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

When a p-n junction diode is forward biased, the applied voltage permits current flow. This section details the movement of charge carriers, specifically electrons and holes, that facilitate this current.

1. Forward Biasing of the P-N Junction:

Forward biasing occurs when the p-type semiconductor connects to the positive terminal and the n-type semiconductor connects to the negative terminal of a power source. This configuration leads to:

• A reduction in the junction's potential barrier, easing carrier passage.
• Repulsion of majority electrons in the n-type region by the negative terminal, directing them towards the junction.
• Repulsion of majority holes in the p-type region by the positive terminal, directing them towards the junction.

2. Movement of Charge Carriers:

The forward bias induces the following carrier movements:

• Electrons: Majority electrons from the n-type region, now unimpeded by the reduced potential barrier, migrate to the p-type region. Upon crossing the junction, they recombine with holes. This n-to-p electron movement is a primary contributor to the circuit's current.
• Holes: Majority holes from the p-type region migrate to the n-type region. After crossing the junction, they recombine with electrons. This p-to-n hole movement also contributes to the current flow.

3. Current Flow in the Circuit:

The flow of electrons from the n-type material to the p-type material and subsequently through the external circuit towards the positive terminal of the battery constitutes an electric current. Similarly, the movement of holes from the p-type to the n-type material also contributes to this current. The combined movement of both carrier types ensures continuous current circulation.

4. Conclusion:

In a forward-biased p-n junction, current generation is a result of electrons moving from the n-type to the p-type material and holes moving from the p-type to the n-type material. The transit of these charge carriers across the junction establishes a sustained current in the external circuit.