How Light Emitting Diodes work (LED)
Have you ever wondered how light emitting diodes work? LEDs emit light over only a relatively small part of the radiation spectrum unlike normal bulbs. The material GaAs is used mostly for the production of light emitting diodes. LEDs are becoming popular for a wide variety of applications due to their extreme long life compared to filament bulbs.
The basic concept of almost all light emitting semiconductors devices is to force a high concentration of electrons and holes in close proximity. Free electrons in the conduction band can recombine with free holes in the valence band and release a photon in the process. In GaAs a significant part of the recombination will result in emission of photon whose wavelength is approximately:
Λ= h x c/Eg = 1.24 eV –um/Eg
h is Planck’s constant
c is the speed of light
Eg is the semiconductor bandgap
The color of the light emitted by a LED depends on the bandgap of the semiconductor is made of. GaP emits green light or red light. GaAsP emits from red to amber depending on the concentration of phosphorus. SiC and GaN can produce blue light.
The earliest practical LEDs were made from GaAsP with band gap of 2.03eV which produces a photon with wavelength 0.611 um, which is the visible red part of the spectrum. Thus, early LEDs were all red; 0.65 um red is now also obtained with AlGaAs.