light-emitting diode
/ lyt ee-MIT-ing DY-ohd /
When a ball rolls off a high shelf onto the floor, it gives up energy with a thud. Inside a light-emitting diode, an electron does something similar but cleaner: it drops from a high-energy state to a low one, and instead of a thud it releases a flash of light. Push enough electrons across and you get a steady glow.
An LED is a p-n junction made from special materials and run in forward bias, so electrons from the n-side and holes from the p-side are driven together and recombine at the junction. Each recombination releases a packet of energy as a photon — a particle of light. The crucial part is that the size of the material's band gap sets how much energy each photon carries, and therefore its color: a wider gap means bluer light, a narrower gap redder light.
LEDs matter because they convert electricity into light far more efficiently than the old glowing-filament bulb, which wastes most of its energy as heat. They underpin modern lighting, screens, and remote controls, and the blue LED that completed the set won a Nobel Prize in 2014. An honest caveat: plain silicon makes a poor LED — its band gap releases energy mostly as heat rather than light — so LEDs rely on compound semiconductors like gallium nitride or gallium arsenide.
A red LED is often made from gallium arsenide phosphide, a blue one from gallium nitride. Mix red, green, and blue LEDs at the right brightness and your eye reads the blend as white — which is how LED screens and white LED bulbs produce every shade.
The band gap of the material picks the color; mixing colors makes white light.
Many 'white' LEDs are not truly white inside — they are blue LEDs coated with a yellow phosphor that absorbs some blue and re-emits yellow, so the combined light looks white to the eye.