Light Emitting Diodes
EE 698A
Kameshwar Yadavalli
Outline
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Basics of Light Emitting Diodes (Electrical)
Basics of Light Emitting Diodes (Optical)
High internal efficiency designs
High extraction efficiency structures
Visible Spectrum LED’s
White-Light LED’s
The promise of solid state lighting
LED-Electrical Properties-PN junctions
• PN junction diode in
forward bias, the
electron-hole
recombination leads
to photon emission
• I = Is(eeV/kT-1)
• Threshold voltage
Vth = Eg/e
• I = IseeV/ηkT
where η is the
ideality factor
Double Heterostructure is used to confine the carriers, improving
the radiative recombination rate
From Light-Emitting Diodes, Fred Schubert.
LED-Electrical Properties-Hetero junctions
Grading of the heterojunction is done to reduce the resistance seen by
carriers
From Light-Emitting Diodes, Fred Schubert.
LED-Electrical Properties-Hetero junctions
From Light-Emitting Diodes, Fred Schubert.
LED-Electrical Properties-Carrier loss
• The confinement barriers are typically several hundred
meV (>>kT)
• Due to Fermi-Dirac distribution of carriers in the active
region, some carriers will have energy higher than that of
the barriers
• In AlGaAs/GaAs and AlGaN/GaN the barriers are high
• In AlGaInP/GaInP the barriers are lower resulting in
higher leakage currents
From Light-Emitting Diodes, Fred Schubert.
LED-Electrical Properties-Blocking layers
• Electron Blocking Layers are required to prevent electron
escape at high injection current densities
From Light-Emitting Diodes, Fred Schubert.
LED-Optical Properties-Efficiency
• ηint = # of photons emitted from active region per second
# of electrons injected in to LED per second
= Pint / (hν)
I/e
• ηextr = # of photons emitted into free space per second
# of photons emitted from active region per second
= P / (hν)
Pint / (hν)
From Light-Emitting Diodes, Fred Schubert.
LED-Optical Properties-Emission Spectrum
• The linewidth of an LED emitting in the visible range is
relatively narrow compared with the entire visible range
(perceived as monochromatic by the eye)
• Optical fibers are dispersive, limiting the bit rate X
distance product achievable with LED’s
• Modulation speeds achieved with LED’s are 1Gbit/s, as the
spontaneous lifetime of carriers in LED’s is 1-100 ns
From Light-Emitting Diodes, Fred Schubert.
LED-Optical Properties-Light Escape Cone
• Total internal reflection at the semiconductor air interface
reduces the external quantum efficiency.
• The angle of total internal reflection defines the light
escape cone.
sinθc = nair/ns
• Area of the escape cone = 2πr2(1-cosθc)
• Pescape / Psource = (1-cosθc)/2 = θc2/4 = (nair2/ns2)/4
From Light-Emitting Diodes, Fred Schubert.
LED-Optical Properties-Emission Spectrum
• Light intensity in air
(Lambertian emission
pattern) is given by
Iair = (Psource/4πr2) X
(nair2/ns2) cosΦ
• Index contrast
between the light
emitting material and
the surrounding
region leads to nonisotropic emission
pattern
From Light-Emitting Diodes, Fred Schubert.
LED-Optical Properties-Epoxy encapsulants
• Light extraction efficiency can be increased by using dome
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shaped encapsulants with a large refractive index.
Efficiency of a typical LED increases by a factor of 2-3 upon
encapsulation with an epoxy of n = 1.5.
The dome shape of the epoxy implies that light is incident at an
angle of 90o at the epoxy-air interface. Hence no total internal
reflection.
From Light-Emitting Diodes, Fred Schubert.
Temperature dependence of emission intensity
• Emission intensity decreases with increasing temperature.
• Causes include non-radiative recombination via deep levels,
surface recombination, and carrier loss over heterostucture
barriers.
From Light-Emitting Diodes, Fred Schubert.
High internal efficiency LED designs
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Radiative recombination probability needs to be increased and non-radiative
recombination probability needs to be decreased.
High carrier concentration in the active region, achieved through double
heterostructure (DH) design, improves radiative recombination.
R=Bnp
DH design is used in all high efficiency designs today.
From Light-Emitting Diodes, Fred Schubert.
High internal efficiency designs
• Doping of the active regions and that of the cladding regions
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strongly affects internal efficiency.
Active region should not be heavily doped, as it causes carrier
spill-over in to the confinement regions decreasing the radiative
efficiency
Doping levels of 1016-low 1017 are used, or none at all.
P-type doping of the active region is normally done due to the
larger electron diffusion length.
Carrier lifetime depends on the concentration of majority
carriers.
In low excitation regime , the radiative carrier lifetime
decreases with increasing free carrier concentration.
Hence efficiency increases with doping.
At high concentration, dopants induce defects acting as
recombination centers.
From Light-Emitting Diodes, Fred Schubert.
P-N junction displacement
• Displacement of the P-N junction causes significant change in
the internal quantum efficiency in DH LED structures.
• Dopants can redistribute due to diffusion, segregation or
drift.
From Light-Emitting Diodes, Fred Schubert.
Doping of the confinement regions
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Resistivity of the confinement regions should be low so that heating is
minimal.
High p-type conc. in the cladding region keeps electrons in the active region
and prevents them from diffusing in to the confinement region.
Electron leakage out of the active region is more severe than hole leakage.
From Light-Emitting Diodes, Fred Schubert.
Non radiative recombination
• The concentration of defects
which cause deep levels in
the active region should be
minimum.
• Also surface recombination
should be minimized, by
keeping all surfaces several
diffusion lengths away from
the active region.
• Mesa etched LEDs and lasers
where the mesa etch exposes
the active region to air, have
low internal efficiency due to
recombination at the
surface.
• Surface recombination also
reduces lifetime of LEDs.
From Light-Emitting Diodes, Fred Schubert.
Lattice matching
• Carriers recombine non-radiatively at misfit dislocations.
• Density of misfit dislocation lines per unit length is proportional to
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lattice mismatch.
Hence the efficiency of LED’s is expected to drop as the mismatch
increases.
From Light-Emitting Diodes, Fred Schubert.
High extraction efficiency structures
• Shaping of the LED die is critical to improve their
efficiency.
• LEDs of various shapes; hemispherical dome, inverted cone,
truncated cones etc have been demonstrated to have
better extraction efficiency over conventional designs.
• However cost increases with complexity.
From Light-Emitting Diodes, Fred Schubert.
High extraction efficiency structures
• The TIP LED employs
advanced LED die shaping
to minimize internal loss
mechanisms.
• The shape is chosen to
minimize trapping of light.
• TIP LED is a high power
LED, and the luminous
efficiency exceeds 100
lm/W.
• TIP devices are sawn using
beveled dicing blade to
obtain chip sidewall angles
of 35o to vertical.
Krames et. al, Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999
Visible spectrum LEDs
The plot charts the gains made in luminous efficiency till date.
From Light-Emitting Diodes, Fred Schubert.
Visible 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.
From Light-Emitting Diodes, Fred Schubert.
White-light LEDs
• White light can be generated in several different ways.
• One way is to mix to complementary colors at a certain power
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ratio.
Another way is by the emission of three colors at certain
wavelengths and power ratio.
Most white light emitters use an LED emitting at short
wavelength and a wavelength converter.
The converter material absorbs some or all the light emitted by
the LED and re-emits at a longer wavelength.
Two parameters that are important in the generation of white
light are luminous efficiency and color rendering index.
It is shown that white light sources employing two
monochromatic complementary colors result in highest possible
luminous efficiency.
From Light-Emitting Diodes, Fred Schubert.
White-light LEDs
• Wavelength converter materials include phosphors,
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semiconductors and dyes.
The parameters of interest are absorption wavelength, emission
wavelength and quantum efficiency.
The overall energy efficiency is given by
η = ηext(λ1/ λ2)
Even if the external quantum efficiency is 1, there is always an
energy loss associated with conversion.
Common wavelength converters are phosphors, which consist of
an inorganic host material doped with an optically active element.
A common host is Y3Al5O12.
The optically active dopant is a rare earth element, oxide or
another compound.
Common rare earth elements used are Ce, Nd, Er and Th.
From Light-Emitting Diodes, Fred Schubert.
White-light LEDs
• Phosphors are stable
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materials and can have
quantum efficiencies of
close to 100%.
Dyes also can have
quantum efficiencies of
close to 100%.
• Dyes can be encapsulated
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in epoxy or in optically
transparent polymers.
However, organic dyes
have finite lifetime. They
become optically inactive
after 104-106 optical
transitions.
From Light-Emitting Diodes, Fred Schubert.
White LEDs based on phosphor converters
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
From Light-Emitting Diodes, Fred Schubert.
Promise of Solid State Lighting
• The use of solid state lighting devices promises huge savings in
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energy consumption.
The electricity for lighting needs is 60GW, over 24 hrs.
About 24 GWyear is consumed by incandescent lamps with a
luminous intensity of 15lm/W.
36 GWyear is consumed by FL/HID lamps with a luminous
intensity of 75lm/W.
Assuming that by year 2020, they are replaced by LEDs with
luminous intensity of 150 lm/W, energy savings are 40 GWyear.
That translates to $40 billion in savings.
At 4Mtons / GWyear of coal consumption, net savings lead to
25% less coal consumption, leading to lesser emissions of green
house gases.
Global savings are projected to be about $140B.
Roland Haitz, Adv. in Solid State Physics