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Difference between Avalanche Breakdown and Zener Breakdown
In a semiconductor diode, the term breakdown implies the short circuit of the diode. As we know, the diode allows the flow of electric current in only one direction (forward direction) and blocks the flow of current in the reverse direction. But, when the applied voltage in the reverse direction exceeds a limit (called breakdown voltage), the diode starts conducting in the reverse direction as well. This stage is called the breakdown in the diode.
The following two types of breakdowns take place in a PN-junction semiconductor diode −
- Zener Breakdown
- Avalanche Breakdown
In this article, we shall study all the significant differences between Zener breakdown and avalanche breakdown. But, before learning about the differences, we will learn that the Zener breakdown and avalanche breakdown are, which makes it easier to understand the differences between them.
What is Zener Breakdown?
The breakdown of PN junction in a semiconductor diode which occurs due to the flow of free electrons across the junction is called Zener breakdown. The Zener breakdown mainly occurs in the heavy doped diodes that have a thin depletion region.
When a high electric field is applied across the PN junction in reverse direction, the charge carriers start flowing across the junction. As a result of it, a heavy current flows in the reverse direction through the diode. The Zener breakdown is temporary breakdown of the PN junction in the diode, which does not destroy the diode. Therefore, once the reverse voltage is removed, the PN junction regains its original state.
The value of reverse voltage at which the Zener breakdown in the PN junction diode takes place is called the Zener voltage. The Zener breakdown takes place in highly doped PN junction diodes.
What is Avalanche Breakdown?
The type of PN junction breakdown in which the applied electric field in the reverse direction across the diode increases the velocity of charge carriers and these charge carriers create a large number of hole-electron pairs by colliding with the atoms of the semiconductor materials is called the avalanche breakdown.
In case avalanche breakdown, the production of hole-electron pairs is continuous which causes an avalanche of free charge carries. The flow of free charge carriers across the junction results a high reverse current in the diode which permanently destroys the PN junction.
The applied reverse voltage at which the avalanche breakdown occurs is called avalanche breakdown voltage. The avalanche breakdown mainly occurs in such PN junction diodes that have thick depletion region. The avalanche breakdown in the diode is permanent, i.e. it cannot regain its original state.
Difference between Zener Breakdown and Avalanche Breakdown
The following table highlights all the major differences between Zener breakdown and Avalanche breakdown −
| Basis of Difference | Zener Breakdown | Avalanche Breakdown |
|---|---|---|
| Definition | The breakdown in a PN junction diode occurs due to the tunneling effect (or field ionization) is known as Zener breakdown. | The breakdown in a PN junction diode that occurs due to impact ionization, i.e. increase in the number of free electrons flowing in the reverse direction is called avalanche breakdown. |
| Breakdown voltage (In reverse direction) | The reverse voltage at which the Zener breakdown occurs is called Zener voltage. It is denoted by VZ and its value typically varies from 5 to 8 volts. | The reverse voltage at which avalanche breakdown takes place is called avalanche breakdown voltage. The value of avalanche breakdown voltage is usually more than 8 volts, which is always greater than Zener voltage. |
| Depletion region | The Zener breakdown occurs in the PN junction diodes having comparatively thin depletion region. | Avalanche breakdown occurs in such PN junction diodes that have thick depletion region. |
| Destruction of junction | The Zener breakdown does destroy the PN junction of the diode. | The avalanche breakdown permanently destroys the PN junction of the diode. |
| Electric field | In the Zener breakdown, the electric field across the junction is strong. | In case of avalanche breakdown, the electric field across the junction is relatively weak. |
| Doping level | Zener breakdown occurs in heavily doped PN junction diodes. | Avalanche breakdown can occur in the diode having any level of doping. |
| Reverse voltage | Zener breakdown takes place at relatively lower reverse voltage. | Avalanche breakdown occurs at a reverse voltage more than Zener voltage. |
| Ionization | In case of Zener breakdown, the ionization is due to electric field. | In case of avalanche breakdown, the ionization is due to collision between charge carries and atoms. |
| Relation between breakdown voltage & temperature | The Zener breakdown voltage is inversely proportional to the temperature. | The avalanche breakdown voltage is directly proportional to the temperature. |
| Change in voltage after breakdown | In case of Zener breakdown, once the breakdown occurs, the reverse voltage across the diode becomes constant. | The voltage across the diode may vary even after the occurrence of avalanche breakdown. |
| Temperature coefficient of voltage | Zener breakdown has a negative temperature coefficient of voltage, i.e. Zener voltage decreases with the increase in temperature. | Avalanche breakdown has a positive temperature coefficient of voltage, which means the avalanche breakdown voltage increases when the temperature decreases. |
| Charge carriers | Zener breakdown generates electrons. | Avalanche breakdown generates holeelectrons pairs. |
| Effect on junction | In case of Zener breakdown, the PN junction regains its original state. | In case of avalanche breakdown, the PN junction does not regain its original state. |
Conclusion
The most significant difference between the two is that the Zener breakdown occurs at a relatively low reverse voltage and is reversible, while the avalanche breakdown occurs at a higher reverse voltage and it is irreversible. Hence, a Zener breakdown does not damage the diode, whereas an Avalanche breakdown may damage the diode permanently.
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