ZENER DIODE
When an ordinary junction diodes are reverse biased , normally a small reverse saturation current I , flows . If the reverse voltage is increased sufficiently the junction breaksdown and a large reverse current flows . This large current could be enough to destroy the junction . A suitably designed diode , which have . stable breakdown voltage over a wide range of reverse currents is called zener diode . A properly doped crystal diode which has a sharp breakdown voltage is known as zener diode . Fig . 1.14 ( a ) is a circuit symbol of zener diode and Fig . 1.14 ( b ) shows equivalent . circuit in breakdown condition . FIG 1.14 : Zener Diode
The main features of zener diodes are :
1. It is a heavily doped diode , whose depletion layer is thin .
2. When forward biased , its characteristics are just that of ordinary diode .
3. A zener diode has sharp breakdown voltage , called zener voltage V₂ .
4. A zener diode is not immediately burnt just because it has entered the breakdown region . As long as the external circuit connected to the diode limits the diode current to less than burn out value , the diode will not burn out .
VI CHARACTERISTICS OF ZENER DIODE
Fig shows VI characteristics of zener diode . Zener diode characteristics are same as PN diode characteristics in forward bias . In reverse characteristics V₂ is zener breakdown voltage , Izk is the test current at which V₂ is measured . Izk is the zener current near the knee of the characteristics the minimum zener current necessary to sustain breakdown . Izm is the maximum zener current which is limited by the maximum power dissipation.
