Breakdown in Diodes

BREAKDOWN IN DIODES

During reverse bias, when the reverse voltage is less than breakdown voltage, the diode current is also very small due to minority carriers and almost constant at I_o. When the reverse voltage is increased beyond certain limit, the diode current will be maximum. The point at which the current increases rapidly is called breakdown and the corresponding voltage is called reverse breakdown voltage.

There are two types of breakdown mechanisms in PN diode. They are

i. Avalanche Breakdown

ii. Zener Breakdown

Avalanche Breakdown

The reverse bias causes a small reverse current to flow in the device. This occurs due to the movement of minority charge carriers i.e., electrons from P-type and holes from N - type, since majority carriers move away from the junction.

When the reverse bias is increased, the minority carriers acquire more energy and this kinetic energy is sufficient to break the covalent bonds of the crystal structure. Thus more valence electrons are released from the crystal structure. If the applied voltage is increased, then velocity is also increased.

As kinetic energy is directly proportional to square of velocity (K.E = 1/2 m V²), the kinetic energy of electron also increases.

If this electron collides with an electron in a covalent bond, then the collision provides enough energy to the valence electron to break its covalent bond. This process is called as impact ionization.

Hence, electron-hole pairs are created. These electron hole pairs in turn participate in collision and generate new electron-hole pairs.

This process is called as Avalanche multiplication or Carrier multiplication. It is a cumulative process and large number of electron - hole pairs are created. The reverse current increases rapidly and the junction is said to be in breakdown region.

The avalanche breakdown occurs only in lightly doped diodes, where the depletion region is very wide and the electric field is very low.

Zener Breakdown

Zener breakdown occurs mainly in heavily doped diodes, where the depletion region is very small. When the diode is reverse biased, the electric field across the depletion region is very large..

Electric field is defined as the ratio of voltage measured to the distance. As the depletion region is narrow, the electric field is very high (: electric field is inversely proportional to distance).

This very high electric field breaks the covalent bonds and pulls the electrons out of the valence bands. Hence, new electron hole pairs are created which increases the reverse current and large amount of reverse current across the junction. This process is called Zener effect. This leads to breakdown in p- n diode, called Zener breakdown.

For diodes with reverse breakdown between 5 V & 6 V, both avalanche and Zener mechanisms occurs if the breakdown voltage is less than 5 V, then Zener breakdown occurs. The breakdown voltage depends upon the doping level of the junction. This value determines the practical safe operating voltage called Peak Inverse Voltage (PIV) of a diode. If the operating voltage is less than PIV rating, the reverse breakdown condition is prevented.

If temperature is increased, the valence electrons acquire high energy levels and minimum voltage is sufficient to pull electrons from covalent bonds. Thus for small voltage, at higher temperature, breakdown occurs.

The breakdown voltage decreases as the temperature increases in Zener breakdown. So, Zener breakdown exhibits negative temperature co-efficient.

In lightly doped diodes, width of depletion region is large. If temperature is increased, the vibration of atoms in the crystal increases. The charge carriers have less opportunity to impart energy between collisions to start carrier multiplication. So, voltage should be increased to create breakdown in diode. Thus breakdown voltage increases as the temperature increases. Hence, Avalanche breakdown has positive temperature co-efficient.