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

Step 1: Understanding the Concept:
A P-N junction consists of a p-type and an n-type semiconductor joined together.
At the interface, holes from the p-side and electrons from the n-side diffuse across and recombine, creating a region devoid of free charge carriers known as the depletion region.
This region contains uncompensated, immobile ions (negative on the p-side, positive on the n-side) that generate an internal electric field (\(E_i\)).
This field creates a built-in potential barrier (\(V_0\)) that opposes further flow of majority carriers.
Forward bias is applied when the positive terminal of an external source is connected to the p-side and the negative terminal to the n-side.
Step 2: Detailed Explanation:
Let's analyze the physical effects of applying a forward bias voltage \(V\):
1. Effect on Barrier Height: The external electric field (\(E_e\)) created by the battery points from the p-side to the n-side. This field is directly {opposite} to the internal built-in field (\(E_i\)). As a result, the net electric field at the junction is weakened. The effective potential barrier becomes \(V_{eff} = V_0 - V\). Because the height of this electrical "hill" is reduced, majority charge carriers can cross the junction much more easily. Thus, the barrier height decreases.
2. Effect on Depletion Width: The positive terminal of the battery repels holes in the p-region toward the junction, and the negative terminal repels electrons in the n-region toward the junction. This forceful push drives majority carriers into the depletion zone, where they neutralize some of the exposed ions. This reduction in the volume of the space-charge region causes the depletion width to shrink or decrease.
As both the barrier and the depletion layer are reduced, the resistance of the diode drops, allowing a significant current to flow through the circuit.
In summary, forward bias "compresses" the junction's electrical obstacles.
Step 3: Final Answer:
Under forward bias, the width of the depletion region decreases and the barrier potential height decreases.