Light-emitting diodes (LEDs) give off light when current is passed through them. They are made from semiconducting materials, with the colour of light depending on the materials used. They are usually gallium-based compounds.
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Christmas isn’t far off now, and whether you’re celebrating it or not, you may well have started seeing Christmas lights starting to appear adorning houses and Christmas trees. How do these lights actually work, and how can they be made to produce such an array of colours? This graphic takes a look at the chemistry.

LED stands for light-emitting diode, and they come in a whole range of colours, from reds and oranges to blues and violets. Though they may look small, they’re packing some serious science – in fact, the 2014 Nobel Prize in physics went to scientists who worked on discovering how to make efficient blue LEDs. Before we discuss that though, let’s begin with the basics.

It makes sense to start by explaining how LEDs can produce light in the first place. LEDs are made of semiconducting materials, materials which conduct electricity under some conditions but not others. Several different semiconducting materials can be used in LEDs, but a lot of them are gallium-based, for example, gallium nitride and gallium phosphide.

LEDs consist of two layers of semiconducting material. The layers are “doped” with impurities, which is to say that atoms of elements other than those originally in the semiconducting material are mixed in. This doping can create different types of layers: p-type layers and n-type layers. The n-type layer has a surplus of electrons, whereas the p-type layer has an insufficient number of electrons, and as such has what are referred to as electron ‘holes’: positions in atoms where an electron could be, but isn’t.

When a current is applied to the LED, the electrons in the n-type layer and the electron ‘holes’ in the p-type layer are driven to an active layer between the two. When the electrons and electron ‘holes’ combine, energy is released, and this is seen as visible light. While this explains how light is produced, we have to look a little more closely at what’s going on to explain how different colours can be obtained.

The colours obtained from LEDs are determined by the semiconducting materials used. As you can see in the graphic, there’s not just one material used for all of the different colours, but a range of possibilities. By using different materials, and adding different impurities to these materials, we can change the size of the band gap – that is, the size of the energy difference between the n-type layer and the p-type layer. The bigger this band gap, the shorter the wavelength of light produced by the LED. So for a red LED, a relatively small band gap is required. For blue LEDs, a larger band gap is needed.

Smaller band gap LEDs were easier to produce, but producing LEDs with the small band gap needed to produce blue light proved more problematic. This was important to work out because red, green and blue LEDs were all needed to produce white light. In the early 1990s, scientists finally worked out how to produce blue LEDs using gallium nitride, and were awarded the 2014 Nobel Prize for their work.

Today LEDs aren’t just found in Christmas lights, but also in many normal light bulbs. They have many advantages over traditional bulbs: they last longer compared to conventional bulbs (up to 100,000 hours compared to 1,000 hours for incandescent bulbs) and they are more energy efficient, requiring less energy to emit the same amount of light. Thanks to LEDs, the electricity bill for that house covered from roof to foundations in Christmas lights isn’t quite as high as it could be!

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