Avalanche Diode

Avalanche Diode Definition

An avalanche diode is a type of semiconductor diode which is designed to experience avalanche breakdown at a specified reverse bias voltage. The pn junction of an avalanche diode is designed to prevent current concentration and resulting hot spots so that the diode is undamaged by the avalanche breakdown.

The avalanche breakdown that occurs is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current when compared to a non-avalanche diode.

The construction of the avalanche diode is similar to the Zener diode, and indeed both Zener breakdown and Avalanche breakdown are present in these diodes. Avalanche diodes are optimized for avalanche breakdown conditions, so they exhibit small but significant voltage drop under breakdown conditions, unlike Zener diodes that always maintain a voltage higher than breakdown.

This feature provides better surge protection than a simple Zener diode and acts more like a gas discharge tube replacement. Avalanche diodes have a small positive temperature coefficient of voltage, where diodes relying on the Zener effect have a negative temperature coefficient.

The normal diode allows an electric current in one direction i.e. forward direction. Whereas, avalanche diode allows the current in both direction i.e. forward and reverse direction but it is specially designed to work in reverse bias condition.

Working Principle

The avalanche diode operates on the principle of avalanche breakdown, where accelerated charge carriers gain enough energy to ionize other atoms, thus creating a chain reaction that significantly increases current flow.

Reverse Bias Configuration

In reverse bias, the diode’s N-region (cathode) connects to the positive terminal of the battery, and the P-region (anode) to the negative terminal.

Now if a diode is lightly doped (i.e. impurities concentration is less), then the width of the depletion region is increased so breakdown voltage occurs at a very high voltage.

At a very high reverse bias voltage, the electric field becomes strong in the depletion region and a point is reached where the acceleration of minority carrier is so much that, when they collide with the semiconductor atoms in the depletion region, they break the covalent bonds.

This process generates electron-hole pairs that are accelerated by the electric field, causing more collisions and further increasing the number of charge carriers—a phenomenon known as carrier multiplication.

This continuous process increases the reverse current in the diode, and hence the diode comes into breakdown condition. This type of breakdown is known as avalanche (flood) breakdown and this effect is known as the avalanche effect.

Applications

• The Avalanche diode is used for the protection of the circuit. When reverse bias voltage increases then up to certain limit diode starts an avalanche effect at a particular voltage and diode breakdown due to avalanche effect.

• It is used to protect the circuit against unwanted voltages.

• It is used in surge protectors to protect the circuit from surge voltage.