Plot and explain I -V characteristics of a PN junction diode. 
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2 Answers

In forward bias mode P-type of semiconductor is connected to positive terminal and Ntype of semiconductor is connected to negative terminal of battery. As voltage increases current starts flowing through diode. When the voltage applied across PN junction reaches to 0.7V (Si) the current flows through the diode i.e. the diode start conducting current as shown above. In reverse bias mode P-type of semiconductor is connected to negative terminal and Ntype of semiconductor is connected to positive terminal of battery. In this current produced is due to minority charge carriers. This current is called leakage current. As the reverse biased voltage is increased at a critical voltage VBR , the reverse current through the diode increases sharply. Most of the diodes have breakdown voltage more than 50 V.

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V-I characteristics of PN junction diode:

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Explanation: 


Forward Bias: 

If the external voltage applied on the silicon diode is less than 0.7 volts, the silicon diode allows only a small negligible electric current. 

* When the external voltage applied on the silicon diode reaches 0.7 volts, the p-n junction diode starts allowing large electric current through it. 

* At this point, a small increase in voltage increases the electric current rapidly. 

* The forward voltage at which the silicon diode starts allowing large electric current is called cut-in voltage. 

* The cut-in voltage for silicon diode is approximately 0.7 volts.


Reverse Bias: 

* Due to thermal energy in crystal minority carriers are produced. 

* These minority carriers are the electrons and holes pushed towards P-N junction by the negative terminal and positive terminal, respectively. 

* Due to the movement of minority carriers, a very little current flows, which is in nano Ampere range (for silicon). This current is called as reverse saturation current. 

* When the reverse voltage is increased beyond the limit and the reverse current increases drastically is called as reverse breakdown voltage. 

* Diode breakdown occurs by two mechanisms: Avalanche breakdown and Zener breakdown.

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