Concept:An LED is a heavily doped PN junction made from direct bandgap semiconductors (e.g., GaAs, GaP) that emits photons via electroluminescence during forward-bias electron-hole recombination.
Formula:$$E_{\text{photon}} = h f = \frac{h c}{\lambda} \approx E_g$$
Solution:- Under forward bias, electrons are injected into the P-region and holes into the N-region.
- These excess minority carriers recombine with majority carriers near the junction, releasing energy as visible or infrared photons.
Why other options are incorrect:- Option B: In reverse bias, carriers are pulled away from the junction, preventing recombination and light emission.
- Option C: In an unbiased diode, net current is zero and no excess recombination occurs.
- Option D: Reverse breakdown damages standard LEDs and does not produce efficient electroluminescence.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.