Zener Diode

ZENER EFFECT

^Ø   In a general purpose PN diode the doping is light; as a result of this the breakdown voltage is high. If a P and N region are heavily doped then the breakdown voltage can be reduced.

^Ø   When the doping is heavy, even the reverse voltage is low, the electric field at barrier will be so strong thus the electrons in the covalent bonds can break away from the bonds. This effect is known as Zener effect.

ZENER DIODE

^Ø   A diode which exhibits the zener effect is called a Zener Diode. Hence it is defined as a reverse biased heavily doped PN junction diode which operates in breakdown region. The zener diodes have been designed to operate at voltages ranging from a few volts to several hundred volts.

^Ø   Zener Breakdown occurs in junctions which is heavily doped and have narrow depletion layers. The breakdown voltage sets up a very strong electric field. This field is so strong enough to break or rupture the covalent bonds thereby generating electron hole pairs.

^Ø   Even a small reverse voltage is capable of producing large number of current carrier. When a zener diode is operated in the breakdown region care must be taken to see that the power dissipation across the junction is within the power rating of the diode otherwise heavy current flowing through the diode may destroy it.

V-I characteristics of Zener diode

^Ø   The illustration above shows this phenomenon in a current vs voltage graph with a zener diode connected in the forward direction .It behaves exactly as a standard diode.

^Ø   In  the  reverse  direction  however  there  is  a  very  small  leakage  current  between  0v

and the zener voltage –i.e. just a tiny amount of current is able to flow.

^Ø   Then, when the voltage reaches the breakdown voltage (vz),suddenly current can flow freely through it.

Application of Zener diode

a) as voltage regulator

b) as peak clippers

c) for reshaping waveforms