A Zener diode is a heavily doped pn-junction diode operated in its breakdown region.
When a pn-junction diode is reverse-biased, a
small & nearly constant current called the reverse
saturation current passes through the diode.
If the magnitude of this reverse-bias is increased further, this current increases very sharply at a certain characteristic reverse-bias voltage called the peak inverse voltage (PIV) or Zener voltage & the diode's valve action breaks down.
Controlling the doping concentrations, Zener diodes are manufactured with peak inverse voltage in the range from about 5V to 200 V.
