LEDs (Light-emitting diodes) are semiconductor diodes that emit incoherent narrow-spectrum light when the p-n junction is forward biased.
The normally empty CB (conduction band) of semiconductors is populated by electrons injected into it by the forward current through the junction. As we saw in chapter 4, when an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. This photon emission is the mechanism by which light is emitted from LEDs. When the electron can undergo the down-ward transition by itself, the photon emission process is called spontaneous emission.
The light is generated when electrons recombine with holes and the wavelength of the light depends on the band gap of the semiconductor material, Eg. The following table shows the colours associated to the wavelengths of the light emitted by LEDs made by different semiconductor materials.
Leds made from semiconductors presenting a direct bandgap emit more light than leds fabricated by using indirect bandgap semiconductors.
Semiconductor |
Colour |
Brightness |
GaAs; GaAlAs |
Infrared |
General (normal) |
GaAs; AlGaAs; GaP |
Red |
General (normal) |
GaN |
Blue |
General (normal) |
GaP |
Green |
General (normal) |
GaAlAs; GaAsP; InGaAlP; |
Red |
High (super & ultra) |
GaN |
Blue |
High (super & ultra) |
GaP ; InGaN |
Green |
High (super & ultra) |
InGaAlP; GaAsP |
Yellow |
High (super & ultra) |
LEDs are applied in many fields such as displays, solid-state lighting, remote control and optical communication systems. In last year’s LEDs are commonly used also in general illumination such as many integrated LED lamps and LED luminaires. Different LED packages can be found in the market depending on the applications. Some kinds of lens are normally included in the packaging of LEDs in order to control the output light beam angle. This is one of the characteristics given by manufacturers in datasheets as well as luminous intensity (mcd), flus (lm), dominate wavelength (nm) and colour. Specific values (typical and thresholds) of electrical parameters as Vf (forward voltage), If (forward current) are also given by manufacturers.
The energy efficiency of a LED, η, is typically characterized in basic terms as the ratio of power input to light output—or more technically: Emitted flux (lumens) divided by power draw (W). Commercial LEDs present efficiencies ranging from 50–70 %.