Zener diode

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Zener diode

It is a junction diode operates in the reverse biasing or basically it is highly doped p-n junction diode in reversed biased. 
A junction diode specially designed to operate only in the reverse breakdown region continuously (without getting damaged) is called a Zener diode. 

Cause of highly doping


As the reversed current flows due to the minority charge carriers therefore high doping increases the no. Of minority charge carriers and hence increases reverses current. 

Symbol of zener diode



I-V characteristics of a Zener diode

The I-V characteristics of a junction diode as shown in diagram. When the applied reverse voltage (V)  reduces to the breakdown voltage (Vz) of the Zener diode, the current suddenly increases.  Thus after the breakdown voltage (Vz), a large change in the current can be produced by almost  insignificant change in the reverse bias voltage. In other words, zener voltage remains constant, even though current through the Zener diode varies over the wide range. This property of the zener diode is used for regulating supply voltages. 


Cause of zener breakdown 

In a zener diode, both p and n sides are heavily doped with acceptor and donor impurities respectively.  Due to this the depletion layer formed is very thin. Even a small reverse bias voltage of 5 V sets up a very high electric field of [5 × (10)of power of 6 ]V/m.  This field is strong enough to pull valence electrons from the host atoms on the p side which are accelerated to n side. These electrons give rise to a large reverse current or breakdown current. The emission of electrons from the host atoms due to high electric field is known as internal field emission or field Ionisation. The breakdown of the diode due to internal field emission is called zener breakdown. 

Zener diode as a voltage regulator 


Principle

When a zener diode is operated in the reverse breakdown region, the voltage across it remains practically constant (equal to the breakdown voltage Vz) for a large change in the reverse current. The use of zener diode as a DC voltage regulator is based on this fact. 

Working

In the diagram shows the circuit for using zener diode as a voltage regulator. Here the zenere diode is connected in reverse vias to a source of fluctuating DC (e.g., the ouput from a rectifier) through a dropping resistor Rs without any change in the voltage across the zener diode. This is because in the breakdown region, zenere voltage remains constant even though the current through the zener diode changes. Similarly, if the input voltage decreases, the current through Rs and Zener diode also decreases. The voltage across Rs decreases without any change in the voltage across the zener diode. Thus any increase/decrease of the voltage drop across Rs without any change in voltage across zener diode. Hence the zener diode acts as a voltage regulator. 



















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