Let there be light
Light-emitting diodes, or LEDs, take electricity and make light. Those familiar with solar cells could think of an LED as a solar cell working backwards. LEDs are diodes typically made of direct bandgap semiconductors like gallium arsenide (GaAs). Though they can be made of indirect semiconductors like gallium phosphide (GaP), they are less efficient and produce more heat.
The light-emitting portion is made up of two doped semiconductors. A semiconductor is a material that conducts electricity poorly unless it is either doped, heated, or illuminated with light. Each one has elements added to it through a process called doping. These dopant elements essentially take away (p-type) or add (n-type) electrons to the bulk of the material, thus forming positive and negative regions, respectively. This can be seen as adding negatively charged electrons and positively charged holes.
When these regions are put together, the opposite charges move across the pn junction to combine with each other and cancel out. This process is called recombination. Recombination results in this segment at the junction having a positive charge on the n-side and negative charge on the p-side. This produces a voltage and thus an electric field that pushes electrons back to the n-side and holes back to the p-side.
Each semiconductor has certain energies in which charge carriers (electrons and holes) can exist. In semiconductors, there is a gap in the middle of the ground layer (valence band, or VB) and conduction layer (conduction band, or CB) called the bandgap. We’ll see soon why this is important. The edges of the VB and CB for the semiconductors bend when the two materials come into contact. An energy hill, or barrier, is formed between materials. Charge carriers need energy to overcome this barrier which can be provided by a voltage source, like a battery.
Applying a voltage in a certain way will make the barrier more shallow. This is called biasing the diode. For an LED, we need to forward bias it. This means that the positive part of the source needs to be in contact with the positive side of the LED (p-side). Not only does forward bias make it easier for charges to move, but the bias voltage forces them to move.
When particles move across the junction, represented as the hill in the band diagram, they recombine and release a photon equal to the bandgap energy. This is due to the fact that electrons and holes will seek out states of lower potential energy. The electrons will be forced up the slope, but will tend to step down into the lower states seen in the valence band.
The bandgap energy released during recombination corresponds to the color of the light they emit. Large bandgaps produce bluer light while smaller bandgaps produce redder light. Some LEDs are designed to emit ultraviolet (UV) light, while some are designed to emit infrared (IR) light.
Since the carriers recombine directly via radiative recombination, very little heat is produced. Ideally, no heat is produced; however, since there will be some carriers that combine indirectly, some heat is produced via the emission of phonons. Phonons are released to conserve momentum during indirect recombination. LEDs are so efficient because of the recombination mechanism. While up to 18% of the electricity going into incandescent bulbs actually produces light, LEDs have been made to produce 2.3 times as much light as electricity going into it. In other words, incandescent bulbs could have an efficiency of 18%, while LEDs could have an efficiency of 230%. Most LEDs we see are not nearly this efficient, and are in the range of 60-98% efficient.
Much of the losses in light output of LEDs don’t have to do with the LEDs themselves, but with losses in the phosphors in the bulbs. LEDs typically produce light in the blue range, so in order to produce other colors, phosphors need to be added. They have different bandgaps than the LEDs. They take the energy from the light of the LED and convert it to light of another wavelength, or color. Since blue light has higher energy than say, yellow, some of the energy in the blue light goes to waste as heat, while the rest of the energy converts to yellow light.