Q.Explain the working of a LED.

[!TLDR] Forward biasing an LED injects electrons and holes that recombine at the junction, releasing energy as photons whose colour is set by the material's band gap; light output grows directly with forward current. [!ANSWER] Forward bias drives electron-hole recombination at the p-n junction; each recombinati …

An LED is a forward-biased p-n junction, heavily n-doped relative to the p-side, encased in a dome-shaped transparent case that lets light escape efficiently in all directions while minimising internal reflection losses. When forward biased, electrons are injected from the n-side (conduction band) across the junction into the p-side, where they RECOMBINE with holes present in the valence band. Each individual recombination event releases its excess energy -- approximately equal to the semiconductor's band gap $E_g$ -- as an emitted PHOTON, rather than as heat, provided the material is a suitable (typically direct-band-gap) compound semiconductor chosen for this purpose. Because this energy corresponds to a fairly well-defined value set by $E_g$, the light produced is essentially MONOCHROMATIC (a single colour), with the specific colour/wavelength determined by the material used (e.g. GaAsP for red/yellow, AlGaP for red/green, ZnSe for blue, AlGaAs for infrared -- by varying the elements and their proportions in the compound). Because the junction region is thin, a substantial fraction of the generated photons can escape the device without being reabsorbed, and the LED is fabricated so recombination happens preferentially near the surface, maximising the light that actually leaves the device. The amount of light OUTPUT is directly proportional to the forward …

Trace the working mechanism step by step: forward bias -> carrier injection -> recombination at the junction -> photon emission (colour se …

Describing LED light emission as simply 'the junction glowing when current flows', without identifying the actual mechanism (electron-hole recombination releasing photons) or explain …