Electrons Become Photons
A light-emitting diode converts electrical energy directly into light through a quantum mechanical process: radiative recombination. When a forward-biased PN junction injects electrons into the conduction band and holes into the valence band, they recombine at the junction. In a direct-bandgap semiconductor, this recombination releases a photon with energy equal to the band gap. The relationship λ = 1240/E_g (nm) maps band gap energy directly to emission color — from infrared through every visible color to ultraviolet.
Spectral Width and Temperature
An LED does not emit a single wavelength. The spectral width arises from the thermal distribution of carrier energies around the band edges. The full width at half maximum (FWHM) scales with kT — roughly 1.8kT in energy terms, which translates to about 20–40 nm in wavelength for visible LEDs at room temperature. Higher temperature broadens the spectrum and red-shifts the peak slightly as the band gap shrinks.
Efficiency: From Electrons to Lumens
The wall-plug efficiency of an LED — optical power out divided by electrical power in — is the product of three factors: internal quantum efficiency (fraction of recombinations that are radiative), extraction efficiency (fraction of photons that escape the chip), and electrical efficiency (fraction of input power that reaches the junction). Modern blue GaN LEDs achieve IQE above 90% and wall-plug efficiencies exceeding 70%, making them the most efficient light sources ever created.
The White LED Revolution
White LEDs, which illuminate homes and streets worldwide, are typically blue GaN LEDs coated with a yellow phosphor (Ce:YAG). The phosphor absorbs some blue photons and re-emits them as broad yellow light; the combination appears white. This invention, which earned the 2014 Nobel Prize in Physics, has reduced global lighting energy consumption by roughly 40% and continues to improve as phosphor engineering and chip architectures advance.