Why does an LED light up but only when connected the right way round?
An LED is a p–n junction that emits light when forward-biased: electrons and holes recombine across the junction and release the energy as photons, with the colour set by the semiconductor's band gap.
An LED is a p–n junction that emits light when forward-biased: electrons and holes recombine across the junction and release the energy as photons, with the colour set by the semiconductor's band gap. Connect it backwards (reverse bias) and no recombination current flows, so it stays dark. That's why LEDs have a fixed polarity, unlike a filament bulb.
Key point
A forward-biased LED emits light as electrons and holes recombine across the junction.
Common mistake
wiring an LED either way and expecting it to glow.
Memory tip
LEDs are polarity-sensitive — only forward bias lights them.
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