Question

A p-n junction is forward biased. Describe the movement of the charge carriers which produce current in it.

Hint:

In forward bias, electrons move from n-type to p-type material, and holes move from p-type to n-type material, producing current in the external circuit.

Answer 1

In a p-n junction diode that is forward biased, the direction of the applied voltage allows current to flow through the diode. Let's discuss the movement of the charge carriers (electrons and holes) that contribute to the current when the p-n junction is forward biased.

1. Forward Biasing of the P-N Junction: 

When the p-n junction is forward biased, the p-type semiconductor is connected to the positive terminal of the battery, and the n-type semiconductor is connected to the negative terminal of the battery. This results in the following effects:

• The applied voltage reduces the potential barrier at the junction, making it easier for charge carriers to cross the junction.
• The electrons in the n-type region (which are the majority carriers in the n-type semiconductor) are repelled by the negative terminal of the battery and are pushed toward the p-n junction.
• The holes in the p-type region (which are the majority carriers in the p-type semiconductor) are repelled by the positive terminal of the battery and are pushed toward the p-n junction.

2. Movement of Charge Carriers:

The forward bias causes the following movements of charge carriers:

• Electrons (Minority carriers in the p-type region): The electrons in the n-type region, which are majority carriers, move towards the p-type region due to the reduced potential barrier. These electrons cross the junction and move into the p-type material, where they recombine with holes. This movement of electrons from the n-type side to the p-type side is what contributes to the current flow in the circuit.
• Holes (Minority carriers in the n-type region): The holes in the p-type region, which are majority carriers, move towards the n-type region. These holes cross the junction and move into the n-type material, where they recombine with electrons. The movement of holes from the p-type side to the n-type side also contributes to the current flow.

3. Current Flow in the Circuit:

As electrons flow from the n-type material into the p-type material, they move through the external circuit towards the positive terminal of the battery, creating a current in the external circuit. Similarly, holes move from the p-type material to the n-type material and also contribute to the current. Thus, the movement of both electrons and holes results in a continuous current flow in the circuit.

4. Conclusion:

In a forward-biased p-n junction, the current is produced by the movement of electrons from the n-type material to the p-type material, and the movement of holes from the p-type material to the n-type material. The flow of these charge carriers (electrons and holes) across the junction leads to a continuous current in the external circuit.