The fact that solid objects can generate light when electrically excited was observed by Henry Joseph Round in a silicon carbide crystal (SiC) as far back as 1907. This phenomenon is also exploited by LED technology, with LED standing for "Light Emitting Diode". A diode denotes an electrical component that only lets current flow in a single direction (forward biased condition) and blocks the current in the other direction (reverse biased condition). If current flows through an LED in a forward direction, it emits energy in the form of visible light, infrared radiation or ultraviolet radiation.

 

• Converting electrical current into light

Light-emitting diodes are based on semiconductor connections, which convert current directly into light. The wavelength of the light emitted by a light-emitting diode depends upon the material, and its energy gap, that is used for the electronic semiconductor. In order to cover the spectral range from infrareds to UVA, materials such as aluminium gallium arsenide (AlGaAs), gallium arsenide phosphide (GaAsP), aluminium indium gallium phosphide (AlInGaP), gallium phosphide (GaP), indium gallium nitride (InGaN) and gallium nitride (GaN) are used for semiconductors.

 

• Wandering electrons

The semiconductor materials have a crystalline structure. A semiconductor crystal contains the so-called valence band, which represents the energy of the atoms' bound electrons, and a higher energy band, called the conduction band, consisting of the energy of the electrons moving freely in the crystal. At room temperature, the thermal energy is high enough to ionise several atoms in the semiconductor material. The result is a minimal but decisive amount of conductivity, which also explains the name "semiconductor".

If sufficient energy is conveyed to an electron to allow it to escape from its parent atom, it moves randomly through the material until it encounters an ionised atom - a so-called hole - with which it recombines. This recombination process causes it to lose energy, which corresponds to the energy gap between the valence band and the conduction band. This energy can be emitted in the form of a photon, and determines the wavelength of the emitted light. Light-emitting diodes are therefore categorised as electroluminescent light sources. The same applies to traditional UV lamps.

 

• Light-emitting diodes almost generate monochrome light

LEDs offer only a small wavelength window of +/-15 nm at the peak wavelength. Current UV LED systems work in the UVA/UVV range between 365 to 405 nm. UV light-emitting diodes can achieve power values of a few watts per square centimetre, and an efficiency rating of between 5 and 25 %, depending on the LED UV system used. As a rough rule of thumb: the shorter the wavelength, the lower the efficiency.