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P-N Junction Diode: Working and Construction

When P-Type and N-Type semiconductors are connected by a special method, and the place where these two semiconductors are connected to each other is called P-N Junction. The device formed after the junction is similar to a diode valve. Hence this device is called P-N Junction Diode.

When both types of impure semiconductors are mixed together, then the charge carriers (free electrons and holes) at the junction start to poison through the junction. P-Type semiconductors emit holes (Hole) into N-Type semiconductors and N-Type semiconductors emit electrons into P-Type semiconductors. After diffusion, these charge carriers neutralize their opposite charge. 

Thus a thin neutral layer is formed on either side of the junction. In this neutral layer, there is no charge carrier of any kind. This neutral layer is called Depletion Layer. The width of this layer is approximately 10–6m.

Due to the diffusion of charge carriers at the junction, the number of positively charged holes in the n-region near the junction and the negatively charged electrons in the p-region increases. 

This results in a Potential Difference between the Depletion Layer across the junction called the Barrier Potential. This difference creates an internal electric field at the junction whose direction is from n-region to P-region. After some time this area becomes so strong that the diffusion of the charge carriers stops.

The flow of electric current through P-N Junction Diode

When an external electrical energy source (battery, cell, etc.) is connected to the terminal of the diode, the current flows through the diode. In the absence of an electrical energy source, no current flows through the diode. The P-N Junction Diode is connected to the electrical energy source in two ways: (a) Forward Biased (b) Reverse Biased.

What is Forward Biased?

In P-N Junction Diode, when the P-Type crystal is connected to the positive terminal of the battery and the N-Type crystal is connected to the negative terminal of the battery, it is called Forward Biased. 

In the case of forwarding Biased, an electric field is established in the diode from the P-region to the n-region and this electric field eliminates the internal electric field. 

In the direction of this new electric field, the holes start moving from the P-region to the N-region and the electrons move from the N-region to the P-region in the opposite direction of the electric field.
Forward Bias Connection of P-N Junction Diode

Holes in the N-region become extinct after being joined by electrons and similarly, holes in the P-region become extinct after being joined by electrons. In this process, electrons come from the battery. 

Due to the lack of holes in the p-region, some new bonds are broken,  and new electrons-holes pairs create, electrons coming out of this bond move towards the positive terminal of the battery, and the holes move towards the negative terminal which causes the current to flow in the diode.

What is Reverse Biased?

In PN Junction Diode, when the P-Type crystal is connected to the negative terminal of the battery and the N-Type crystal is connected to the positive terminal of the battery, it is called Reverse Biased

The power field generated at the junction after the Revers Bias is directed from the N-region to the P-region and helps with the internal electric field. Thus the width of the Depletion Layer increases in Reverse Bias as being connected to Opposite Polarity, the holes in the P-region run towards the negative terminal of the battery and the electron-positive terminal in the N-region, causing the charge to run away from the carrier junction. .
Reverse Bias Connection of P-N Junction Diode

Some electrons and holes remain in the P and N regions due to thermal disturbances. These electrons and holes present in the P and N region are called Minority Charge Carriers. Due to these minority Charge Carriers, a very small value of current flows through the diode in Reverse Bias which is called Reverse Current. 

When the Potential Difference between the terminals of the diode is increased in Reverse Bias, no current flows from the diode to a large value of Reverse voltage, but if the Potential Difference is increased continuously then the current of the diode suddenly starts to flows after a fixed value of the reverse voltage. This value of voltage is called Zenor Voltage or breakdown Voltage and denoted by  Vz
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