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Question
Explain why elemental semiconductor cannot be used to make visible LEDs.
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Solution
Specially designed diodes, give out light radiations when forward biases. LEDs are made of GaAsp, Gap etc.
These are forward-biased P-N junctions which emit spontaneous radiation.
In element semiconductor, the band gap is such that the emission is in the infrared region and not in the visible region.
`λ = (hc)/E_g = (1242 eV nm)/E_g`
For Si; `E_g = 1.1 eV, λ = 1242/1.1` = 1129 nm
For Ge; `E_g = 0.7 eV, λ = 1242/0.7` = 1725 nm
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