Q.18 Explain the working of an LED and list its applications.
Best answer
Answer: An LED is a forward-biased p-n junction in which electrons and holes recombine and release energy as light (electroluminescence).
Working: when the LED is forward biased, electrons from the n-region and holes from the p-region cross the junction. When a free electron in the conduction band recombines with a hole in the valence band, it falls to a lower energy level and gives up the energy difference (about the band gap \(E_g\)) as a photon. In LEDs this recombination is radiative because they are made of direct band gap compound semiconductors (GaAs, GaAsP, GaP, GaN), not silicon. The colour depends on the band gap: \(\lambda=\dfrac{hc}{E_g}\) (GaAs: infrared, GaAsP: red/yellow, GaN: blue). Forward voltage is about 1.5–3.5 V and a series resistor limits the current.
Applications: indicator and pilot lamps, seven-segment displays, LED TV backlights and display panels, domestic and street lighting, traffic signals, automobile lights, IR LEDs in TV remote controls and optocouplers, optical-fibre communication.
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