Zener Diode

A Zener diode allows current to flow from its anode to its cathode like a normal semiconductor diode, but it also permits current to flow in the reverse direction when its "Zener voltage" is reached. 

Circuit Symbol

Working

•       Zener diodes have a highly doped p-n junction.

•       When junction is reverse biased a small reverse saturation current flows Is.

•       When sufficient voltage is given the junction breakdown and large reverse current flows.

Diode connection

•       If a resistor is connected in series, then the current can be limited and power dissipation can be controlled so that the device does not damage.

•       The diode may operate continuously under such condition.

•       The current returns to normal level if voltage is reduced below breakdown level.

IV Characteristics

It is used as:

•       Voltage reference sources

ü  the breakdown voltage is designed to remain stable for a wide range of current

Breakdown Mechanism

ü  Zener Breakdown

ü  Avalanche Breakdown

Zener Breakdown

•       In a very narrow depletion region electric filed strength is high.

•       High electric filed intensity cause the electrons to break from their atoms .

•       Thus the depletion region starts to conduct.

•       It usually occurs when reverse bias voltage is <5 V.

•       This effect is also called as Ionization by electric field.

Avalanche Breakdown

•       When depletion region is wide, the reverse saturation current gives energy for the electrons to collide with other electrons in the depletion region.

•       Avalanche effect is termed as production of more free electrons by the Collison of electrons.

•       This is also termed as Ionization by Collision

This breakdown happens >5V