Write a short note on a Zener diode.

(N/A) Zener diode is a special-purpose semiconductor diode,named after its inventor,$C$. Zener.
The symbol for a Zener diode is shown in figure $(a)$.
$A$ Zener diode is fabricated by heavily doping both the $p$ and $n$-sides of the junction. Due to this,the depletion region formed is very thin $\left(< 10^{-6} \ m\right)$ and the electric field at the junction is extremely high $\left(\approx 5 \times 10^{6} \ V/m\right)$ even for a small reverse bias voltage of about $5 \ V$.
The $I-V$ characteristics of a Zener diode are shown in figure $(b)$.
It is observed that when the applied reverse bias voltage $(V)$ reaches the breakdown voltage $\left(V_{Z}\right)$ of the Zener diode,there is a large change in the current.
Note that after the breakdown voltage $V_{Z}$,a large change in the current can be produced by an almost insignificant change in the reverse bias voltage. Thus,the Zener voltage remains constant.
This property of the Zener diode is used for regulating supply voltages so that they remain constant.
We know that the reverse current is due to the flow of electrons (minority carriers) from $p \rightarrow n$ and holes from $n \rightarrow p$.