Zener diode is fabricated by heavy doping of the p-side and n-side of the p-n junction diode. As a result, the depletion region is very thin. The electric field of the junction is extremely high even for a small reverse bias voltage.

The current in the reverse biasing of a pn junction diode is only due to the minority carriers.  When the electric field connected to the zener diode is increased, produces a very large change in current through the circuit but the voltage across the zener remains constant. This is an important property which helps in using zener diode as a voltage regulator.