Light Emitting Diode, LED: how does a LED work

The semiconductor technology and materials used are the key to understanding how a LED works.

Light Emitting Diode Tutorial Includes:
Light emitting diode     How does a LED work     How a LED is made     LED datasheet specifications     LED lifetime     LED packages     High power / brightness LEDs     LED lighting technology     Organic LEDs, OLEDs    

Other diodes: Diode types    

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.

 Light emitting diode, LED circuit symbol
Light emitting diode, LED circuit symbol

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.

 Light emitting diode operation
How a light emitting diode works

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.

LED materials and light colours

Range (nm)
Colour VF @ 20mA Material
< 400 Ultraviolet 3.1 - 4.4 Aluminium nitride (AlN)
Aluminium gallium nitride (AlGaN)
Aluminium gallium indium nitride (AlGaInN)
400 - 450 Violet 2.8 - 4.0 Indium gallium nitride (InGaN)
450 - 500 Blue 2.5 - 3.7 Indium gallium nitride (InGaN)
Silicon carbide (SiC)
500 - 570 Green 1.9 - 4.0 Gallium phosphide (GaP)
Aluminium gallium indium phosphide (AlGaInP)
Aluminium gallium phosphide (AlGaP)
570 - 590 Yellow 2.1 - 2.2 Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium phosphide (GaP)
590 - 610 Orange / amber 2.0 - 2.1 Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaUInP)
Gallium phosphide (GaP)
610 - 760 Red 1.6 - 2.0 Aluminium gallium arsenide (AlGaAs)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium phosphide (GaP)
> 760 Infrared < 1.9 Gallium arsenide (GaAs)
Aluminium gallium arsenide (AlGaAs)

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