what a voltage regulator is- It is a system wherein a constant voltage level is maintained either using positive feedback or negative feedback.
The Zener diode acts as a conventional diode when it is connected in the forward direction. The current flows without much restriction while there can be a voltage drop of about 0.3 to 0.7 Volts.
However, when the reverse voltages are applied, or reverse feedback is provided, a tiny leakage current is observed between the zener voltage and 0V. After it attains a voltage VZ, there is unrestricted current flow in the system. This voltage at which seamless current flow begins is known as the breakdown voltage. This is how zener diode is used as voltage regulator.
Zener Breakdown -
When the applied reverse bias voltage reaches closer to the Zener voltage, the electric field in the depletion region gets strong enough to pull electrons from their valence band.
At the Zener breakdown region, a small increase in the voltage results in the rapid increase of the electric current.
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