The diode is a unilateral electronic device. It has two terminals termed – Cathode and Anode. When the diode is forward biased i.e., the anode is kept at a higher potential than the cathode, then ideally, the diode acts as a short circuit (zero resistance) and a large current will pass through it even if a very small voltage is applied across its terminals. But when the diode is reverse biased i.e., the cathode is kept at a higher potential than the anode, then ideally, the diode acts as an open circuit, and no current flows.
But in practice, we see that, after a certain reverse bias voltage, the diode no longer acts as an open circuit, and a large current flows in the reverse direction i.e., from cathode to anode. This event is called the breakdown of the diode and the value of reverse bias voltage for which breakdown occurs is called breakdown voltage of the diode. The breakdown of a diode is primarily of two types – Zener Breakdown and Avalanche Breakdown. In this article, we going to discuss the differences between Zener and Avalanche breakdown.
Differences Between Zener Breakdown and Avalanche Breakdown
In the following table, we have pointed out some key differences between Zener and Avalanche breakdown.
| Zener Breakdown | Avalanche Breakdown |
| Zener breakdown occurs due to a high electric field which causes electrons in the valance band to come into the conductance band. | Avalanche breakdown occurs due to the collision of free electrons and atoms due to a high electric field. |
| The potential barrier is not destroyed in this case. | The potential barrier is destroyed in this case. |
| The regions have to be highly doped for Zener breakdown. | The regions are lightly doped for Avalanche breakdown. |
| The magnitude of Zener breakdown voltage is relatively low. | The magnitude of Avalanche breakdown voltage is relatively high. |
| The slope of the VI curve of Zener breakdown is sharper. | The slope of the VI curve of Avalanche breakdown is not as sharp as the Zener breakdown. |
| It has a negative temperature coefficient, i.e., with the increase in temperature, Zener breakdown voltage decreases. | It has a positive temperature coefficient i.e., with the increase in temperature, Avalanche breakdown voltage increases. |
What is Zener Breakdown?
Zener breakdown is noticed where the diode is highly doped. This causes the depletion layer to be thin. Now if the reverse bias voltage reaches Zener breakdown voltage, then a large number of free electrons are generated due to the transfer from the valence band to the conductance band because of the bias. Then due to the electric field and a thin junction, these free electrons cause a large reverse bias current. This event is called Zener breakdown.
What is Avalanche Breakdown?
Avalanche breakdown is a breakdown mechanism. When the reverse bias potential reaches breakdown voltage, the free electrons in the p-region, enter the depletion layer and collide with atoms. This collision cause electron from the valence band of the atom to come to the conductance band due to the kinetic energy of the first electron. Hence, the electron of the atom becomes a free electron. Then due to the electric field, these free electrons collide with more atoms and create more free electrons which move in the effect of the electric field. So, suddenly a high current is generated due to the reverse bias potential and diode the breaks down. This event is known as Avalanche breakdown.
Conclusion
Avalanche and Zener breakdown primarily differ in the mechanism. In case of avalanche breakdown, the junction is destroyed but in case of, Zener breakdown the junction is not destroyed. That is why an Avalanche breakdown is not desired but Zener breakdown has many applications such as in over-voltage protection circuits.
Author
Subhrajyoti Choudhury
