This question explores the behavior of a p-n junction when it is subjected to reverse bias. Understanding how reverse biasing affects the depletion layer and the overall resistance is crucial in semiconductor physics.

Reverse Biasing a p-n Junction Explained

A p-n junction is formed when a p-type semiconductor (having an excess of holes) is joined with an n-type semiconductor (having an excess of electrons). At the junction, some electrons diffuse into the p-side and holes diffuse into the n-side, creating a region depleted of mobile charge carriers, known as the depletion layer or depletion region. This layer has an associated potential barrier.

Reverse biasing occurs when the positive terminal of an external voltage source is connected to the n-type material and the negative terminal is connected to the p-type material. This is opposite to the condition for forward biasing.

Effect of Reverse Bias on the Depletion Layer

When a p-n junction is reverse biased:

• The applied external voltage opposes the natural potential barrier at the junction.
• The negative potential on the p-side repels the holes (majority carriers in p-type) away from the junction.
• The positive potential on the n-side repels the electrons (majority carriers in n-type) away from the junction.
• As the majority charge carriers are pulled further away from the junction, the region depleted of these mobile carriers (the depletion layer) becomes wider.
• This increase in the width of the depletion layer results in a higher potential barrier.

Therefore, when a p-n junction is reverse biased, the depletion layer increases in width.

Impact on Resistance

The increased width of the depletion layer under reverse bias means there are fewer free charge carriers available to conduct current across the junction. This significantly increases the opposition to current flow, meaning the junction offers a high resistance.

Analyzing the Options

Let's examine the given options in the context of a reverse biased p-n junction:

• Depletion layer decreases: This is incorrect. Reverse bias widens the depletion layer, it does not decrease it.
• It offers low resistance: This is incorrect. Reverse bias leads to high resistance due to the widened depletion layer.
• Depletion layer increases: This is correct. As explained above, the movement of majority carriers away from the junction under reverse bias causes the depletion layer to widen.
• Electron mobility increases: This is generally not the primary or direct effect described for reverse biasing. While electric fields exist, the dominant impact is on carrier movement across the widened barrier, not an increase in intrinsic electron mobility.

The primary and most significant effect of reverse biasing a p-n junction is the expansion of the depletion layer.