**Correct Option: B. Narrows** ### Detailed Explanation: A semiconductor junction diode consists of two types of semiconductor materials: p-type (which has an abundance of holes) and n-type (which has an abundance of electrons). When these two materials are joined together, they form a p-n junction, which creates a depletion region or barrier layer at the interface. This depletion region is characterized by a lack of charge carriers (electrons and holes) due to recombination. #### Forward Biasing the Diode: 1. **Understanding Forward Bias**: When a diode is forward-biased, a positive voltage is applied to the p-type material and a negative voltage to the n-type material. This external voltage reduces the potential barrier created by the depletion region. 2. **Effect on the Depletion Region**: - In the absence of an external voltage, the depletion region is at its maximum width due to the electric field created by the fixed charges (ionized donor and acceptor atoms). - When forward bias is applied, the external voltage opposes the built-in potential barrier. This causes the energy bands in the semiconductor to bend downwards, allowing charge carriers (electrons from the n-side and holes from the p-side) to move across the junction more easily. - As a result, the depletion region narrows because the applied voltage effectively reduces the width of the barrier that charge carriers must overcome to recombine. 3. **Charge Carrier Movement**: With the depletion region narrowed, more electrons from the n-side can recombine with holes on the p-side, allowing current to flow through the diode. This is the principle behind the operation of a forward-biased diode. ### Why Other Options Are Incorrect: - **Option A: Widens**: This is incorrect because widening of the depletion region occurs under reverse bias, not forward bias. In reverse bias, the applied voltage increases the potential barrier, leading to a wider depletion region. - **Option C: Remains Constant**: This is also incorrect. The depletion region does not remain constant when forward bias is applied; it actively narrows as the external voltage reduces the barrier. - **Option D: Widens then Narrows**: This option is misleading. While it might suggest a transient behavior, in a steady-state forward bias condition, the depletion region consistently narrows as long as the forward voltage is applied. There is no phase where it widens under forward bias. ### Summary of Key Points: - **Depletion Region**: The area in a p-n junction where charge carriers are depleted, creating a barrier. - **Forward Bias**: Applying a positive voltage to the p-side and negative to the n-side reduces the potential barrier. - **Effect of Forward Bias**: The depletion region narrows, allowing charge carriers to flow and current to increase. - **Reverse Bias**: In contrast, reverse bias widens the depletion region, preventing current flow. ### Revision Summary: - In a forward-biased diode, the depletion region narrows due to the applied voltage. - This narrowing allows for increased charge carrier movement and current flow. - The depletion region widens under reverse bias, which is the opposite effect. - Understanding the behavior of the depletion region is crucial for analyzing diode operation.