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zener diode

Introduction :


Zener diodes are a special type of semiconductor diode which is operated in reverse bias.it conducts current in bot forward and reverse bias. this device is named after clarance Melvin zener who discovered zener effect.


Zener-Diode-Symbol.png


In forward bias it will act as normal P-N junction diode and it will not turn till more than 0.7 volts are provided.


 when it is operated in reverse bias it will allow small amount of current till reverse voltage reaches Zener voltage. If you increase reverse voltage beyond Zener voltage (vz) leakage current increases. It is called Zener break down or zener effect and this voltage is called breakdown voltage.


Break down mechanisms are two type


1.  Avalenche break down.


 2.Zener break down



Avalanche breakdown :


A P-type semiconductor in contact with an N-type semiconductor forms a depletion region when it is operated in reverse bias, as bias voltage increases width of this depletion region also increases. In reverse bias condition very little amount of current flows due to minority carriers.  This tiny current is called as reverse saturation current.


Reverse saturation current:  minority carriers in p-side will move to n-side and minority carriers in n-side will move to p-side in reverse bias condition.


As magnitude of reverse voltage increases the kinetic energy of electrons also increases if electron hits the atom new electron will release forming a hole. This released electron also in the same field and gains more kinetic energy and hits another atom. In this way more and much more electrons will be released by breaking their covalent bonds. It process is called carrier multiplication which results in increase of minority carriers (both holes and electrons). As minority carriers increases depletion region reduces finally depletion region will vanish as a result large reverse current starts flows through device if this reverse current is not limited by a resistor in series the junction may permanently destroy due to excessive generation of heat.



Zener break down :


It is heavily doped P-N junction diode where depletion region is very narrow. As reverse bias voltage increases more electric field is produced as this field is enough to break covalent bonds and produces high current.


zener

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