Light Emitting Diodes EE 698A Kameshwar Yadavalli.

Light Emitting DiodesLight Emitting Diodes

EE 698AEE 698A

Kameshwar YadavalliKameshwar Yadavalli

OutlineOutline

• Basics of Light Emitting Diodes (Electrical)Basics of Light Emitting Diodes (Electrical)• Basics of Light Emitting Diodes (Optical)Basics of Light Emitting Diodes (Optical)• High internal efficiency designsHigh internal efficiency designs• High extraction efficiency structuresHigh extraction efficiency structures• Visible Spectrum LED’sVisible Spectrum LED’s• White-Light LED’s White-Light LED’s • The promise of solid state lightingThe promise of solid state lighting

LED-Electrical Properties-PN junctionsLED-Electrical Properties-PN junctions

From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

• PN junction diode in PN junction diode in forward bias, the forward bias, the electron-hole electron-hole recombination recombination leads to photon leads to photon emissionemission

• I = II = Iss(e(eeV/kTeV/kT-1)-1)

• Threshold voltage Threshold voltage VVthth = E = Egg/e/e

• I = II = IsseeeV/eV/ηηkTkT

where where ηη is the is the ideality factorideality factor

Double Heterostructure is used to confine the carriers, improving the radiative recombination rate

LED-Electrical Properties-Hetero junctionsLED-Electrical Properties-Hetero junctions


Grading of the heterojunction is done to reduce the resistance seen by carriers

LED-Electrical Properties-Hetero junctionsLED-Electrical Properties-Hetero junctions


LED-Electrical Properties-Carrier lossLED-Electrical Properties-Carrier loss

• The confinement barriers are typically several hundred meV The confinement barriers are typically several hundred meV (>>kT)(>>kT)

• Due to Fermi-Dirac distribution of carriers in the active region, Due to Fermi-Dirac distribution of carriers in the active region, some carriers will have energy higher than that of the barrierssome carriers will have energy higher than that of the barriers

• In AlGaAs/GaAs and AlGaN/GaN the barriers are highIn AlGaAs/GaAs and AlGaN/GaN the barriers are high

• In AlGaInP/GaInP the barriers are lower resulting in higher In AlGaInP/GaInP the barriers are lower resulting in higher leakage currentsleakage currents


LED-Electrical Properties-Blocking LED-Electrical Properties-Blocking layerslayers

• Electron Blocking Layers are required to prevent electron Electron Blocking Layers are required to prevent electron escape at high injection current densitiesescape at high injection current densities


LED-Optical Properties-EfficiencyLED-Optical Properties-Efficiency • ηηintint = = # of photons emitted from active region per # of photons emitted from active region per

secondsecond # of electrons injected in to LED per second# of electrons injected in to LED per second

= = PPintint / (h / (hνν)) I / eI / e • ηηextrextr = = # of photons emitted into free space per second# of photons emitted into free space per second # of photons emitted from active region per second# of photons emitted from active region per second = = P / (hP / (hνν))

PPintint / (h / (hνν))


LED-Optical Properties-Emission LED-Optical Properties-Emission SpectrumSpectrum

• The linewidth of an LED emitting in the visible range is The linewidth of an LED emitting in the visible range is relatively narrow compared with the entire visible range relatively narrow compared with the entire visible range (perceived as monochromatic by the eye)(perceived as monochromatic by the eye)

• Optical fibers are dispersive, limiting the Optical fibers are dispersive, limiting the bit rate X bit rate X distancedistance product achievable with LED’s product achievable with LED’s

• Modulation speeds achieved with LED’s are 1Gbit/s, as Modulation speeds achieved with LED’s are 1Gbit/s, as the spontaneous lifetime of carriers in LED’s is 1-100 nsthe spontaneous lifetime of carriers in LED’s is 1-100 ns


LED-Optical Properties-Light Escape LED-Optical Properties-Light Escape ConeCone

• Total internal reflection at the semiconductor air Total internal reflection at the semiconductor air interface reduces the external quantum efficiency.interface reduces the external quantum efficiency.

• The angle of total internal reflection defines the light The angle of total internal reflection defines the light escape cone.escape cone.

sinsinθθcc = n = nairair/n/nss

• Area of the escape cone = 2Area of the escape cone = 2ππrr22(1-cos(1-cosθθcc))• PPescape escape / P/ Psourcesource = (1-cos = (1-cosθθcc)/2 = )/2 = θθcc

22/4 = (n/4 = (nairair22/n/nss

22)/4)/4


LED-Optical Properties-Emission LED-Optical Properties-Emission SpectrumSpectrum

• Light intensity in air Light intensity in air (Lambertian (Lambertian emission pattern) is emission pattern) is given bygiven by

IIairair = (P = (Psourcesource/4/4ππrr22) X ) X (n(nairair

22/n/nss22) cos) cosΦΦ

• Index contrast Index contrast between the light between the light emitting material emitting material and the surrounding and the surrounding region leads to non-region leads to non-isotropic emission isotropic emission patternpattern


LED-Optical Properties-Epoxy encapsulantsLED-Optical Properties-Epoxy encapsulants

• Light extraction efficiency can be increased by using dome Light extraction efficiency can be increased by using dome shaped encapsulants with a large refractive index.shaped encapsulants with a large refractive index.

• Efficiency of a typical LED increases by a factor of 2-3 upon Efficiency of a typical LED increases by a factor of 2-3 upon encapsulation with an epoxy of n = 1.5.encapsulation with an epoxy of n = 1.5.

• The dome shape of the epoxy implies that light is incident at The dome shape of the epoxy implies that light is incident at an angle of 90an angle of 90oo at the epoxy-air interface. Hence no total at the epoxy-air interface. Hence no total internal reflection.internal reflection. From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

Temperature dependence of emission Temperature dependence of emission intensityintensity• Emission intensity decreases with increasing temperature.Emission intensity decreases with increasing temperature.

• Causes include non-radiative recombination via deep levels, Causes include non-radiative recombination via deep levels, surface recombination, and carrier loss over heterostucture surface recombination, and carrier loss over heterostucture barriers.barriers.


High internal efficiency LED designsHigh internal efficiency LED designs

• Radiative recombination probability needs to be increased and non-Radiative recombination probability needs to be increased and non-radiative recombination probability needs to be decreased.radiative recombination probability needs to be decreased.

• High carrier concentration in the active region, achieved through double High carrier concentration in the active region, achieved through double heterostructure (DH) design, improves radiative recombination.heterostructure (DH) design, improves radiative recombination.

R=BnpR=Bnp

• DH design is used in all high efficiency designs today.DH design is used in all high efficiency designs today.


High internal efficiency designsHigh internal efficiency designs

• Doping of the active regions and that of the cladding Doping of the active regions and that of the cladding regions strongly affects internal efficiency.regions strongly affects internal efficiency.

• Active region should not be heavily doped, as it causes Active region should not be heavily doped, as it causes carrier spill-over in to the confinement regions decreasing carrier spill-over in to the confinement regions decreasing the radiative efficiencythe radiative efficiency

• Doping levels of 10Doping levels of 101616-low 10-low 101717 are used, or none at all. are used, or none at all.

• P-type doping of the active region is normally done due to P-type doping of the active region is normally done due to the larger electron diffusion length. the larger electron diffusion length.

• Carrier lifetime depends on the concentration of majority Carrier lifetime depends on the concentration of majority carriers. carriers.

• In low excitation regime , the radiative carrier lifetime In low excitation regime , the radiative carrier lifetime decreases with increasing free carrier concentration. decreases with increasing free carrier concentration.

• Hence efficiency increases with doping.Hence efficiency increases with doping.

• At high concentration, dopants induce defects acting as At high concentration, dopants induce defects acting as recombination centers.recombination centers.


P-N junction displacementP-N junction displacement

• Displacement of the P-N junction causes significant change in Displacement of the P-N junction causes significant change in the internal quantum efficiency in DH LED structures.the internal quantum efficiency in DH LED structures.

• Dopants can redistribute due to diffusion, segregation or Dopants can redistribute due to diffusion, segregation or drift.drift.


Doping of the confinement regionsDoping of the confinement regions

• Resistivity of the confinement regions should be low so that heating is Resistivity of the confinement regions should be low so that heating is minimal.minimal.

• High p-type conc. in the cladding region keeps electrons in the active High p-type conc. in the cladding region keeps electrons in the active region and prevents them from diffusing in to the confinement region.region and prevents them from diffusing in to the confinement region.

• Electron leakage out of the active region is more severe than hole Electron leakage out of the active region is more severe than hole leakage. leakage.


Non radiative recombinationNon radiative recombination

• The concentration of defects The concentration of defects which cause deep levels in which cause deep levels in the active region should be the active region should be minimum.minimum.

• Also surface recombination Also surface recombination should be minimized, by should be minimized, by keeping all surfaces several keeping all surfaces several diffusion lengths away from diffusion lengths away from the active region.the active region.

• Mesa etched LEDs and Mesa etched LEDs and lasers where the mesa etch lasers where the mesa etch exposes the active region to exposes the active region to air, have low internal air, have low internal efficiency due to efficiency due to recombination at the recombination at the surface. surface.

• Surface recombination also Surface recombination also reduces lifetime of LEDs.reduces lifetime of LEDs.


Lattice matchingLattice matching

• Carriers recombine non-radiatively at misfit dislocations.Carriers recombine non-radiatively at misfit dislocations.

• Density of misfit dislocation lines per unit length is proportional Density of misfit dislocation lines per unit length is proportional to lattice mismatch.to lattice mismatch.

• Hence the efficiency of LED’s is expected to drop as the Hence the efficiency of LED’s is expected to drop as the mismatch increases. mismatch increases.


High extraction efficiency structuresHigh extraction efficiency structures

• Shaping of the LED die is critical to improve their efficiency.Shaping of the LED die is critical to improve their efficiency.

• LEDs of various shapes; hemispherical dome, inverted cone, LEDs of various shapes; hemispherical dome, inverted cone, truncated cones etc have been demonstrated to have better truncated cones etc have been demonstrated to have better extraction efficiency over conventional designs.extraction efficiency over conventional designs.

• However cost increases with complexity.However cost increases with complexity.


High extraction efficiency structuresHigh extraction efficiency structures

• The TIP LED employs The TIP LED employs advanced LED die shaping advanced LED die shaping to minimize internal loss to minimize internal loss mechanisms.mechanisms.

• The shape is chosen to The shape is chosen to minimize trapping of light.minimize trapping of light.

• TIP LED is a high power TIP LED is a high power LED, and the luminous LED, and the luminous efficiency exceeds 100 efficiency exceeds 100 lm/W.lm/W.

• TIP devices are sawn TIP devices are sawn using beveled dicing using beveled dicing blade to obtain chip blade to obtain chip sidewall angles of 35sidewall angles of 35oo to to vertical.vertical.

Krames et. al, Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999

Visible spectrum LEDsVisible spectrum LEDs


The plot charts the gains made in luminous efficiency till date.

Visible spectrum LEDsVisible spectrum LEDs


• The emission spectrum of the blue, green and red LEDs indicate that the green LED has a wider spectrum.• Alloy broadening leads to spectral broadening that is greater than 1.8 kT linewidth.

White-light LEDsWhite-light LEDs

• White light can be generated in several different ways.White light can be generated in several different ways.

• One way is to mix to complementary colors at a certain One way is to mix to complementary colors at a certain power ratio.power ratio.

• Another way is by the emission of three colors at certain Another way is by the emission of three colors at certain wavelengths and power ratio.wavelengths and power ratio.

• Most white light emitters use an LED emitting at short Most white light emitters use an LED emitting at short wavelength and a wavelength converter.wavelength and a wavelength converter.

• The converter material absorbs some or all the light The converter material absorbs some or all the light emitted by the LED and re-emits at a longer wavelength.emitted by the LED and re-emits at a longer wavelength.

• Two parameters that are important in the generation of Two parameters that are important in the generation of white light are luminous efficiency and color rendering white light are luminous efficiency and color rendering index.index.

• It is shown that white light sources employing two It is shown that white light sources employing two monochromatic complementary colors result in highest monochromatic complementary colors result in highest possible luminous efficiency.possible luminous efficiency.


White-light LEDsWhite-light LEDs• Wavelength converter materials include phosphors, Wavelength converter materials include phosphors,

semiconductors and dyes.semiconductors and dyes.

• The parameters of interest are absorption wavelength, The parameters of interest are absorption wavelength, emission wavelength and quantum efficiency.emission wavelength and quantum efficiency.

• The overall energy efficiency is given byThe overall energy efficiency is given by

ηη = = ηηextext((λλ11/ / λλ22))

• Even if the external quantum efficiency is 1, there is always Even if the external quantum efficiency is 1, there is always an energy loss associated with conversion.an energy loss associated with conversion.

• Common wavelength converters are phosphors, which consist Common wavelength converters are phosphors, which consist of an inorganic host material doped with an optically active of an inorganic host material doped with an optically active element.element.

• A common host is YA common host is Y33AlAl55OO1212..

• The optically active dopant is a rare earth element, oxide or The optically active dopant is a rare earth element, oxide or another compound.another compound.

• Common rare earth elements used are Ce, Nd, Er and Th.Common rare earth elements used are Ce, Nd, Er and Th.


White-light LEDsWhite-light LEDs

• Phosphors are stable Phosphors are stable materials and can have materials and can have quantum efficiencies of quantum efficiencies of close to 100%.close to 100%.

• Dyes also can have Dyes also can have quantum efficiencies of quantum efficiencies of close to 100%.close to 100%.


• Dyes can be encapsulated Dyes can be encapsulated in epoxy or in optically in epoxy or in optically transparent polymers.transparent polymers.

• However, organic dyes However, organic dyes have finite lifetime. They have finite lifetime. They become optically inactive become optically inactive after 10after 1044-10-1066 optical optical transitions.transitions.

White LEDs based on phosphor convertersWhite LEDs based on phosphor converters


A blue GaInN/GaN LEDand a phosphor wavelengthconverter suspended in a epoxy resin make a white Light LED.The thickness of the phosphorcontaining epoxy and the concentration of the phosphor determine the relative strengths of the two emissionbands

Promise of Solid State LightingPromise of Solid State Lighting

• The use of solid state lighting devices promises huge savings The use of solid state lighting devices promises huge savings in energy consumption.in energy consumption.

• The electricity for lighting needs is 60GW, over 24 hrs.The electricity for lighting needs is 60GW, over 24 hrs.

• About 24 GWyear is consumed by incandescent lamps with a About 24 GWyear is consumed by incandescent lamps with a luminous intensity of 15lm/W.luminous intensity of 15lm/W.

• 36 GWyear is consumed by FL/HID lamps with a luminous 36 GWyear is consumed by FL/HID lamps with a luminous intensity of 75lm/W.intensity of 75lm/W.

• Assuming that by year 2020, they are replaced by LEDs with Assuming that by year 2020, they are replaced by LEDs with luminous intensity of 150 lm/W, energy savings are 40 luminous intensity of 150 lm/W, energy savings are 40 GWyear.GWyear.

• That translates to $40 billion in savings.That translates to $40 billion in savings.

• At 4Mtons / GWyear of coal consumption, net savings lead to At 4Mtons / GWyear of coal consumption, net savings lead to 25% less coal consumption, leading to lesser emissions of 25% less coal consumption, leading to lesser emissions of green house gases.green house gases.

• Global savings are projected to be about $140B.Global savings are projected to be about $140B.

Roland Haitz, Adv. in Solid State Physics

Light Emitting Diodes EE 698A Kameshwar Yadavalli.

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