Question

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

Answer 1

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