Transcript 2007S-JLAB MagLite -Nicholas & Freno

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
Holes
 Electrons
are pumped from the valence
band to the conduction band.
 Upon
relaxation, light is emitted to
conserve energy.


 Direct gap semiconductor materials are
Applied voltage across a p-n junction
causes electrons to be promoted to the needed for optoelectronic devices.
conduction band, leaving holes behind.
 Momentum must be also conserved.
The electrons and holes will then be
 For momentum to be conserved in an
swept towards each other where they
indirect gap semiconductor, phononrecombine, emitting light.
assisted transitions must take place.

This area is referred to as the depletion
region.

Three particle processes are very
improbable.
Schematics
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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
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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.
InxGa1-xN
Ce:YAG
References
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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