How does an LED work?

source:wikipedia.org

When you have an isolated atom of semiconductor, each electron in that atom occupies a discreet energy level. You can think of these energy levels like individual seats from a hockey stadium, and all atoms of the same element, when they are far apart from each other, have identical available energy levels. But when bring multiple atoms together to form a solid, something interesting happens. The outermost electrons now feel the pole, not only of their own nucleus but of all the other nuclei as well. As a result, their energy levels shift. So instead of being identical, they become a series of closely spaced, but separate energy levels. An energy band. Electrons with the highest energy band are known as the valence band, and the next higher energy band is called the conduction band. You can think of it like the balcony level. In conductors, the valence band is only partially occupied. This means with a little bit of thermal energy, electrons can jump into nearby unfilled seats, and if an electric field is applied, they can jump from one unfilled seat to the next and conduct current through the material. In insulators, the valence band is full, and the energy difference between the valance band and the conduction bond is too large. So when an electric field is applied, no electrons can move. There are no vacant seats within the valence band and the band gap is too big for electrons to jump into the conduction band which brings us to semiconductors. Semiconductors are similar to insulators, except the band gap is much smaller. This means at room temperature, a few electrons will have sufficient energy to jump into the conduction band, and now they can easily access nearby empty seats and conduct current. Not only that, but the empty seats they left behind in the valence band can also move. Indeed, it is the adjacent electrons jumping into those empty seats. But if you look from afar, it appears as if the vacant seat or void is in motion, resembling a positive charge moving in the opposite direction to the electrons within the conduction band. By themselves, pure semiconductors are not that useful. To make them way more very functional, you have to add impurity atoms into the lattice. This is known as doping. For example, in silicon, you can add a small amount of phosphorus atoms. The phosphorus atom is similar to a silicon atom, that why it is easily fits into the lattice, but it brings with it one extra valence electron. This electron exists at a donor level just beneath the conduction band. So with little amount of thermal energy, all these electrons can jump into the conduction band and conduct current. Since most of the charges that can move in this type of semiconductor are negative electrons, this sort of semiconductor is called n-type, n for negative, but It is important to highlight that the semiconductor remains neutral. It's just that most of the moving(mobile) charge carriers are negative. They're electrons. So there is also another type of semiconductor where most of the mobile charge carriers are positive, and it's called p-type. To make p-type silicon, you need to add a small number of atoms of, boron. Boron fits into the lattice but brings with it one fewer valence electrons than silicon. So it creates an empty acceptor level just above the valence band. And with a bit of thermal energy, electrons can jump out of the valence band, leaving behind holes. These positive holes are mostly responsible for carrying current in the p-type semiconductor. Again, the material overall is uncharged, it's just that most of the mobile charge carriers are positive holes. Where things get interesting is when you put a piece of p-type and n-type together. In the p-type semiconductor without even connecting to a circuit, some electrons will diffuse from n to p and fall into the holes. This makes the p-type a slight negatively charged, and the n-type a slight positively charged. So there is now an electric field inside an inert piece of material. Electrons keep diffusing until the electric field becomes so large, that it prevents them from crossing over And now we have established the depletion region, an area depleted of mobile charge carriers. There are no electrons in the conduction band and no holes in the valence band. Suppose you connect a battery the wrong way to this diode. In that case, it simply expands the depletion region until its electric field perfectly opposes that of the battery and no current flows. But if you flip the polarity of the battery, then the depletion region shrinks, the electric field decreases, and electrons can flow from n to p. When an electron falls from the conduction band into a hole in the valence band, that band gap energy can be emitted as a photon. The energy change of the electron is emitted as light, and this is how a light-emitting diode might work. The size of the band gap determines the color of the emitted photon. In pure silicon, the band gap is only 1.1 electron volts. So the photon released isn't visible range, it's infrared light. These LEDs are actually used in remote controls for your TV, and you can capture them on camera. Moving up the spectrum, you can see why the first visible light LEDs were red and then green, and why blue was so hard. A photon of blue light requires more energy, and therefore a larger band gap.