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Chapter 18: Problem 43

Briefly describe electron and hole motions in a \(p-n\) junction for forward and reverse biases; then explain how these lead to rectification.

Short Answer

Expert verified
Question: Explain how the majority carrier motion changes in a p-n junction under forward and reverse biases and how it leads to rectification. Answer: In a p-n junction under forward bias, the applied voltage reduces the barrier height, allowing majority carriers (holes from the p-type semiconductor and electrons from the n-type semiconductor) to flow across the junction and recombine, leading to current flow. However, under reverse bias, the applied voltage increases the barrier height, inhibiting the flow of majority carriers across the junction and resulting in little or no current flow. This characteristic of the p-n junction diode, allowing current flow primarily in the forward bias direction and blocking it in reverse bias, results in rectification.

Step by step solution

01

Define p-n junction and its properties

A p-n junction is formed by bringing a p-type semiconductor (having a higher concentration of holes as majority carriers and doped with acceptor atoms) and an n-type semiconductor (having a higher concentration of electrons as majority carriers and doped with donor atoms) together. When they come into contact, holes from the p-type region diffuse into the n-type region, while electrons from the n-type region diffuse into the p-type region. This diffusion leads to the formation of a depletion region at the junction, which is devoid of majority carriers but contains charged ions. The electric field generated in the depletion region opposes the further movement of carriers across the junction.
02

Forward bias and associated electron-hole motions

When a p-n junction is forward biased, the positive terminal of the battery is connected to the p-type semiconductor, and the negative terminal is connected to the n-type semiconductor. This external voltage reduces the barrier created by the depletion region and helps the majority carrier electrons in the n-region to overcome the barrier and move into the p-region, and holes from the p-region to move into the n-region. The movement of electrons and holes recombines in the junction and constitutes current flow from the p-type to the n-type side.
03

Reverse bias and associated electron-hole motions

Under reverse bias, the negative terminal of the battery is connected to the p-type semiconductor, and the positive terminal is connected to the n-type semiconductor. In this case, the applied voltage increases the barrier height in the depletion region, and it opposes the movement of majority carriers (electrons from n-region to p-region and holes from p-region to n-region). Thus, the overall current flow in the junction is minimal in reverse bias, and the majority carrier movement is almost absent.
04

Rectification in p-n junction diode

Rectification is the process through which a p-n junction diode allows the passage of current primarily in one direction (forward bias) while blocking it in the opposite direction (reverse bias). In forward bias, the barrier height is reduced due to the applied voltage, and majority carriers flow across the junction and recombine, leading to current flow. Conversely, in reverse bias, the barrier height increases, and majority carriers cannot overcome it to flow across the junction, leading to little or no current flow. This ability of the p-n junction diode to allow current flow in one direction while inhibiting it in the other results in rectification.

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Most popular questions from this chapter

Briefly tell what is meant by the drift velocity and mobility of a free electron.

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