Light-Emitting Diodes (LED)
LED is a semiconductor PN junction diode designed to emit light when forward-biased. It is one of the most popular optoelectronic source. LEDs consume very little power and are inexpensive.
When PN junction is forward biased, the electrons in the N-type material and the holes in the P-type material travel towards the junction. Some of these holes and electrons recombine with each other and in the process radiate energy. The energy will be released either in the form of photons of light.
Gallium Phosphide (GaP), Gallium Arsenide (GaAs) and Gallium arsenide Phosphide (GaAsP) are used in the construction of LEDs. In the absence of an externally applied voltage, the N-type material contains electrons while the Ptype material contains holes that can act as current carriers. When the diode is forward-biased, the energy levels shift and there is significant increase in the concentration of electrons in the conduction band on the N-side and that of holes in valance band on the P-side. The electrons and holes combine near the junction to release energy in the form of photons. The process of light emission in LED is spontaneous, i.e., the photons emitted are not in phase and travel in different directions. The energy of the photon resulting from this recombination is equal to the bandgap energy of the semiconductor material and is expressed by: 1240 Where λ is the wavelength (nm) and ΔE is the bandgap energy (eV).
LEDs are operated in the forward-biased mode. As the current through the LED changes very rapidly with change in forward voltage above the threshold voltage, LEDs are current-driven devices. The resistor (R) is used limit the current flowing through the device. A silicon diode can be placed inversely parallel to the LED for reverse polarity voltage protection. The current that will flow through the LED is given by VCC -VF ) R