A Zener diode is a special kind of diode made from semiconductor materials that allow for a large volume of current to pass through it in reverse bias while keeping the voltage constant. This reliability in the flow of backwards current (in reverse bias) is very helpful in constructing voltage regulators. The constant voltage is also called the Zener voltage of the diode, named after Clarence Zener, who discovered this effect (also called the Zener effect).
In normal diodes, current can only flow in forward bias. Forward bias is a condition where the p side of the p-n junction of the diode is on a higher potential. The reverse condition is called reverse bias, where the n side is on higher potential. In reverse bias, the diode offers significant resistance to the flow of the current. But if we keep on increasing the voltage across the diode, then a breakdown voltage is reached, where suddenly a large current starts flowing through the diode in reverse bias. This condition is called the avalanche condition. The diode may be permanently damaged due to overheating if this condition is allowed to persist.
But in a Zener diode, the breakdown current does not damage the diode. Instead, the current can be varied as desired while keeping the voltage constant.
Zener diodes are made with a variety of Zener voltages. In some Zener diodes, the Zener voltage can be changed. Diodes with low Zener voltage have a heavily doped p-n region inside them and the reverse current is conducted through the process of quantum tunnelling. This happens in the space between the p and n junction of the diode, and this effect is called the Zener effect. For higher Zener voltages, the p-n region is moderately doped to allow avalanche breakdown to happen. Though both the processes happen in all Zener does, the avalanche breakdown is more noticeable in Zener diodes with high voltages and the Zener effect is noticed more in lower voltages.
Zener diodes are an important part of many electronic devices and are basic to their operation. They are useful in managing the voltage fluctuations in circuits. They are also used as a small step down DC transformer to convert a large voltage into a small and stable voltage. Overheating and voltage overload can be prevented by using a Zener diode.
A simple example of voltage regulation by the Zener diode can be given by considering voltage protection. A Zener diode is connected to a device in parallel. The diode is in reverse bias, so it does not conduct any current and does not interfere with the functioning of the device. The diode is earthed. If the device experiences voltage fluctuations, the Zener diode is designed to have an avalanche breakdown at a certain voltage. When the fluctuations exceed that voltage, the avalanche breakdown conditions of the diode are met and the diode starts conducting backwards current. All the harmful and damaging current is conducted to the earth and the device is prevented from overheating.
These diodes are referred to as clamping diodes. This is due to the fact that they maintain the voltage across a device to a certain limit and prevent it from exceeding the recommended value. A clamping diode may be used again depending on if the energy of the fluctuating voltage caused damage to the diode. If the diode experiences overheating, then the diode is damaged and a new diode is required to be installed in the device.
A Zener diode is made by heavy doping of its p-n junction. As a result, the depletion region of the diode is very thin and a very strong electric field is present in the junction. The width of the depletion region is generally of the order of 1 micrometre or less and the strength of the electric field is estimated to be about 500 kV/m. At such conditions, even a small voltage is enough to induce a current in the diode. The conduction takes place due to a phenomenon called ‘tunnelling’.
Tunnelling happens when the electrons from the valence band are suddenly sucked into the conduction band on the other side of the junction due to the very high electric field present in the environment and the reduced distance between the p-type and the n-type material. This happens between the electrons of the p-type material who are tunnelled to the conduction band of the n-type material.
Breakdown voltages of several different Zener diodes can range from 1.2 volts to 200 volts.