A Zener diode is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "zener knee voltage" or "zener voltage" or "Avalanche point".
Zener Diode was discovered by a scientist named Clarence Zener. A Zener diode is one in which the reverse breakdown is due to electron quantum tunnelling under high electric field strength—the Zener effect. However, many diodes described as "Zener" diodes rely instead on avalanche breakdown as the mechanism. Both types are used with the Zener effect predominating under 5.6 V and avalanche breakdown above. Common applications include providing a reference voltage for voltage regulators, or to protect other semiconductor devices from momentary voltage pulses.
Zener diodes are mainly used as voltage sources. Since they provide a reasonably constant voltage drop, even when current changes, they are useful in small power supplies as voltage regulators. The power dissipation of a zener diode equals the product of its voltage and current. PZ =VZ IZ . When zener is forward biased it works as a diode and drop across it is 0.7V when it works in breakdown region the voltage across it is constant Vz and the current through it is decided by external resistance. Zener diode is used for regulating the the dc voltage. It maintains the output voltage constant even through the current through changes.
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