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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

pulfrey@ece.ubc.ca

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,

1

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