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Question
Explain why elemental semiconductor cannot be used to make visible LEDs.
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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