Zener diode is Highly doped diode, generally used in reverse bias. Due to highly doping, the depletion layer width is very small.
If P-N junction diode in reverse bias then No current passing through circuit. so it is in OFF state.
whereas in Zener diode, the operated in Reverse bias. A constant output voltage generated in reverse bias.
So that Zener diodes used as Voltage regulators.
Working of Zener diode
The Zener diode operates just like the normal diode when in the forward-bias mode, and has a turn-on voltage of between 0.3 and 0.7 V. However, when connected in the reverse mode, which is usual in most of its applications, a small leakage current may flow. As the reverse voltage increases to the predetermined breakdown voltage (Vz), a current starts flowing through the diode. The current increases to a maximum, which is determined by the series resistor, after which it stabilizes and remains constant over a wide range of applied voltage.
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