How zener diode is different from PN junction diode?

Normal PN junction diodes are used to allow the flow of current in one direction (forward biased – current flows from anode to cathode) and to block the reverse flow of current (reverse biased – positive of the supply connected to the cathode and negative to the anode); these diodes are used in rectifiers, demodulators, and similar circuits.

A normal diode can prevent the reverse flow up to its PIV voltage (Peak Inverse Voltage). Therefore, while designing the circuit, the diodes are always selected with a higher PIV value than the maximum possible voltage in the circuit that might be applied to it in the reverse bias. This is because once a voltage above the PIV is applied, an avalanche breakdown will occur and permanently destroy the diode junction.

Zener diode

The Zener diode is a type of diode that operates under reverse bias, which breaks down when the applied voltage reaches a knee voltage or a particular point of voltage under reverse bias. It is a specially designed diode with a specified reverse breakdown voltage and a heavily doped p-n junction. Zener diode allows the current to flow like a normal PN junction diode when it is forward biased and blocks the reverse flow of current during the reverse bias up to the breakdown voltage. That is, it has a narrow depletion region, resulting in a low breakdown voltage, typically in a few volts range. Beyond the breakdown voltage or Zener voltage, the Zener diode will permit the reverse flow of current, and the voltage across the Zener (that is, the Zener voltage) remains constant for a wide range of reverse current.

This happens due to the Zener effect, a type of electrical breakdown that occurs when the reverse voltage increases the electric field, enabling the tunneling of electrons from the valence to the conduction band. It increases the number of free charge carriers, leading to a sudden increase in reverse current. A Zener breakdown occurs before the avalanche breakdown. A Zener breakdown is a temporary breakdown, and the junction recovers when the voltage is withdrawn.