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Light-emitting DiodesLight-emitting Diodes
for general lighting applications for general lighting applications
D.L. Pulfrey
Department of Electrical and Computer EngineeringUniversity of British ColumbiaVancouver, B.C. V6T1Z4, Canada
http://nano.ece.ubc.ca
Day 2, May 28, 2008, Pisa
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Examples of colour lightingExamples of colour lighting
EFS: Regensburg bridge
EFS:
18 million LEDs in New York city
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How much energy is used for How much energy is used for lighting?lighting?
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Lighting: growth and costsLighting: growth and costs
Tsao
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LED roadmapLED roadmap
Tsao
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Basic operationBasic operation
Radiative recombination
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Recombination in direct- and indirect-Recombination in direct- and indirect-bandgap materialsbandgap materials
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GaP is indirect !GaP is indirect !
How can this work?
EFS
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Competing NON-radiative recombination Competing NON-radiative recombination processes in direct-bandgap materialsprocesses in direct-bandgap materials
Pierret
Phonons
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Competing NON-radiative recombination Competing NON-radiative recombination processes in direct-bandgap materialsprocesses in direct-bandgap materials
Which of these 2 mechanisms is more likely to occur?
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Minority carrier Minority carrier recombination recombination
lifetimelifetime
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DnpCpAugere
2,
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Minority carrier recombination lifetimeMinority carrier recombination lifetimedue to non-radiative processesdue to non-radiative processes
SRH Auger
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http://www.eng.yale.edu/posters150/pdf/woodall.pdf
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Optical Output PowerOptical Output Power
How do we relate this to current ?
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What are these terms?
LED efficienciesLED efficiencies
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Current efficiencyCurrent efficiency
EFS
Fraction of LED current due to electrons recombining in the semiconductor (as opposed to at the contacts)
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Improving the current Improving the current efficiencyefficiency
EFS
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Carrier capture, recombination, and Carrier capture, recombination, and escapeescape
EFS
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Choice of material for Choice of material for heterostructureheterostructure
2.5
2
1.5
1
0.5
05.4 5.5 5.6 5.7 5.8 5.9 6 6.1
Lattice Constant, A
Ban
dg
ap, e
V
Si
Ge
GaAs
GaP
AlP
AlAs
InAs
InP90
8070
6050
4030
2010
a = 5.6533 Amatched to GaAs
°
°
a = 5.8688 Amatched to InP
°
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AlGaAs/GaAs DH LEDAlGaAs/GaAs DH LED
What is the algorithm for drawing band diagrams?
What is the doping type of the active region?
EFS
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Heterojunction Band Diagrams
e.g., n-Al0.3Ga0.7As (Eg=1.80eV, =3.83eV) on p-GaAs (Eg=1.42eV, =4.07eV)
Separated system Joined system
E0, El
EC
EF
EV
• e-barrier < h-barrier
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• Need short radiative lifetime - choose material
What are the B values for Si and GaAs ?
• Need long non-radiative lifetime
How do we get this?
Radiative efficiencyRadiative efficiency
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Extraction efficiencyExtraction efficiency
Solve #2 with wide bandgap "cladding"
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TIRTIR
nr is about 3.5 for GaAs-family materials
What is the critical angle?
What is a typical value for ext in a cheap LED?
EFS
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Creative TIRCreative TIR
EFS
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ReflectorsReflectors
Put reflector on top and use transparent substrate
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Contact blockingContact blocking
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Current spreading layerCurrent spreading layer
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Photon "voltage"Photon "voltage"
What is the photon energy?
EFS
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Intensity spectrumIntensity spectrum
EFS
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The range of AlGaInP LEDsThe range of AlGaInP LEDs
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The range of AlInGaN LEDsThe range of AlInGaN LEDs
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Operating voltageOperating voltage
How can the LED survive being operated at VD Eg/q ?
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Current controlCurrent control
Why is it important to minimize T ?
EFS
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Effect of T on Effect of T on radrad
EFS
What about the effect of T on non-rad ?
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Thermal resistanceThermal resistance
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Brighter and brighterBrighter and brighter
EFS
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http://astro-canada.ca/_en/a3300.html
• Achromatic
• Impression of white light via RGB
White lightWhite light
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Generating LED white lightGenerating LED white light
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Perception of lightPerception of light
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Eye sensitivity functionEye sensitivity function
Wha
t is
lum
inou
s ef
ficac
y?
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Colour matching functionsColour matching functions
Cone stimulation
Chromaticity coordinates
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Chromaticity diagramChromaticity diagram
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Additive colour mixingAdditive colour mixing
What is the colour gamut?
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Chromaticity and LEDsChromaticity and LEDs
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Additive LEDsAdditive LEDs
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Additive possibilitiesAdditive possibilities
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Dichromatic LEDDichromatic LED
Not quite complementary, but broadening (35nm for blue and 50nm for green) give possibility of white light.
Li et al.,JAP, 94, 2167, 2003
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White light using phosphorsWhite light using phosphors
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Blue/yellow phosphor LEDsBlue/yellow phosphor LEDs
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Colour renderingColour rendering
Illuminated by:
(a) high-CRI source
(b) low-CRI source
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Colour rendering with LED Colour rendering with LED arrayarray
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1/683 W @ 555nm
LI = 1cdIntegrate over sphere LF = 1 lm
50W halogen = 900 lm
Illuminance is LI/area
1 lm/m2 = 1 lux
desk light 500 lux
sunlight = 100,000 lux
LI = luminous intensity
LF = luminous flux
LE = luminous efficiency (lm/Welectrical)
Photometric unitsPhotometric units
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Incandescent bulbsIncandescent bulbs
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Fluorescent bulbsFluorescent bulbs
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Challenges to white LED Challenges to white LED technologiestechnologies
And of course
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What is wall-plug efficiency?
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White-light LEDs are here!White-light LEDs are here!
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http://www.physorg.com/news93198212.html
Osram announces 1000 lm LEDOsram announces 1000 lm LED
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The shape of things to comeThe shape of things to come
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ReferencesReferences
• EFS http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/
• LUMILEDS http://www.ele.uva.es/~pedro/optoele/LEDs/LEDilumination.pdf
• Tsao, J.Y., http://www.sandia.gov/~jytsao/
• Pierret, R.F., "Advanced Semiconductor Fundamentals", Addison-Wesley, 1987
• Fonstad, C., http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-772Spring2003/FDFCCD41-4572-4628-B733-6A535E375BCE/0/Lecture18v2.pdf
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TerminologyTerminology
