Sketch the current voltage characteristic curve of a PN diode and explain its shape.

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When the external voltage applied is zero, the potential barrier at the junction does not allow current to flow and therefore circuit current is zero as shown by point O.

When the junction is forward biased, the potential barrier is reduced. At some forward voltage of i.e 0.7 v , the potential barrier is eliminated completely and the current starts flowing in the circuit and as a result the current begins to increase with corresponding increase in forward voltage as shown by region AB on the curve where the PN junction behaves as an ordinary resistor.

When the junction is reverse biased, the potential barrier is increased and hence the resistance of the junction and hence practically no current flows through the circuit , a very small current flows through the circuit with reverse bias as shown by the reverse characteristic. This is called reverse saturation current and it is due to flow of minority charge carriers.

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