How LED Lighting works

Artificial lighting is so widespread we take it for granted , major savings in energy would  be  realized  if incandescent  lights  can  be replaced by light-emitting diodes (LEDs). Because LEDs are
made  from  semiconductors, this  is  an  appropriate  place  to  take  a closer look at the operation of an LED.The heart of an LED is a p–n diode, which is formed by bringing an n-type semiconductor in contact with a p-type semiconductor. In the junction where they meet there are very few electrons or holes to carry the charge and the conductivity decreases. When an appropriate voltage is applied, it drives electrons from the conduction band of the n-doped side into the junction, where they meet holes that are pushed in from the valence band of the p-doped side. The electrons fall  into  the  empty  holes, and  their  energy  is  converted  into  light whose  photons  have  energy  equal  to  the  band  gap . In this way electrical energy is converted into optical energy.Because the wavelength of light that is emitted depends on the band gap of the semiconductor, the color of light produced by the
LED can be controlled by appropriate choice of semiconductor. Most red LEDs are made of a mixture of GaP and GaAs. The band gap of GaP is 2.26 eV  , which corresponds to a green photon
with a wavelength of 549 nm, while GaAs has a band gap of 1.43 eV, which corresponds to an infrared photon with a wavelength of 867 nm. •  By forming solid solutions of these two compounds, with stoichiometries of ,the  band  gap  can  be  adjusted  to  any  intermediate  value. Thus,
is the solid solution of choice for red, orange, and yellow LEDs. Green  LEDs  are  made  from mixtures  of GaP  and  AlP Red  LEDs  have  been  in  the  market  for  decades, but  to  make white  light  an  efficient  blue  LED  was  needed. The  first  prototype bright blue LED was demonstrated in Japan in 1993. In 2006, only 13 years later, over $4 billion worth of blue LEDs were sold worldwide. The  blue  LEDs  are  based  on  combinations  of GaN  and InN. With the availability  of blue  LEDs  there  are  various  strategies  for  making  white LEDs. In  some  cases, light  is  combined from blue, green, and red LEDs. More commonly  a  blue  LED  is  coated  with phosphor, a material that converts some of the  blue  light  into  yellow  light. In  either case  the  combined  colors  appear  white  to the  eye.

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