Kameshwar Light Emitting Diode LED Device

LED

1. Light Emitting DiodesLight Emitting Diodes
EE 698AEE 698A
Kameshwar YadavalliKameshwar Yadavalli

2. 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

3. 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 leads to recombination leads to
photon emissionphoton emission
• I = II = I
ss(e(e
eV/kTeV/kT
-1)-1)
•Threshold voltage VThreshold voltage V
thth
= E= E
gg/e/e
• I = II = I
ssee
eV/eV/ηηkTkT
where where ηη is the ideality is the ideality
factorfactor
Double Heterostructure is used to confine the carriers, improving
the radiative recombination rate

4. LED-Electrical Properties-Hetero junctionsLED-Electrical Properties-Hetero junctions
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.
Grading of the heterojunction is done to reduce the resistance seen by
carriers

5. LED-Electrical Properties-Hetero junctionsLED-Electrical Properties-Hetero junctions
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

6. LED-Electrical Properties-Carrier lossLED-Electrical Properties-Carrier loss
•The confinement barriers are typically several hundred meV (>>kT)The confinement barriers are typically several hundred meV (>>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 leakage In AlGaInP/GaInP the barriers are lower resulting in higher leakage
currentscurrents
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

7. LED-Electrical Properties-Blocking layersLED-Electrical Properties-Blocking layers
•Electron Blocking Layers are required to prevent electron escape at Electron Blocking Layers are required to prevent electron escape at
high injection current densitieshigh injection current densities
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

8. LED-Optical Properties-EfficiencyLED-Optical Properties-Efficiency
•ηη
intint = = # of photons emitted from active region per second# of photons emitted from active region per second
# of electrons injected in to LED per second# of electrons injected in to LED per second
= = PP
intint / (h / (hνν))
I / eI / e

•ηη
extrextr = = # 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))
PP
intint / (h / (hνν))
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

9. LED-Optical Properties-Emission SpectrumLED-Optical Properties-Emission Spectrum
•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 the Modulation speeds achieved with LED’s are 1Gbit/s, as the
spontaneous lifetime of carriers in LED’s is 1-100 nsspontaneous lifetime of carriers in LED’s is 1-100 ns
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

10. LED-Optical Properties-Light Escape ConeLED-Optical Properties-Light Escape Cone
•Total internal reflection at the semiconductor air interface Total internal reflection at the semiconductor air interface
reduces the external quantum efficiency.reduces the external quantum efficiency.
•The angle of total internal reflection defines the light escape The angle of total internal reflection defines the light escape
cone.cone.
sinsinθθ
cc = n = n
airair/n/n
ss
•Area of the escape cone = 2Area of the escape cone = 2ππrr
22
(1-cos(1-cosθθ
cc))
•PP
escape escape / P/ P
sourcesource = (1-cos = (1-cosθθ
cc)/2 = )/2 = θθ
cc
22
/4 = (n/4 = (n
airair
22
/n/n
ss
22
)/4)/4
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

11. LED-Optical Properties-Emission SpectrumLED-Optical Properties-Emission Spectrum
•Light intensity in air Light intensity in air
(Lambertian emission (Lambertian emission
pattern) is given bypattern) is given by
II
airair = (P = (P
sourcesource/4/4ππrr
22
) X ) X
(n(n
airair
22
/n/n
ss
22
) cos) cosΦΦ
•Index contrast Index contrast
between the light between the light
emitting material and emitting material and
the surrounding the surrounding
region leads to non-region leads to non-
isotropic emission isotropic emission
patternpattern
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

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

13. Temperature dependence of emission intensityTemperature dependence of emission intensity
•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.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

14. High internal efficiency LED designsHigh internal efficiency LED designs
•Radiative recombination probability needs to be increased and non-radiative Radiative recombination probability needs to be increased and non-radiative
recombination probability needs to be decreased.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.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

15. High internal efficiency designsHigh internal efficiency designs
•Doping of the active regions and that of the cladding regions Doping of the active regions and that of the cladding regions
strongly affects internal efficiency.strongly affects internal efficiency.
•Active region should not be heavily doped, as it causes carrier spill-Active region should not be heavily doped, as it causes carrier spill-
over in to the confinement regions decreasing the radiative over in to the confinement regions decreasing the radiative
efficiencyefficiency
•Doping levels of 10Doping levels of 10
1616
-low 10-low 10
1717
are used, or none at all. are used, or none at all.
•P-type doping of the active region is normally done due to the P-type doping of the active region is normally done due to the
larger electron diffusion length. larger electron diffusion length.
•Carrier lifetime depends on the concentration of majority carriers. Carrier lifetime depends on the concentration of majority carriers.
•In low excitation regime , the radiative carrier lifetime decreases In low excitation regime , the radiative carrier lifetime decreases
with increasing free carrier concentration. 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.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

16. 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.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

17. 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 region High p-type conc. in the cladding region keeps electrons in the active region
and prevents them from diffusing in to the confinement region.and prevents them from diffusing in to the confinement region.
•Electron leakage out of the active region is more severe than hole leakage. Electron leakage out of the active region is more severe than hole leakage.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

18. Non radiative recombinationNon radiative recombination
•The concentration of defects The concentration of defects
which cause deep levels in the which cause deep levels in the
active region should be 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 lasers Mesa etched LEDs and lasers
where the mesa etch exposes where the mesa etch exposes
the active region to air, have the active region to air, have
low internal efficiency due to low internal efficiency due to
recombination at the surface. recombination at the surface.
•Surface recombination also Surface recombination also
reduces lifetime of LEDs.reduces lifetime of LEDs.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

19. 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 to Density of misfit dislocation lines per unit length is proportional to
lattice mismatch.lattice mismatch.
•Hence the efficiency of LED’s is expected to drop as the mismatch Hence the efficiency of LED’s is expected to drop as the mismatch
increases. increases.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

20. High extraction efficiency structuresHigh extraction efficiency structures
•Shaping of the LED die is critical to improve their Shaping of the LED die is critical to improve their
efficiency.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 truncated cones etc have been demonstrated to have
better extraction efficiency over conventional designs.better extraction efficiency over conventional designs.
•However cost increases with complexity.However cost increases with complexity.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

21. 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 using TIP devices are sawn using
beveled dicing blade to beveled dicing blade to
obtain chip sidewall angles obtain chip sidewall angles
of 35of 35
oo
to vertical. to vertical.
Krames et. al, Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999

22. Visible spectrum LEDsVisible spectrum LEDs
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.
The plot charts the gains made in luminous efficiency till date.

23. Visible spectrum LEDsVisible spectrum LEDs
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.
• 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.

24. 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 power One way is to mix to complementary colors at a certain power
ratio.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 emitted by The converter material absorbs some or all the light emitted by
the LED and re-emits at a longer wavelength.the LED and re-emits at a longer wavelength.
•Two parameters that are important in the generation of white Two parameters that are important in the generation of white
light are luminous efficiency and color rendering index.light are luminous efficiency and color rendering 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 possible monochromatic complementary colors result in highest possible
luminous efficiency.luminous efficiency.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

25. 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, emission The parameters of interest are absorption wavelength, emission
wavelength and quantum efficiency.wavelength and quantum efficiency.
•The overall energy efficiency is given byThe overall energy efficiency is given by
ηη = = ηη
extext((λλ
11/ / λλ
22))
•Even if the external quantum efficiency is 1, there is always an Even if the external quantum efficiency is 1, there is always an
energy loss associated with conversion.energy loss associated with conversion.
•Common wavelength converters are phosphors, which consist of Common wavelength converters are phosphors, which consist of
an inorganic host material doped with an optically active element.an inorganic host material doped with an optically active element.
•A common host is YA common host is Y
33AlAl
55OO
1212..
•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.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.

26. 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%.
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.
•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 have However, organic dyes have
finite lifetime. They finite lifetime. They
become optically inactive become optically inactive
after 10after 10
44
-10-10
66
optical optical
transitions.transitions.

27. White LEDs based on phosphor convertersWhite LEDs based on phosphor converters
From Light-Emitting Diodes, Fred Schubert.From Light-Emitting Diodes, Fred Schubert.
A blue GaInN/GaN LED
and a phosphor wavelength
converter suspended in a
epoxy resin make a white
Light LED.
The thickness of the phosphor
containing epoxy and the
concentration of the phosphor
determine the relative
strengths of the two emission
bands

28. Promise of Solid State LightingPromise of Solid State Lighting
•The use of solid state lighting devices promises huge savings in The use of solid state lighting devices promises huge savings in
energy consumption.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 GWyear.luminous intensity of 150 lm/W, energy savings are 40 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 green 25% less coal consumption, leading to lesser emissions of green
house gases.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