P-N Junction Diode and Depletion Layer

A P-N junction diode is a fundamental semiconductor device formed by joining P-type and N-type semiconductor materials. At the interface of these two materials, a region known as the depletion layer is formed.

Depletion Layer Formation

The depletion layer is created due to the diffusion of majority charge carriers across the junction. Holes from the P-side diffuse to the N-side, and electrons from the N-side diffuse to the P-side. When they cross the junction, they recombine, leaving behind uncompensated immobile ions (positive ions on the N-side and negative ions on the P-side). This region, which is depleted of free charge carriers, acts as an insulating layer and presents a built-in barrier potential.

Effect of Bias on Depletion Layer Width

The width of the depletion layer is significantly influenced by the type of external bias voltage applied across the P-N junction diode.

Forward Bias and Depletion Layer

• When a forward bias is applied, the positive terminal of the external voltage source is connected to the P-type material, and the negative terminal is connected to the N-type material.
• This external voltage opposes the built-in barrier potential of the junction.
• As a result, the effective barrier potential is reduced.
• The majority charge carriers gain enough energy to easily cross the junction.
• Consequently, the width of the depletion layer decreases, allowing a large current to flow through the diode.

Reverse Bias and Depletion Layer

• When a reverse bias is applied, the negative terminal of the external voltage source is connected to the P-type material, and the positive terminal is connected to the N-type material.
• This external voltage adds to the built-in barrier potential of the junction.
• The effective barrier potential across the junction increases significantly.
• This increased barrier potential makes it very difficult for majority charge carriers to cross the junction.
• To sustain this higher potential difference, the region devoid of free charge carriers, i.e., the depletion layer, must expand.
• Therefore, the width of the depletion layer increases when a reverse bias is applied. This increased width acts as a higher resistance to current flow, allowing only a very small leakage current.

Summary of Bias Effects on Depletion Layer Width

Based on the analysis, the width of the depletion layer in a P-N junction diode increases when a reverse bias is applied because the reverse bias adds to the built-in barrier potential, requiring the depletion region to widen to accommodate the larger potential difference.