A Zener diode is a sort of diode that allows current not only in the forward direction such as a normal diode, but as well in the reverse direction if the voltage is larger than the breakdown voltage termed as "Zener knee voltage" or "Zener voltage". The device was entitled later than Clarence Zener, who found this electrical property.
A conventional solid-state diode will not permit major current if it is reverse-biased below its reverse breakdown voltage. While the reverse bias breakdown voltage is go beyond, a conventional diode is subject to high current because of avalanche breakdown. If not this current is limited through circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode shows a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends upon the semiconductor material and the doping concentrations.
A Zener diode shows almost the same properties, except the device is particularly designed so as to have a very much reduced breakdown voltage, the supposed Zener voltage. By contrast with the conventional device, a reverse-biased Zener diode will show a controlled breakdown and permit the current to keep the voltage across the Zener diode at the Zener voltage. For instance, a diode with a Zener breakdown voltage of 3.2 V will show a voltage drop of 3.2 V even if reverse bias voltage applied across it is much more than its Zener voltage. Hence the Zener diode is ideal for applications such as the generation of a reference voltage (for example- for an amplifier stage), or like a voltage stabilizer for low-current applications.