Question

Why width of depletion layer decreases in forward biasing?

Answer 1

In a diode, the width of the depletion layer changes depending on whether the diode is in forward bias or reverse bias. Let's break down why the width of the depletion layer decreases when the diode is forward biased.

### Understanding the Depletion Layer

1. **Depletion Layer**: This is the region around the p-n junction in a diode where mobile charge carriers (holes in the p-type and electrons in the n-type) have recombined, creating a zone devoid of free charge carriers. This region is charged with immobile, fixed positive (in the n-type) and negative (in the p-type) ions.

2. **Forward Biasing**: Forward biasing a diode means connecting the positive terminal of the power supply to the p-type material and the negative terminal to the n-type material. This setup reduces the barrier for charge carrier movement across the junction.

### Why the Depletion Layer Width Decreases in Forward Bias

1. **Reduction in Built-In Potential**: The p-n junction has an inherent built-in potential barrier due to the difference in work functions between the p-type and n-type materials. When a diode is forward biased, the external voltage applied reduces this built-in potential barrier. This decrease in the barrier height allows more charge carriers to cross the junction.

2. **Decrease in Electric Field**: The external forward bias voltage opposes the electric field of the depletion region. As a result, the effective electric field across the depletion region is reduced. A weaker electric field means that fewer immobile charges (ions) need to be neutralized, leading to a narrowing of the depletion region.

3. **Increased Carrier Injection**: With forward bias, holes from the p-type side and electrons from the n-type side are pushed toward the junction. These carriers move into the depletion region, where they recombine with the immobile charges. This recombination reduces the region's width because the electric field that was maintaining the separation of charges is reduced.

4. **Recombination and Charge Neutralization**: As more charge carriers cross the junction and recombine in the depletion region, the width of the region decreases. Essentially, the positive and negative ions that were part of the depletion region get neutralized by the influx of mobile carriers, resulting in a smaller depletion region.

### Summary

When a diode is forward biased, the external voltage reduces the built-in potential barrier, weakens the electric field, and increases the number of charge carriers moving across the junction. This leads to recombination of carriers and reduction of the depletion layer width. In other words, the forward bias effectively narrows the depletion region by allowing more charge carriers to neutralize the fixed charges within that region.

Answer 2

In a semiconductor diode, the width of the depletion layer changes with different types of biasing. Here’s a detailed explanation of why the width of the depletion layer decreases when a diode is forward-biased:

### Depletion Layer Basics

In a diode, the depletion layer (or depletion region) is the area around the p-n junction where mobile charge carriers (electrons and holes) have diffused away, leaving behind fixed, immobile ions. This region is characterized by an electric field that opposes the movement of charge carriers.

### Forward Biasing

When a diode is forward-biased, the positive terminal of the power source is connected to the p-type material, and the negative terminal is connected to the n-type material. This setup reduces the barrier potential of the diode. Here’s what happens in detail:

1. **Reduction of Barrier Potential:**
   - In the absence of an external voltage, the built-in potential (or barrier potential) across the junction prevents charge carriers from crossing the junction. This potential is due to the electric field created by the fixed charges in the depletion region.
   - When forward bias is applied, it effectively reduces the built-in potential barrier. This is because the external voltage counteracts the electric field created by the fixed charges in the depletion region.

2. **Increase in Carrier Injection:**
   - Forward biasing provides energy to the electrons in the n-type region and holes in the p-type region. This energy allows more charge carriers to overcome the reduced barrier and recombine across the junction.
   - As more electrons move from the n-type region to the p-type region (and vice versa for holes), the number of carriers in the depletion region increases, which leads to a decrease in the width of the depletion region.

3. **Reduction in Electric Field:**
   - The forward bias voltage decreases the electric field within the depletion region. Since the electric field is directly related to the width of the depletion region, reducing the electric field leads to a narrowing of the depletion region.

4. **Recombination and Depletion of Fixed Charges:**
   - With the reduction in barrier potential, more carriers are injected into the junction, and recombination occurs. This recombination reduces the effect of the fixed charges that originally widened the depletion region.

### Summary

In summary, when a diode is forward-biased:
- The applied voltage reduces the barrier potential, making it easier for charge carriers to cross the junction.
- This reduced barrier allows more carriers to move across the junction, leading to a decrease in the width of the depletion region.
- The electric field in the depletion region weakens, further contributing to the narrowing of the depletion layer.

This behavior is fundamental to the operation of diodes in various electronic circuits, where controlling the width of the depletion region allows for the regulation of current flow.