Avalanche: forward bias; Zener: reverse bias
   Answer: B. Collision multiplication versus direct field emission. (Key point: Cause)

2. Which pairing is correct?
   A. Avalanche: light doping, wide region, above about 6 V; Zener: heavy doping, narrow region, below about 5 V
   B. Avalanche: heavy doping, below 5 V
   C. Zener: light doping, above 6 V
   D. Both: heavy doping, same voltage
   Answer: A. Doping sets depletion width and thus mechanism. (Key point: Doping and voltage)

3. Which is correct?
   A. Avalanche negative, Zener positive
   B. Both positive
   C. Both zero
   D. Avalanche positive, Zener negative
   Answer: D. VBR rises with temperature in avalanche and falls in Zener. (Key point: Temperature coefficient)

4. What is the knee current Izk?
   A. The maximum safe current
   B. The forward threshold current
   C. The minimum current needed to stay in the breakdown region
   D. The leakage current at 0 V
   Answer: C. Iz(max) = Pz(max)/Vz is the upper limit. (Key point: Knee current)

## Related in Electronics Devices and Circuit

- [Q-point of diode; avalanche vs Zener breakdown](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Adiode-q-point-avalanche-zener) — Electronics Devices and Circuit, chapter 1
- [Zener breakdown and Zener regulator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Azener-breakdown-regulator) — Electronics Devices and Circuit, chapter 6, asked 2×
- [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
- [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
- [PN and Zener I-V graphs; diode ac resistance](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Adiode-iv-ac-equivalent-resistance) — Electronics Devices and Circuit, chapter 1, asked 2×

## Related in other subjects

- [Thyristor V-I, avalanche and forward breakdown](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-breakdown) — Power Electronics, chapter 1
- [Zener diode as voltage regulator](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Azener-voltage-regulator) — Basic Electrical and Electronics Engineering, chapter 3