PN JNCTION DIODE UNDER REVERSE BIAS

1.When the pn junction is reverse biased the negative teriminal attracts the holes in the p region, away from the junction.

2.The positive terminal attracts the free electrons in the n region away from the junction.

3.No charge carrier is able to cross the junction.As electrons and holes both move away from the junction, the depletion region widens.

4.As depletion region widens, barrier potential across the junction also increases.

5.The positive side drags the electrons from p region towards the positive of battery.Similarly negative side of barrier potential drags the holes from n region towards the negative of battery.

6.The electrons on p side and holes on n side are minority charge carriers,which constitute the current in reverse biased condition.Thus reverse conduction takes place

7.The reverse current flows due to minority charge carriers which are small in number.Hence reverse current is always very small.This reverse current is called reverse saturation current.

PN JNCTION DIODE UNDER FORWARD BIAS

BIASING: Applying external d.c voltage to any electronic device is called  biasing.

     If an external voltage is applied in such a way that the p type semiconductor is connected to the positive terminal  of the dc voltage and the n type semiconductor to the negative terminal of the dc voltage.

Add caption

OPERATION OF FORWARD BIASED DIODE:

Add caption

1.When the pn junction is forward biased as long as the applied voltage is less than the barrier potential ,there is no current flow(conduction).

2.When the applied voltage becomes more than the barrier potential,the negative terminal repels the free electrons against barrier potential from n to p region.

3.similarly positive terminal repels the free holes against barrier potential from p to n region and cross the junction.

4.Thus the applied voltage overcomes the barrier potential.This reduces the width of depletion region.

5.As forward voltage increased,at a particular value the depletion region becomes very much narrow  such that large number of majority carriers can cross the junction.

6.The large number of majority carriers constitute a current called forward current.

7.The overall forward current is due to the majority charge carriers.

8.These majority carriers can then travel around the closed circuit and a relatively large current flows.

9.The direction of flow of electrons is from negative to positive of the battery.

10.While the direction of the conventional current is from positive to negative of the battery.

SYMBOL OF DIODE

        The p region acts as anode while the n region acts as cathode.The 

arrow in the symbol indicates the direction of the conventional current.