An old-fashioned light bulb works by brute force: push electricity through a thin wire until it glows white-hot, wasting most of the energy as heat. An LED does something far cleverer and cooler. It's built from a special material, a semiconductor, in which electrons can sit at two different energy levels with a gap in between, like two shelves with a step down from one to the other. When you push electrons across the join inside the LED, they drop from the high shelf to the low one, and each electron pays for its fall by spitting out a single tiny packet of light called a photon. No heat needed. Below a certain push, the turn-on voltage, the electrons can't make the jump, so the LED stays dark. The size of the step decides the colour: a small step makes red light, a bigger step makes blue. Slide the voltage and the gap in the simulator and watch the colour change.
Most people think an LED makes light by getting hot, like a tiny bulb. In fact it stays cool: light comes from electrons dropping across the band-gap, each emitting a single photon whose energy sets the colour.
What's actually happening
For more than a century, making light meant making heat. The incandescent bulb runs current through a tungsten filament until it reaches a few thousand degrees and glows, which works but is wildly wasteful: the overwhelming majority of the energy leaves as warmth, not light. The light-emitting diode threw that whole approach out. An LED can run cool to the touch and still shine, because it doesn't coax light out of hot atoms at all. It pulls light directly out of the behaviour of electrons in a carefully engineered crystal, one electron and one photon at a time.
The crystal is a semiconductor, and the key to it is a feature called the band-gap. Electrons in the material are allowed to occupy a lower energy band or a higher one, but not the range of energies in between — that forbidden zone is the gap. Picture two shelves with a clean step between them. An LED is built as a junction between two slightly different versions of the semiconductor, and when you connect a battery the right way round, you drive electrons across that junction. As each electron crosses, it tumbles from the upper band down into a vacancy in the lower band, and the energy it sheds in that single quantum step is emitted as one photon of light. This is the crucial idea: the light comes from electrons making a defined drop, not from anything getting hot. There's a threshold, though. If the push from the battery is too gentle, electrons can't get across the junction at all, no drops happen, and the LED stays completely dark. Only once the voltage passes the turn-on value, set by the size of the gap, does current flow and light pour out.
The most elegant part is the colour. Because every photon carries exactly the energy of one electron's drop, and that drop equals the band-gap, the band-gap alone decides the colour of the light. A small gap means a low-energy photon, which is red or even invisible infrared; a larger gap means a higher-energy photon, climbing through yellow and green to blue and beyond into ultraviolet. Engineers pick the colour by choosing the semiconductor's composition to set the gap. Red and green LEDs arrived decades ago, but blue, needing a wide gap and a difficult material, held out until the 1990s, when Shuji Nakamura and his colleagues finally cracked it — work so important it earned the 2014 Nobel Prize in Physics and, by completing the trio of red, green and blue, made white LED lighting and modern screens possible. In the simulator you can push the voltage past turn-on and stretch the gap, watching the emitted photon march across the whole spectrum.
An LED turns electricity straight into light by letting electrons drop across a band-gap, and the size of that gap chooses the colour.
- 1Find a simple LED on a circuit kit or a clear gadget and a way to vary the voltage across it, such as a potentiometer and a small battery.
- 2Turn the voltage up slowly from zero. Notice that nothing happens at all for a while, then the LED suddenly lights once you pass its turn-on voltage — the energy needed to push electrons across the gap.
- 3Compare a red LED with a blue one: the blue lights at a noticeably higher voltage, because its wider band-gap demands more energy per electron, exactly as the simulator's two sliders show.
Common questions
An incandescent bulb makes light by heating a filament until it glows, wasting most of its energy as heat. An LED instead releases light when individual electrons drop across the band-gap, so it can produce light directly and efficiently without needing to get hot.
Each photon carries exactly the energy an electron loses as it drops across the gap, and that energy fixes the colour through E = hc/λ. A small gap gives a low-energy red or infrared photon; a wider gap gives a higher-energy blue or ultraviolet one.
Blue light needs a wide-gap semiconductor that proved extremely difficult to grow and dope. Red and green LEDs appeared decades earlier, but a bright blue LED arrived only in the 1990s, a breakthrough that won the 2014 Nobel Prize in Physics and enabled white LED lighting.