What is a Zener diode (voltage regulator diode, constant-voltage diode)?

Zener diode is a type of pn junction diode. Fig. 1 shows the V–I curve of the pn junction diode. Typical pn junction diodes are used with a forward bias whereas Zener diodes are used with a reverse bias as they are designed to operate in the breakdown region. There are the following two breakdown mechanisms depending on the dopant concentration of the p-type and n-type semiconductors. Since both breakdown events occur at a fixed voltage when temperature is constant, Zener diodes are also called voltage regulator diodes and constant-voltage diodes.

• Zener breakdown:
  If both the p-type and n-type semiconductors have high dopant concentration, the depletion region that extends around the junction becomes narrow. In this case, electrons pass over a potential barrier by quantum tunneling, causing breakdown. Zener breakdown occurs at relatively low voltage.
• Avalanche breakdown:
  This occurs when either the p-type or n-type semiconductor is more heavily doped than the other. The reverse voltage across the diode is applied across the depletion region that extends around the junction, causing an electric field to have the maximum strength at the pn junction. When the electric field exceeds a certain level, avalanche breakdown occurs, causing current to pass through a diode. Avalanche breakdown occurs at a higher voltage than Zener breakdown.

In practice, there are no distinct regions in which these breakdown events occur. Zener breakdown is more dominant below 6 V whereas avalanche breakdown is more dominant above 6 V.