Understanding the p-n Junction and Reverse Bias

A p-n junction is formed when a p-type semiconductor and an n-type semiconductor are joined together. In the p-type material, the majority charge carriers are holes, while in the n-type material, they are electrons. When these two types are brought into contact, electrons from the n-side diffuse into the p-side, and holes from the p-side diffuse into the n-side. This diffusion leaves behind immobile ions near the junction — positive ions on the n-side and negative ions on the p-side. This region, devoid of free charge carriers, is called the depletion region.

Within the depletion region, the immobile ions create an internal electric field that opposes further diffusion of charge carriers. This internal field also establishes a potential difference across the junction, known as the built-in potential or barrier potential.

What Happens Under Reverse Bias in a Semiconductor Diode?

Reverse bias is a condition applied to a p-n junction (often called a semiconductor diode) where the positive terminal of an external voltage source is connected to the n-type side and the negative terminal is connected to the p-type side. This connection is opposite to the direction that allows current to flow easily (forward bias).

When a p-n junction is subjected to a reverse voltage, the applied external electric field is in the same direction as the built-in internal electric field within the depletion region. This combined field pulls the majority carriers (holes from the p-side, electrons from the n-side) away from the junction interface.

Effect of Reverse Bias on the Depletion Region Width

As the majority carriers are pulled away by the applied reverse voltage, more immobile ions on both the p-side and n-side become exposed. This effectively increases the thickness of the region that is depleted of free charge carriers. Therefore, the depletion region becomes wider under reverse bias.

A wider depletion region means the barrier potential across the junction increases, making it even harder for majority carriers to cross. This is why only a very small leakage current (due to minority carriers) flows under reverse bias in a p-n junction.

Analyzing the Options

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

• becomes wider: As explained above, the applied reverse voltage adds to the built-in potential, sweeping majority carriers away from the junction and expanding the depletion region. This option is consistent with the physics of a reverse-biased semiconductor diode.
• becomes narrower: This happens under forward bias, where the external voltage opposes the built-in potential and pushes majority carriers towards the junction, reducing the depletion region width.
• disappears: The depletion region does not disappear, even under strong forward bias. It only becomes very thin. Under reverse bias, it becomes wider.
• does not change: The depletion region width is highly dependent on the biasing condition (forward or reverse voltage) applied to the p-n junction. It certainly changes.

Based on the behavior of a semiconductor diode under reverse bias, the depletion region significantly increases in width.