Quantum in LEDs – let there be light!

Cost-effective, efficient and durable – light-emitting diodes or LEDs for short have almost completely replaced conventional light bulbs. But what is behind the miracle lamps? Plain and simple: (quantum) physics!

After we have dealt in the last part with the birth of quantum physics and its basic ideas, this time we will deal with concrete applications in our everyday life.

Moving charge = current

The heart of an LED is a tiny semiconductor crystal. In principle, each material can be divided into one of three categories: Insulator, semiconductor and metal. The classification is determined by how well the material can conduct electricity. Since electricity is nothing more than moving charge (usually the light, negatively charged electrons), the more electrons can move freely in a material, the better it conducts.

Not every atom wants to share its electron

However, different kinds of atoms handle their electrons differently. While some like to share their electrons with other atoms, others are reluctant to let them go and keep them captive. Quantum physical considerations can be used to find out which type of atom acts how. This is because both the atoms and the electrons are so small that they follow the rules of quantum physics.

Metals, semiconductors, insulators

This finally results in the three categories. One extreme case are metals whose electrons can stay freely in the material and therefore conduct electricity very well. The other are insulators, which actually do not have any free electrons and do not conduct electricity at all. In between are the semiconductors.

With semiconductors, the external conditions determine whether they conduct electricity or not. This is because, although atoms basically keep their electrons with them, they willingly let them go when they are given a kick. This kick is energy that you can give them in the form of heat or light, for example.

In an LED exactly the opposite effect is used. The aim is to specifically induce electrons that are already moving freely in the semiconductor to bind to an atom. This is because energy is released – in the form of light. But for this one uses a little trick.

New properties due to impurities

This is because semiconductors can be further influenced in their electrical conductivity by giving them additional electrons or taking them away. For this purpose, impurities are specifically introduced into the material, i.e. other types of atoms with slightly different compositions – this is called doping. A distinction is made between n-doped semiconductors, which receive additional electrons through this process, and p-doped semiconductors, which thus lack electrons (missing electrons are called “holes”).

How the LED works

The trick is the following: Light can be generated by bringing free electrons into a material with many holes. This is done by bringing an n-doped semiconductor into contact with a p-doped semiconductor. The n-doped semiconductor provides the free electrons that are to be transported to the holes in the p-doped semiconductor. To do this, however, they must first overcome a barrier that forms when the two materials come into contact.

In order to pass over this barrier, the electrons need a sufficiently high speed. You give it to them by applying a voltage. The electrons then enter the p-layer where they meet the holes. But these are nothing else than free places for electrons. The originally free electron takes this place and is bound to an atom. Because energy is released in the process, light is created – with a different colour depending on the material!

Structure of an LED. When a voltage is applied, an electron current flows from n- to p-layer. During the recombination of the electrons with the holes of the p-layer light is generated.

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