It is a specific type of semiconductor diode, which is made to operate in the reverse breakdown region. The following figure depicts the crystal structure and the symbol of a Zener diode. It is mostly similar to that of a conventional diode. However, small modification is done to distinguish it from a symbol of a regular diode. The bent line indicates letter ‘Z’ of the Zener.

The most significant difference in Zener diodes and regular PN junction diodes is in the mode which they are used in circuits. These diodes are normally operated only in the reverse bias direction, which implies that the anode must be connected to the negative side of the voltage source and the cathode to the positive.

If a regular diode is used in the same way as Zener diode, it will be destroyed due to excessive current. This property makes the Zener diode less significant.

The following illustration shows a regulator with a Zener diode.

The Zener diode is connected in reverse bias direction across unregulated DC supply source. It is heavily doped so that the reverse breakdown voltage is reduced. This results in a very thin depletion layer. Due to this, the Zener diode has sharp reverse breakdown voltage V_z.

As per the circuit action, breakdown occurs sharply with a sudden increase in current as shown in the following figure.

Voltage V_z remains constant with an increase in current. Due to this property, Zener diode is widely used in voltage regulation. It provides almost constant output voltage irrespective of the change in current through the Zener. Thus, the load voltage remains at a constant value.

We can see that at a particular reverse voltage known as knee voltage, current increases sharply with constant voltage. Due to this property, Zener diodes are widely used in voltage stabilization.