Breakdown Mechanism in Diodes

Breakdown Mechanism: 

Breakdown is the mechanism in which charge carriers present in junction in the form of immobile ions, take participation in the conduction of current when diode is working in reverse bias condition.

The minimum reverse voltage at which this occurs, is called Breakdown voltage.

The maximum reverse voltage at which diode does not undergoes into the breakdown region, is called Peak Inverse Voltage. So we can see that both are the similar values with a very fine difference.

For example: Let breakdown voltage is 5 Volt.

Then PIV is just previous value which might be 4.9999 volt.

4.9999 volt ~ 5 Volt

So numerically we can represent Breakdown Voltage and Peak Inverse Voltage are equal.

There are two types of Breakdown Mechanism

Avalanche breakdown and Zener breakdown.

Avalanche Breakdown Mechanism 

This breakdown occurs in low doped diode. Due to low doping, depletion width becomes very large. As we increase voltage across the diode in reverse condition, charge carriers are energized hence accelerated. So charge carries collide with the junction with a high velocity. Due to collision, temperature in the junction increases which leads to provide energy to the charge carriers present in the junction in the form of ions. As we increase reverse voltage, the temperature and hence the energy to charge carries present in the form of ions also increases.

As we increase reverse voltage further, a condition arise when an electron escape by leaving from the junction. This high energy electron again collide to the junction to escape another electron from the junction. These two electrons again collide to make four electrons free. In this way, we can see carriers are multiplied from 1 to 2 then 2 to 4 then 4 to 8 then 8 to 16 then ……………………. And so on.

The process of increasing number of carriers in diode in this way is called Carrier Multiplication or Avalanche multiplication.

So a large current  flows in the diode even in reverse bias condition.

Zener Breakdown Mechanism

This breakdown occurs in highly doped diode. The junction diode which are highly doped (i.e. formed using highly doped P and N type materials), is called Zener Diode. Therefore there exists an intense electric field across the PN junction diode. So high force is exerted on the valence electrons of atoms, tending to separate them from corresponding nucleus.

So large electron hole pairs are generated due to which there is a reverse breakdown, causing large current to flow.

In this case, the slop of breakdown characteristics is very sharp. Normally this happens at low voltages like 6 volt and below.

Comparison of Avalanche and Zener  Breakdown Mechanism:

Comparison Table

SN	Avalanche Breakdown	Zener Breakdown
1	It happens in low doped diodes	It happens in highly doped diodes.
2	Due to Avalanche Multiplication	Due to intense electric field
3	Due to collision of carrier with junction which increases temperature	Due to breaking of bond because of intense electric field
4	Temperature coefficient of resistance is negative.	Temperature coefficient of resistance is positive.
5	It happens at high voltages (> 8V)	It happens at low voltages (< 6V)
6	Not as Sharp Slope as shown below	Very Sharp Slope as shown below
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