How does a light-emitting diode (LED) work in a circuit?

An LED works in a circuit by emitting light when an electric current passes through it in the correct direction.

In more detail, a light-emitting diode (LED) is a special type of diode that emits light when it is forward-biased. This means that the current is flowing from the positive (anode) side to the negative (cathode) side of the diode. The LED is made from a semiconductor material, typically gallium arsenide, which is doped to create a P-N junction.

When a voltage is applied across the LED, electrons from the N-type material start moving towards the P-type material. At the same time, holes (which are essentially vacancies where an electron could exist) in the P-type material move towards the N-type material. When an electron and a hole meet at the junction, they combine and the electron drops into a lower energy level. This drop in energy is released in the form of a photon, which we see as light.

The colour of the light emitted by an LED depends on the energy gap of the semiconductor material used. For example, a larger energy gap will produce a higher energy photon, which corresponds to a shorter wavelength of light (towards the blue end of the spectrum). Conversely, a smaller energy gap will produce a lower energy photon, which corresponds to a longer wavelength of light (towards the red end of the spectrum).

It's important to note that LEDs are polarised, meaning they only allow current to flow in one direction. If you connect an LED the wrong way round in a circuit (i.e., reverse-biased), no current will flow and the LED will not light up. This is why it's crucial to connect the LED correctly, with the longer leg (anode) connected to the positive side of the circuit and the shorter leg (cathode) connected to the negative side.