LEDs are a type of diode, which emit light when an electric current is passed through them. They are made of semiconducting materials, which only allow current to flow one way. The movement of electrons to different energy levels is the reason for the light being emitted.
Diodes contain p-type and n-type semiconductor. The n-type semiconductor has an excess of electrons whereas the p-type has too few electrons so also contains regions called ‘holes’. Between the two layers is what is known as the depletion layer.
When a positive terminal is connected to the p-type semiconductor, the holes will be repelled and the electrons from the n-type semiconductor will be attracted. This causes the depletion layer to shrink and eventually disappear. A current can flow, this is forward bias:
When a negative terminal is connected to the p-type semiconductor, the holes will be attracted and the electrons from the n-type semiconductor will be repelled. This causes the depletion layer to grow. A current cannot flow, this is reverse bias:
Why do LEDs emit light?
When a current flows in an LED, electrons from the n-type semiconductor fill the holes in the p-type. Because they move from a higher energy level to a lower energy level, the excess energy is emitted in the form of photons (light).
Different colours can be produced by using different materials. This is because the gap between energy levels changes so the photons have different amounts of energy, therefore changing the wavelength and therefore the colour.
What advantages do LEDs have over filament bulbs?
LEDs firstly use a lot less energy than filament bulbs. This is due to the fact that filament bulbs use heat in order to produce light and therefore a huge amount of energy is wasted in heating – around 80% – so they are much less efficient.
LEDs also usually last much longer and are more durable. Another useful feature is that they can be very small, so they can be used in circuit boards easily and can be put together to produce a screen.