Avalanche breakdown is due to ?

Hint:Before answering the question let us first talk about diode. A diode is a semiconductor device that functions as a current one-way switch. It allows current to flow freely in one direction while severely restricting current flow in the other.

Complete answer:
A Zener Diode, also known as a Breakdown Diode, is a highly doped semiconductor device that operates in the opposite direction. The junction breaks down and current flows in the opposite direction when the voltage through the terminals of a Zener diode is inverted and the potential exceeds the Zener Voltage (knee voltage). The Zener Effect is the name given to this phenomenon.

Avalanche breakdown: When a high reverse voltage is applied across the diode, the avalanche breakdown occurs. The electric field across the junction increases as the applied reverse voltage is increased. The electrons at the junction are pushed out of their covalent bonds by this electric field. These free electrons begin to move through the junction at a high rate, colliding with the other atoms and producing more free electrons.

As a consequence, net current rapidly increases. In Zener diodes, both of these breakdowns occur. When a large amount of reverse voltage is applied to the PN junction, the free electrons accumulate enough energy to accelerate rapidly. These high-velocity free electrons collide with other atoms, knocking off more electrons.

As the electric current in the diode rapidly increases as a result of this continuous collision, a large number of free electrons are produced. A normal diode may be permanently destroyed by this sudden rise in electric current, but a Zener diode is engineered to work under avalanche breakdown and can withstand the sudden spike in current. In Zener diodes with a zener voltage \[\left( {Vz} \right)\]greater than \[6V,\] avalanche breakdown occurs.

Note:The avalanche breakdown in the p-n junction is caused by the conduction band electron's cumulative effect. In reverse bias, electrons gain enough energy from the applied electric field to free more electrons bound to the atom. For conduction, a large number of electron-hole pairs are formed, which has a cumulative effect.