CBSE Class 12-science Answered

Zener diode is used for regulating the unregulated voltage supply.

In Zener diode, both p and n sides are heavily doped. Due to high dopant density, the depletion junction width is small and the junction field is high. Under large reverse bias, the high junction field may strip an electron from the valence band which can tunnel to the n side quantum mechanically through the thin depletion layer.

Fig (a) Unbiased p⁺-n⁺ junctions Fig (b) Reverse biased p⁺- n⁺ junctions of Zener diode

This takes place after a certain critical voltage is crossed, and is called 'internal field emission', which gives rise to a high reverse or 'breakdown' current. Such a breakdown is called 'Zener breakdown'.

Fig (b) V-I characteristics of a Zener diode

After breakdown, a large current can be produced by an almost insignificant change in the reverse bias voltage. So, for widely different currents, the voltage across the Zener-diode stays constant. This leads it to be used in voltage regulation of DC voltages.