Light-emitting diode (LED)

An LED is a diode that turns electricity directly into light. Like any diode it lets current flow one way, but it's made from special semiconductors (such as gallium nitride or gallium arsenide) where every electron crossing the junction can drop into a 'hole' and release its leftover energy as a single particle of light — a photon. No glowing-hot filament, no waste heat bonfire: the current is converted to colour almost straight from the electrons, which is why a 9 W LED bulb can outshine a 60 W incandescent.

The colour is set by the chip's bandgap — the energy gap an electron falls across — not by temperature. A wide gap gives a blue photon, a narrow one gives red; engineers tune the alloy to pick the wavelength. White LEDs cheat: a blue LED is coated with a yellow phosphor that re-emits part of the blue light, and blue + yellow reads as white to the eye. The 2014 Nobel Prize in Physics went to the inventors of the efficient blue LED precisely because it was the missing piece that finally made white solid-state lighting possible.