Question
Explain why elemental semiconductor cannot be used to make visible LEDs.
✓
Answer
Specially designed diodes, which give out light rediations when forward biases. LED's are made of GaAsp, Gap etc.
There are forwad biased P-N junctions which emits spontaneous fadiation. In elemental semiconductor, the band gap is such that the emission are in infrared region and not in visible region. $\lambda=\frac{\text{hc}}{\text{E}_\text{g}}$ For Si; $\text{E}_\text{g}= 1.1\text{eV},\lambda=\frac{1242}{1.1}=1129\text{nm}$ For Ge; $\text{E}_\text{g}=0.7\text{eV},\lambda=\frac{1242}{0.7}=1725\text{nm}$
There are forwad biased P-N junctions which emits spontaneous fadiation. In elemental semiconductor, the band gap is such that the emission are in infrared region and not in visible region. $\lambda=\frac{\text{hc}}{\text{E}_\text{g}}$ For Si; $\text{E}_\text{g}= 1.1\text{eV},\lambda=\frac{1242}{1.1}=1129\text{nm}$ For Ge; $\text{E}_\text{g}=0.7\text{eV},\lambda=\frac{1242}{0.7}=1725\text{nm}$
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