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Working principle of P-N Junction in Forward Bias Condition

Working principle of P-N Junction in forward Bias

When the P-N Junction is forward Bias, the current from the P-N Junction does not flow until the voltage applied between the ends of the P-N Junction exceeds the Barrier Potential

When the applied voltage between the ends of P-N Junction exceeds the Barrier Potential, electrons from the negative end of the battery start moving from the N-region to the P-region.

In contrast, holes from the P-region begin to move towards the N-region. The applied voltage between the ends reduces the effect of the Barrier Potential thus the width of the Depletion Layer gets reduce.

As the width of the depletion layer decreases, the charge carrier easily crosses the P-N  junction. As the application voltages increase, the width of the Depletion Layer becomes thinner, causing a greater amount of charge to cross the carrier junction.

Due to the increase in the number of charge carriers, the current starts to flows through the P-N Junction. This current is called Forward Current. Electrons acting as charge carriers in the N-region, called Conduction electrons, act as Valence electrons when they enter the P-region.

These electrons run from one hole to another, from second to third, towards the positive terminal of the battery. These electrons that run towards the positive terminal of the battery push the holes in the P-region backward, causing the holes to move in the opposite direction of the electron towards the negative terminal of the battery.

Hence holes are the majority charge carriers in the P-region and electrons are the majority charge carriers in the N-region. Forward current at the P-N Junction flows from the positive terminal of the battery to the negative terminal. And charge carrier electrons move in the opposite direction of this current.

Effect of forward bias on depletion layer

Due to the Forward Bias, a large number of electrons go into the Depletion area, so that the positive holes present in the Depletion layer are eliminated by combining or reducing their number, and similarly, the flow of more holes from the P-area to the Depletion layer.

 Being on the side, the electrons present in the depletion region terminate or decrease in number. This reduces the width of the Depletion Layer. In the picture below, the Depletion Layer is shown in Un Biased and Forward condition.

Effect of forwarding Voltage on Barrier Potential

Due to the forward voltage being applied to the terminal of the P-N junction, the free electrons present in the crystal receive energy equal to the Barrier Potential from the applied external energy source so that they can easily cross the junction.

This part of the energy received by the free electron gives a voltage drop in forwarding voltage. This voltage drop is equal to the Barrier Potential.
Unbiased PN Junction
Depletion layer after Forward Voltage

The polarity of the voltage drop on either side of the P-N Junction is opposite to the Barrier potential. Additionally, there is a voltage drop due to the internal resistance of the P-N Junction.

Hence, the total voltage drop in the forward bias of any P-N Junction diode is equal to the sum of two types of voltage drop: -
  • Voltage Drop (VBp) due to Barrier Potential
  • Voltage Drop (IfRf) due to internal resistance
If the total voltage drop Vf in forwarding Bias is

Vf = IfRf +VBp

V-I Characteristics of P-N Junction Diode in forwarding Bias

When the diagram shows the value of current flowing through the P-N Junction Diode for various Forward Voltage charged in forwarding Bias, it is called the V-I Characteristics of the P-N junction Diode.

The figure below is a diagram drawn between the Forward Voltage and Forward Current of the P-N Junction Diode.
Barrier Potential

The diagram shows that the current flowing through the P-N Junction Diode starts after a certain voltage. The minimum value of applied Forward Voltage after which the current starts through the P-N Junction Diode is called Cut-in Voltage.

The value of the cut-in voltage is shown at point P, on the horizontal line. This point is called Knee Point. There is a sudden increase in the Forward Current by increasing the voltage slightly above the cut-in Voltage.
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