A PN Junction is a semiconductor block consisting of P-type and N-type material joined together to form a useful structure. Here, holes are majority carriers in p-type and electrons are majority carriers in n-type.

PN Junction

Figure: PN Junction

A two-terminal semiconductor device formed by the junction of p-type and n-type semiconductor material which allows the flow of electric current in only one direction and blocks the electric current in opposite direction is known as PN junction diode.

BIASING OF PN JUNCTION

FORWARD BIASED

When a positive potential is applied to the p-type and negative potential is applied to the n-type then it is said to be forward-biased.

PN Junction Forward Biased

Figure: Forward Biased

When the PN junction is forward biased, the positive potential on the P side causes the holes to be repelled towards the junction. Similarly, the negative potential on the N side causes the electron to be repelled towards the junction. As a result, the depletion width is reduced along with the barrier potential. When the applied voltage (V) is increased subsequently the barrier potential gets smaller and smaller. When the barrier potential is effectively reduced and gets disappears then the charge carrier can easily flow across the junction. For a PN junction made up of Silicon, the barrier potential is approximately 0.7V and for Germanium, the barrier potential is approximately 0.3V.

REVERSE BIASED

When a negative potential is applied to the P-type and positive potential is applied to the N-type, then it is said to be reverse biased.

PN Junction Reverse Biased

Figure: Reverse Biased

When the PN junction is reverse biased, the negative potential on the P-side causes the holes to be attracted towards the negative terminal of the battery. Similarly, the electrons on the N-side are attracted towards the positive terminal. As a result, the size of the depletion width is increased which in turn increases the barrier potential. With the increase in depletion width and a significant increase in barrier potential, there is no possibility of flow of current across the junction due to majority charge carriers. But there is the flow of current across the junction due to minority charge carriers. This current is known as reverse current or drifts current.


MORE ON BASICS OF ELECTRICAL ENGINEERING. CLICK HERE.


ENGINEERING NOTES ONLINE: NORTON’S THEOREM

LEARN ELECTRICAL ENGINEERING

OPERATIONAL AMPLIFIER