Zeners

A Zener Diode is a specific type of diode that is designed to allow current to flow forwards and backwards. Current flows backwards when a level of voltage known as zener voltage is reached. The P-N junction is heavily doped, which allows the current to flow backward. 

Zener diodes are commonly used for voltage regulation. Specifically they can be found in clipper circuits and switching applications.  They provide reference voltage for circuits and also protect circuits from too much voltage and electrostatic discharge. 

Zener diodes are named after Clarence Zener , an American physicist who originally discovered the Zener effect in 1938.