Assertion (A): A p-n junction has high resistance in reverse direction.
Reason (R): When a reverse bias is applied to p-n junction, the width of the depletion layer increases.

The given assertion and reason state that a p-n junction has high resistance in the reverse direction, and the reason behind this is the increase in the width of the depletion layer when a reverse bias is applied. Let's analyze whether both the assertion and reason are true and whether the reason correctly explains the assertion.

Understanding the p-n Junction:
A p-n junction is formed when a p-type semiconductor (which has excess holes or positive charge carriers) is connected to an n-type semiconductor (which has excess electrons or negative charge carriers). The region near the junction where the excess carriers recombine is called the depletion region, and it lacks free charge carriers. The depletion region acts as a barrier to the flow of current.

A p-n Junction in Forward Bias:
When a forward bias is applied to a p-n junction, the positive terminal of the voltage source is connected to the p-side, and the negative terminal is connected to the n-side. This forward bias reduces the width of the depletion layer, allowing the majority charge carriers to flow across the junction. As a result, the resistance of the p-n junction decreases, and current can flow easily in the forward direction.

A p-n Junction in Reverse Bias:
When a reverse bias is applied to a p-n junction, the positive terminal of the voltage source is connected to the n-side, and the negative terminal is connected to the p-side. This reverse bias increases the width of the depletion layer, creating a larger barrier for the majority charge carriers. As a result, the resistance of the p-n junction increases significantly, and only a very small reverse saturation current can flow.

Analysis of the Assertion and Reason:
A: The assertion that a p-n junction has high resistance in the reverse direction is true. In reverse bias, the width of the depletion layer increases, limiting the flow of majority charge carriers and thus increasing the resistance.

R: The reason given for the assertion, that the width of the depletion layer increases when a reverse bias is applied, is also true. In reverse bias, the electric field across the junction increases, pushing the majority charge carriers away from the junction and widening the depletion region.

Explanation:
The reason (R) correctly explains the assertion (A). When a reverse bias is applied to a p-n junction, the width of the depletion layer increases due to the increased electric field across the junction. This increase in the width of the depletion layer leads to a higher resistance in the reverse direction. Therefore, both the assertion and the reason are true, and the reason correctly explains the assertion.

Conclusion:
In conclusion, a p-n junction has high resistance in the reverse direction because the width of the depletion layer increases when a reverse bias is applied. This increase in width is due to the increased electric field across the junction, which limits the flow of majority charge carriers and results in a higher resistance.