GEL–4203 / GEL–7041

OPTOÉLECTRONIQUE

Auxiliaire d’enseignement Nicolas Ayotte

2012-03-15

GEL−4203 / GEL−7041 Optoélectronique

IX – LEDS

Carrier recombination

Luminescence

Diode

Optical power

From radiometry to photometry

Miscellaneous

Lighting applications

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IX – LEDS

2006: first production cars to use LED headlights

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Lexus LS 600h L

V.2 – SEMICONDUCTORS

Light-matter interaction

Three possible processes

Absorption →

photodetectors

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E

x

absorption

electron-hole pair

V.2 – SEMICONDUCTORS

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E

x

spontaneous emission stimulated emission E

x

Emission

Stimulated laser

Spontaneous LED

IX.1 – CARRIER RECOMBINATION

Radiative mechanisms

1 electron-hole pair recombines

and generates 1 photon

Emission over energies

(frequencies) higher than band gap (~4-6%)

Non-radiative mechanisms

Electron-hole pairs recombine,

energy going into heat

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E

x

spontaneous emission

Carrier lifetimes in

the ns range

nrr

rq

11

1int,

Internal quantum

efficiency

IX.2 – LUMINESCENCE

Recombination at the same wavelength from different

charge carriers

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hn

IX.2 – LUMINESCENCE

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1.4 1.5 1.6 1.7 1.80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

photon energy [eV]

lum

inescence [

a.u

.]

700 750 800 850 9000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

photon wavelength [nm]

lum

inescence [

a.u

.]

GaAs T = 300 K

E

Eg

TkB5.3~hc

TkB

2

5.3~

IX.3 – DIODE

Light-Emitting Diode (LED)

Same basic structure as photodiode

Photon generation

Dominated by spontaneous emission (random process)

Emission > absorption

Generation rate (forward bias)

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© 1999 S. O. Kasap, Optoelectronics (Prentice Hall)

Light output

Insulator (oxide)p

n+ Epit axial layer

A schematic illustration of typical planar surface emitting LED devices . (a) p-layergrown epitaxially on an n+ substrate. (b) First n+ is epitaxially grown and then p regionis formed by dopant diffus ion into the epitaxial layer.

Light output

pEpit axial layers

(a) (b)

n+

Subst rat e Subst rat e

n+

n+

Metal electrode

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

e

IP q int,intopt, ]ph/s[

IX.3 – DIODE

Light-Emitting Diode (LED)

Unidirectional emission

Reflection at surface

Limited by total internal

reflection

Critical angle of 16o for

GaAs

“Unpolarized” light

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© 1999 S. O. Kasap, Optoelectronics (Prentice Hall)

2

2 1

2 1

r r

r r

n nR

n n

1 1

2

sin rc

r

n

n

IX.3 – DIODE

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2ln

i

adj

n

NN

e

kTV

kT

ETn

g

i exp32

IV curve

IX.3 – DIODE

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Uniform Lambertian source

The radiant intensity observed is directly

proportional to the cosine of the angle between

the line of sight and the surface normal

Produces a constant radiance Le on the surface

, , , coscos

se s s sA

e s s

I L r dAL A

IX.4 – OPTICAL POWER

Total optical power

LI curve

Linear with current

Slope gives external

quantum efficiency

~< 500 mW

higher power in arrays

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Ie

hcP q

ext,opt

IX.4 – OPTICAL POWER

Electrical efficiency (or external efficiency or wall-plug

efficiency)

~< 20%

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opt el

el

elP P

VI

Popt

I

I

V

IX.4 – OPTICAL POWER

Thermal budget

Examples:

Low-power LED Thermal resistance of 250 K/W

Optical power of 18 mW

Electrical power of 1.6 V × 50 mA = 80 mW

→ heating of 16 K

High-power LED Thermal resistance of 9 K/W

Optical power of 52000 mlm / (0.71 x 683 lm/W) = 107 mW

Electrical power of 3.8 V × 350 mA = 1.3 W

→ heating of 11 K

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IX.4 – OPTICAL POWER

Radiant intensity (W/sr)

Power (W)

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2R

A

2

0

2/

0opt ddsin),(IP

+azimuthal symmetry

1/2

IX.4 – OPTICAL POWER

Power spectral density (W/nm)

Spectral characteristics

Central wavelength

Line width (~< 5%)

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d)(0

optopt

PP

IX.4 – OPTICAL POWER

Band gap engineering

To find alloys with proper

band gap energy

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© 1999 S. O. Kasap, Optoelectronics (Prentice Hall)

© 2003 http://cnx.rice.edu/content/m1011/latest/

IX.5 – FROM RADIOMETRY TO PHOTOMETRY

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IX.5 – FROM RADIOMETRY TO PHOTOMETRY

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IX.5 – FROM RADIOMETRY TO PHOTOMETRY

Photopic (day vision)

Cones

Scotopic (night vision)

When luminance ~<

0.03 cd/m2

Rods

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d)()(0

eph VIKS m

d)()( '

0e

'

sc VIKS m

1978 CIE

IX.5 – FROM RADIOMETRY TO PHOTOMETRY

Radiometric quantities Photometric quantities

Flux or radiant power (W)

{Flux énergétique}

Luminous flux (lm)

{Flux lumineux}

Radiant intensity (W/sr)

{Intensité énergétique}

Luminous intensity (cd = lm/sr)

{Intensité lumineuse}

Irradiance (W/m2)

{Éclairement énergétique}

Illuminance (lx = lm/m2)

{Éclairement lumineux}

Exitance (W/m2)

{Exitance énergétique}

Luminous exitance (lx = lm/m2)

{Exitance lumineuse}

Radiance (W/sr m2)

{Luminance énergétique}

Luminance (cd/m2 = lm/sr m2)

{Luminance lumineuse}

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© 1995 J. Singh, Semiconductor optoelectronics (McGraw-Hill)

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© 2003, Nédélec et Vianden, Universität Bonn

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© 2002, Harris, Stanford University

IX.6 – MISCELLANEOUS

Modulation bandwidth

Limited to 10-100 MHz

Carrier lifetime (~ns)

Driving electronics

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IX.6 – MISCELLANEOUS

Structures

Surface-emitting vs. edge-emitting

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(a) Surface emitting LED (b) Edge emitting LED

Double

heterostructure

Light

Light

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)© 1999 S. O. Kasap, Optoelectronics (Prentice Hall)

IX.6 – MISCELLANEOUS

Super luminescent LEDs

Spontaneous emission (LED)

Light amplification by the semiconductor medium

(see the Laser section)

Bi-colour LED / RGB LED

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IX.6 – MISCELLANEOUS

Standard packaging

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IX.6 – MISCELLANEOUS

High power packaging

(Luxeon)

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IX.6 – MISCELLANEOUS

High-power device

(Luxeon V Emitter)

500 mW at 455 nm

(royal blue) => ~10 %

efficiency

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IX.6 – MISCELLANEOUS

Generating LED white light (Luxeon)

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IX.6 – MISCELLANEOUS

Mounting

Through hole

Surface Mounted Device (SMD)

Low power High power

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© 2007 EREA sa (http://www.erea.be)

IX.6 – MISCELLANEOUS

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see Navigant Consulting, Inc., “Solid-State Lighting Research and Development Portfolio – Multi-Year Program Plan FY’07-FY’12” (http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_multiyear_plan.pdf)