LEDs are semiconductor devices. Like transistors, and other diodes, LEDs are
made out of silicon. What makes an LED give off light are the small amounts of
chemical impurities that are added to the silicon, such as gallium, arsenide,
indium, and nitride.
When current passes through the LED, it emits photons as a byproduct. Normal
light bulbs produce light by heating a metal filament until it's white hot.
Because LEDs produce photons directly and not via heat, they are far more
efficient than incandescent bulbs.
Not long ago LEDs were only bright enough to be used as indicators on dashboards
or electronic equipment. But recent advances have made LEDs bright enough to
rival traditional lighting technologies. Modern LEDs can replace incandescent
bulbs in almost any application.
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