AIIMS 2006 Physics Diodes MCQ Question

To understand the behavior of a p-n diode when it is reverse biased, let’s clarify some key concepts about diodes, their structure, and how they respond to external voltages.

Explanation of Reverse Biasing in a p-n Diode

A p-n diode is formed by joining p-type and n-type semiconductors. The p-type semiconductor contains an abundance of holes (positive charge carriers), while the n-type contains an abundance of electrons (negative charge carriers). At the junction between these two materials, electrons from the n-side and holes from the p-side recombine, creating a depletion region that is devoid of charge carriers. This region acts as an insulator and is characterized by a potential barrier.

When a p-n diode is reverse biased, a voltage is applied such that the p-side is connected to the negative terminal of the power supply and the n-side is connected to the positive terminal. This setup has the following effects:

1. Increased Depletion Region: The reverse bias causes additional charge carriers to move away from the junction, which widens the depletion region. This is because the electric field created by the applied reverse voltage pushes holes from the p-side deeper into the p-material and electrons from the n-side deeper into the n-material, effectively increasing the width of the depletion region.

2. Potential Barrier: The potential barrier, which is the energy barrier that prevents the flow of charge carriers across the junction, actually increases under reverse bias conditions. The applied reverse voltage adds to the built-in potential barrier, making it more difficult for charge carriers to cross the junction.

Analysis of the Options

Now, let’s analyze the provided options:

A) No current flows

• This statement is partially correct but misleading. In reverse bias, the current that flows is extremely small (due to minority carrier movement) and is usually negligible compared to the forward-biased condition. Therefore, this option does not capture the essence of the changes occurring in the diode.

B) The depletion region is increased

• This statement is correct. Under reverse bias, the depletion region does indeed increase due to the movement of charge carriers away from the junction. However, this is not the answer we are looking for as per the question which states that the height of the potential barrier is reduced (which is incorrect).

C) The depletion region is reduced

• This statement is incorrect. As explained, the depletion region widens under reverse bias, not reduces.

D) The height of the potential barrier is reduced

• This statement is, in fact, incorrect. In reverse bias, the height of the potential barrier increases due to the additional reverse voltage applied across the diode. So, this option is misleading as the actual change is an increase, not a decrease.

Conclusion

The correct understanding of reverse biasing leads us to conclude that:

• The correct answer should reflect that the potential barrier increases, and the depletion region widens.
• Therefore, none of the provided options accurately describe the behavior of the diode under reverse bias. The statement about the height of the potential barrier being reduced is fundamentally incorrect.

Summary

In summary, when a p-n diode is reverse biased:

• The depletion region increases.
• The potential barrier height increases, not decreases.

This behavior is crucial for understanding the operation and characteristics of diodes in electronic circuits, particularly in applications like rectifiers and voltage regulators.