white light-emitting diodes
DESCRIPTION
White Light-Emitting Diodes. Robert Nicholas & Daniel Freno Junior Lab Spring 2007. White LEDs. The structural and operational characteristics of white LEDs will be discussed. Spectra will be taken from both a traditonal lamp and a white LED for comparison. - PowerPoint PPT PresentationTRANSCRIPT
White Light-Emitting Diodes
Robert Nicholas & Daniel Freno
Junior Lab
Spring 2007
White LEDs
The structural and operational characteristics of white LEDs will be discussed.
Spectra will be taken from both a traditonal lamp and a white LED for comparison http://www.hebeiltd.com.cn/leds/Mr.Led.jpg
Energy Band Diagrams
Electrons are pumped from the valence band to the conduction band.Upon relaxation, light is emitted to
conserve energy. Direct gap semiconductor materials are
needed for optoelectronic devices.Momentum must be also conserved.For momentum to be conserved in an
indirect gap semiconductor, phonon-assisted transitions must take place.
Three particle processes are very improbable.
Holes
Applied voltage across a p-n junction causes electrons to be promoted to the conduction band, leaving holes behind.
The electrons and holes will then be swept towards each other where they recombine, emitting light.
This area is referred to as the depletion region.
Schematics
Physical structure of a white LED. Primary emission is from an InxGa1-xN LED.
Primary blue light is partially absorbed by a phosphor [Ce:YAG ( Cerium doped Yttrium Aluminum Garnet)] layer. Results in secondary, redshifted phosphorescence.
Wavelengths are mixed and appear to the human eye as white.
Spectra of phosphor based white LED vs. filament lamp
InxGa1-xN
Ce:YAG
The intensity of the LED is larger, as expected. The lower energy peak of the white LED is from the InGaN diode. The broader redshifted peak is from the secondary emission due to the phosphor.
References http://micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html http://en.wikipedia.org/wiki/LED http://www.hebeiltd.com.cn/leds
http://upload.wikimedia.org/wikipedia/commons/c/cb/RBG-LED.jpg