Light Emitting Diode
A light emitting diode glows only when its junction is forward biased inside a display panel, and an engineer explains the physical process producing the light.
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About This Question
- Subject
- physics
- Chapter
- electronic devices
- Topic
- light emitting diode
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
Electrons and holes recombine at the junction releasing energy as photons
NCERT explains that a light emitting diode is a heavily doped p-n junction operated in forward bias. Under forward bias, electrons from the n-side and holes from the p-side are injected across the junction and recombine near it. Each recombination releases energy roughly equal to the band gap, and in suitable semiconductor compounds this energy is emitted as a photon of light, so the diode glows. The colour of the light depends on the band gap of the material used. The option about reverse breakdown is wrong because an LED emits under forward bias through recombination, not through reverse breakdown, which would damage an ordinary diode. The option about a heated filament is wrong because an LED has no filament and does not rely on incandescence; it is a cold light source based on electroluminescence. The option about absorption and re-emission is wrong because the LED generates light through recombination, not by absorbing external photons. As stated in NCERT Class 12, Chapter 14, forward-biased recombination in an LED produces light of energy near the band gap. A consistency check confirms that a larger band gap yields higher-energy, shorter-wavelength light, matching how LED colour depends on the material.
This easy difficulty physics question is from the chapter electronic devices, covering the topic of light emitting diode. It appeared in the 2025 exam.
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