Question

When an external operating bias voltage is applied to a standard P-N junction semiconductor diode under a forward bias configuration, how do the width of the internal depletion region and the height of the contact barrier potential change?

• A. \( \text{The depletion width increases and the barrier height increases.} \)
• B. \( \text{The depletion width decreases and the barrier height decreases.} \)
• C. \( \text{The depletion width increases and the barrier height decreases.} \)
• D. \( \text{The depletion width decreases and the barrier height increases.} \)

Hint:

Think of forward bias as an assisting force that compresses the junction: it shrinks the depletion width and lowers the potential barrier. Reverse bias does the opposite, pulling carriers away to widen the depletion layer and raise the barrier.

Answer 1

The Correct Option is B

Concept: A clean P-N junction forms an internal depletion layer at its interface. Free electrons from the n-type region diffuse across the junction into the p-type region, where they recombine with available holes. This migration leaves behind uncompensated positive donor ions on the n-side edge and negative acceptor ions on the p-side edge. This localized region of uncovered ions is depleted of free charge carriers, creating the depletion region. These opposite ions generate an internal electric field (\(E_i\)) pointing from the n-side to the p-side. This field creates a built-in potential barrier (\(V_0\)) that stops further diffusion, stabilizing the junction.

Step 1: Analyze how forward biasing affects the internal electric field. In a forward bias setup, you connect an external battery so its positive terminal attaches to the p-type side and its negative terminal attaches to the n-type side. This external voltage sets up an electric field (\(E_e\)) that points from the p-side to the n-side. Because this external field runs directly opposite to the built-in internal electric field (\(E_i\)), it weakens the overall field at the junction.

Step 2: Evaluate changes to the barrier potential height. Since the external voltage opposes the built-in potential barrier, it lowers the net barrier height. If the built-in barrier potential is \(V_0\) and you apply a forward voltage \(V\), the effective potential barrier drops to: \[ V_{\text{effective}} = V_0 - V \] This reduction lowers the electrical barrier, making it much easier for majority charge carriers to cross the junction.

Step 3: Evaluate changes to the depletion region width. The negative terminal of the external battery repels free electrons in the n-region toward the junction, while the positive terminal repels holes in the p-region toward the junction. This forcing action drives majority carriers into the depletion zone, neutralizing some of the exposed boundary ions. As a result, the width of the depletion layer shrinks. Consequently, both the depletion region width and the barrier potential height decrease under a forward bias configuration.