How do LEDs REALLY work?
(c)1997 William J. Beaty

Light Emitting Diodes (LEDs) create light in much the same way as flourescent tubes or neon signs. In an LED's crystal the electrons of its atoms are pumped up to higher energy states. When they fall back down again, each atom gives off a particle/wave of light. However, the electrons in an LED are not exactly the same as the ones in gas molecules in a neon sign. LED electrons aren't in orbitals stuck to individual atoms. Instead the electrons occupy a contiguous "sea of charge," and they continually wander among all the atoms in the crystal material. But while they do this, they maintain a particular energy level just like they do when stuck to individual atoms. It's as if each electron in an LED crystal was "orbiting" among all the atoms of the substance as a whole, and each electron always "orbits" at a particular "height" above each of the atoms it passes.

Be aware that *all* substances contain electrons. The electrons I'm discussing here are not supplied by the battery or power supply. Instead they occur naturally in the wires, in crystals, etc. They are in the LED all the time, even when a battery is not connected. Don't make my original mistake by imagining that electrons are injected into the LED by the power supply. In fact, they're already in the material, and the power supply simply forces them to flow along.

To create LED light, first we connect two conductive crystals of different characteristics together. Both types of crystal contain movable electrons. In one type of crystal the electrons "orbit" naturally at a high energy level, and in the other, they always "orbit" low. When a voltage is applied across the joined crystals, the electrons inside are forced to flow across the boundary between the pair of crystals. If the flow direction is correct, electrons in the "high" crystal flow into the "low" crystal and must begin orbiting at the lower energy level. As they fall to the lower energy level, they give off light. The frequency of the light (which we see as color) is determined by the difference in energy levels between the two crystals. By manufacturing different types of crystals having different natural energy levels, various colors of light can be created. Crystals with similar levels create low-energy photons of red light or even infrared light. With a larger difference in energy levels, green light can be created. An even larger energy-step can create blue light, or violet, or UV.

The "high" and "low" crystals are usually called "n-type" and "p-type." In n-type crystals the movable electrons wander around while staying at the upper energy level of an unfilled outer atomic orbital. During an electric current they travel at this level. In "p-type" crystals the mobile electrons naturally exist at a deeper orbital level. When the two crystals are connected to each other and then connected properly in a circuit with a battery, the battery provides a voltage-difference which creates a current in the entire circuit. It sucks electrons out of the end of the p-type crystal and into the wire. At the same time it pushes electrons into the far end of the n-type crystal. The electrons present inside the n-type crystal then are forced to flow across the crystal junction, fall down in energy, emit light, and end up back in the p-type crystal.

Where did the electrons get the energy to emit light? How do they get to a higher energy level so they can enter the n-type crystal? Well, in order for the battery to push electrons through the LED, it had to apply electrical attraction and repulsion forces to the electrons in the crystal. To apply force to the electrons in the crystal, it had to apply a force to the electrons in the negative wire. This squeezes all the electrons on the surface of the negative wire together, which raises the voltage of the entire wire. (If electrons were like water, then the wire is like a long trough. The battery pumps water into one end of the trough, and this makes the water level 'voltage' rise everywhere in the trough.) When the negative wire's electrons get to the energy level equal to the n-type crystal, they start flowing into the crystal and falling "down" the junction, emitting light as they go. (This analogy is incomplete: at the same time that the battery was pumping up the "water level" of the negative wire to match the n-type crystal level, it also was REDUCING the "water level" of the positive wire so that the low-energy electrons of the p-type crystal could be sucked into the wire.)

Here's another way to visualize LEDs. In a neon sign, the electrons orbiting around each neon atom get pumped up in energy as they're whacked by incoming high-speed electrons. In an LED the battery pumps up the atoms' electrons directly. In a neon sign, each atom emits light when an electron spontaneously falls back to its original energy level. In an LED, the whole crystal junction emits light as electrons drop back to a lower level. Therefor an LED resembles ...a gigantic neon atom! An LED/atom is so large that we can use wires to connect its electron cloud directly to a battery. It's so large that we can build in different orbital characteristics, and change the color of its flourescence.

Light Emitting Diodes are much like solar cells. Both devices use n-type and p-type crystals, but in solar cells the process runs backwards: instead of electrons falling down in energy and emitting light, the light hitting the solar cell causes electrons in the p-type crystal to jump upwards in energy. If these electrons are near the crystal junction, they can end up in the n-type crystal, and they can flow through wires to the outside world, falling down in energy as they do. In fact, if light shines on an LED, the LED behaves as a tiny, inefficient solar cell. And conversely, if a battery is used to create a current in a solar cell, the solar cell can emit a very tiny amount of (mostly infrared) light. An LED gives light when charges are pumped through it, and when light shines on a solar cell, the solar cell becomes a charge-pump.

Light Emitting Diodes are also like thermocouples. N-type and p-type crystals are not the only materials whose electrons "orbit" at different energy levels. Different metals have different levels too. If a copper wire is twisted together with an iron wire, a junction is formed between them which contains an energy-step like that of an LED. The energy-step in a thermocouple is much smaller than in an LED. If electrons are forced to flow across the thermocouple's energy step, they fall down in energy level and emit energy. But what do they emit? Longwave Infrared light and crystal vibrations. Together we call these by the name "heat energy". The energy step in a thermocouple is too small, so it cannot emit photons of visible light. Instead it creates "heat." And conversely, if heated, a thermocouple can create an electric current. When operated one way, a thermocouple is a bit like an LED which emits heat. When operated the other way, it acts a bit like a "solar battery:" it becomes a "heat-powered battery."

Don't you love the way that different parts of physics can hang together and seem the same?



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