What is an LED and How Do LEDs Work?

LED technology is taken for granted as LEDs are in widespread use. However the technology and the materials used are key to understanding how a LED works.

Although the basic PN junction had been in use for many years, it was not until 1962 that the LED was developed and its action started to be understood.

LED technology: how a LED works

The LED is a specialised form of PN junction that uses a compound junction. The semiconductor material used for the junction must be a compound semiconductor. The commonly used semiconductor materials including silicon and germanium are simple elements and junction made from these materials do not emit light. Instead compound semiconductors including gallium arsenide, gallium phosphide and indium phosphide are compound semiconductors and junctions made from these materials do emit light.

These compound semiconductors are classified by the valence bands their constituents occupy. For gallium arsenide, gallium has a valency of three and arsenic a valency of five and this is what is termed a group III-V semiconductor and there are a number of other semiconductors that fit this category. It is also possible to have semiconductors that are formed from group III-V materials.

The light emitting diode emits light when it is forward biased. When a voltage is applied across the junction to make it forward biased, current flows as in the case of any PN junction. Holes from the p-type region and electrons from the n-type region enter the junction and recombine like a normal diode to enable the current to flow. When this occurs energy is released, some of which is in the form of light photons.

It is found that the majority of the light is produced from the area of the junction nearer to the P-type region. As a result the design of the diodes is made such that this area is kept as close to the surface of the device as possible to ensure that the minimum amount of light is absorbed in the structure.

To produce light which can be seen the junction must be optimised and the correct materials must be chosen. Pure gallium arsenide releases energy in the infra read portion of the spectrum. To bring the light emission into the visible red end of the spectrum aluminium is added to the semiconductor to give aluminium gallium arsenide (AlGaAs). Phosphorus can also be added to give red light. For other colours other materials are used. For example gallium phoshide gives green light and aluminium indium gallium phosphide is used for yellow and orange light. Most LEDs are based on gallium semiconductors.