Zener diode is a special purpose semiconductor diode, named after its inventor $C$. Zener.

The symbol for zener diode is as below.

Zener diode is fabricated by heavily doping both $p$ and $n$-side of junction. Due to this depletion region formed is very thin $\left(<10^{-6} \mathrm{~m}\right)$ and the electric field of the junction is extremely high $\left(-5 \times 10^{6} \frac{\mathrm{V}}{\mathrm{m}}\right)$ even for a small reverse bias voltage about $5 \mathrm{~V}$.

The $I \rightarrow V$ characteristics of a zener diode is shown in figure $(b)$.

It is seen that when the applied reverse bias voltage $(\mathrm{V})$, reaches the breakdown voltage $\left(\mathrm{V}_{\mathrm{Z}}\right)$ of the zener diode, there is a large change in the current.

Note that after the breakdown voltage $\mathrm{V}_{\mathrm{Z}}$, a large change in the current can be produced by almost insignificant change in the reverse bias voltage. So zener voltage remains constant.

This property of the zener diode is used for regulating supply voltages so that they are constant.

We know that reverse current is due to the flow of electrons (minority carriers) from $p \rightarrow n$ and holes from $n \rightarrow p$.