LED
Fundamentals
Prof. Yiannis Kaliakatsos
Dept of Electronics, T.E.I. of Crete
giankal@chania.teicrete.gr
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In this lecture we will review the mechanisms of
light generation from semiconductor materials
and we will focus on Light Emitting Diodes
(LEDs)
The aim of the lecture is to give you some
fundamental concepts of light generation from
inorganic semiconductor materials in order to have a
comparison between them and similar devices from
organic semiconductor materials
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INTRODUCTION
Photonic semiconductor devices are those in which the photon
plays a major role
They are divided into three groups:
• Devices as light sources (LED, Laser Diode)
• Devices as light detectors (Photodiode)
• Devices as light converters (Solar Cell)
In this lecture we will examine devices of the first group
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LED and Laser Diode belong to the luminescent device group.
Luminescence is the emission of optical radiation as a result of
electronic radiation in a device or material, excluding any radiation
that is purely the result of the temperature of the material
(incandescence).
A semiconductor emits electromagnetic radiation (in form of light),
commonly, when excess electrons that have been injected into the
conduction band of a semiconductor fall into the valence band
releasing the energy difference as photons
Next figure demonstrates schematically the basic recombination
transitions of excess carriers in a semiconductor
Not all transitions occur in the same material or under the same
conditions and not all the transitions are radiative
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EC
ED
Et
Eg
EA
EV
Band-to-band
recombination
Recombination through
impurities or defects
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Auger
transition
LIGHT EMISSION FROM SEMICONDUCTORS
The following phenomena related with light emission are the
important for our discussion
•
Absorption
•
Spontaneous Emission
•
Stimulated Emission
These are shown schematically in the next slide
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most
Figure 1. Radiative recombination processes:
a) absorption, b) spontaneous emission, c) stimulated emission.
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Spontaneous emission
When an electron in an excited state in the conduction band falls
back into the valence band releases its excess energy in the
form of a photon with an energy given by:
The photon emitted by the electron during this process has a
random phase and direction.
The rate at which excited electrons will spontaneously emit
photons is given by,
𝑅𝑠𝑝0 = 𝐵𝑟 𝑛0 𝑝0
Where Br is the transition probability of an excited particle falling
into a vacant lower state.
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In equilibrium, the rate at which electrons are excited into the
conduction band is equal to the rate of electrons falling back to the
valence band.
If the concentration of electron-hole pairs increase
𝑛 = ∆𝑛 + 𝑛0 ,
𝑝 = ∆𝑝 + 𝑝0
The recombination rate also increase
𝑅𝑠𝑝 = 𝐵𝑟 ∆𝑛 + 𝑛0 )( ∆𝑝 + 𝑝0 = 𝐵𝑟 𝑝0 𝑛0 + 𝑛0 △ 𝑝 + 𝑝0 △ 𝑛 +△ 𝑛 △ 𝑝
= 𝑅𝑠𝑝0 + 𝑅𝑠𝑝𝑒𝑥
where
𝑅𝑠𝑝𝑒𝑥 = 𝐵𝑟 𝑛0 △ 𝑝 + 𝑝0 △ 𝑛 +△ 𝑛 △ 𝑝
is the excess recombination rate producing the light
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If we would like to have continuous light emission we need to
keep this excess recombination rate
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Interband transitions, (Band-to-band)
A semiconductor is always one of two types, a direct band gap
or an indirect band gap.
The minimal-energy state in the conduction band, and the
maximal-energy state in the valence band, are each
characterized by a certain k-vector.
If the k-vectors are the same, it is called a "direct gap". If they
are different, it is called an "indirect gap".
You can see this schematically in the next figure
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Conduction
Band
Valence
Band
Energy
Energy
Valence
Band
Conduction
Band
Momentum
Momentum
Direct gap
semiconductor
Indirect gap
semiconductor
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Either for photon absorption or emission the conventional theory for
optical transitions between the valence and the conduction bands
of the semiconductor is based on the so-called k-selection rule,
i.e. the transition must conserve the total wave vector of the system
(conservation of energy and crystal momentum)
If the electron is near the bottom of the conduction band and the hole
is near the top of the valence band (as is usually the case), this
process is possible in a direct band gap semiconductor, but
impossible in an indirect band gap one, because conservation of
crystal momentum would be violated.
For radiative recombination to occur in an indirect band gap material,
the process must also involve the absorption or emission of a
phonon, where the phonon momentum equals the difference
between the electron and hole momentum.
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The involvement of the phonon makes this process much less likely
to occur in a given span of time, which is why radiative
recombination is far slower in indirect band gap materials than
direct band gap ones.
This is why light-emitting and laser diodes are almost always made
of direct band gap materials, and not indirect band gap ones
If for other reasons, i.e. color emission, we use indirect band gap
semiconductors, we need to be 'altered„ them in order to enhance
their radiative processes. T
This is usually accomplished by introducing specific impurities (such
as nitrogen) into the indirect band-gap semiconductor to form efficient
radiative recombination centers within them.
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Band-to-band recombination depends on the density of available
electrons and holes. Both carrier types need to be available in the
recombination process. Therefore, the rate is expected to be
proportional to the product of n and p.
Also, in thermal equilibrium, the recombination rate must equal the
generation rate since there is no net recombination or generation.
As the product of n and p equals ni2 in thermal equilibrium, the net
recombination rate can be expressed as:
𝑅21 = 𝐵21 (𝑛𝑝 − 𝑛𝑖2 )
where B21 is the bimolecular recombination constant.
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Trap assisted recombination
The net recombination rate for trap-assisted recombination is given by
𝑅𝑡 =
𝑝𝑛 − 𝑛𝑖2
𝐸 − 𝐸𝑡
𝑝 + 𝑛 + 2𝑛𝑖 𝑐𝑜𝑠ℎ 𝑖
𝑘𝑇
𝑁𝑡 𝜐𝑡ℎ 𝜎
The derivation of this equation is beyond the scope of this text
This expression can be further simplified for n >> p to:
𝑝𝑛 − 𝑝𝑛0
1
𝑈𝑝 = 𝑅𝑝 − 𝐺𝑝 =
𝑤ℎ𝑒𝑟𝑒 𝜏𝑝 = 𝜏𝑛 =
𝜏𝑝
𝑁𝑡 𝜐𝑡ℎ 𝜎
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Excitation mechanism for LEDs and Diode Lasers
Both LEDs (Light Emitting Diodes) and Diode Lasers are in
principle p-n semiconductor diodes.
Injection electroluminescence is the most important method of
excitation in p-n junction.
In this type of devices when a forward bias is applied to the p-n
junction the injection of minority carriers across the junction can
give rise to efficient radiative recombination since electric energy
can be converted directly into photons.
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The aforementioned are shown schematically in the following figures
p
p
n+
eV0
n+
EV
EV
Eg
EF
Eg
hν
hν=Eg
hν
hν
eV0
EC
EC
V
+
p-n junction without bias
-
forward bias p-n junction
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The previous figure shows the energy band diagram of an unbiased
pn+ junction device in which the n-side is more heavily doped than the
p-side. The band energy diagram is drawn to keep the Fermi level
uniform through the device which is a requirement of equilibrium with
no applied bias.
The depletion region in a pn+ device extends mainly on the p-side.
There is a potential energy barrier eVo , where Vo is the built-in
potential. The higher concentration of free electrons in the n-side
encourages the diffusion of conduction electrons from the n to the pside. This net electron diffusion is prevented by the electron barrier
eVo.
As soon as a forward bias V is applied, this voltage drops across the
depletion region since this is the most resistive part of the device. As a
result the built-in potential is reduced to Vo - V which allows the
electrons from the n+ side to diffuse, or become injected into the pside, as in figure b
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In equilibrium the minority carriers in p and n regions are given by
np0 and pn0 respectively (in our case with p-n+ semiconductor pn0 ≈ 0)
and with a forward bias we have an excess of minority carriers in the
p region given by:
∆𝑛𝑝 = 𝑛𝑝 − 𝑛𝑝0 =
𝑞𝑉
𝑛𝑝0 (𝑒 𝑘𝑇
− 1);
∆𝑝 ≈ 0
and thus:
𝑅𝑠𝑝𝑒𝑥 = 𝐵𝑟 𝑛0 △ 𝑝 + 𝑝0 △ 𝑛 +△ 𝑛 △ 𝑝 ≈ 𝐵𝑟 𝑝0 △ 𝑛 =
=
𝑞𝑉
𝑅𝑠𝑝 𝑒 𝑘𝑇
−1
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𝑞𝑉
𝐵𝑟 𝑝0 𝑛𝑝0 𝑒 𝑘𝑇
−1
As the forward current through the p-n junction is:
𝐼=
𝑞𝑉
𝐼𝑠 𝑒 𝑘𝑇
−1
Hence the recombination
rate increases linearly with
the forward current:
𝑅𝑠𝑝𝑒𝑥 ~𝐼
As shown in the figure
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LED STRUCTURE
There are various types of LED structure.
It depends on the application what is the favor structure.
In any case we attempt to have the highest luminous efficiency.
We prefer the most of radiative recombination to take place from
the side of the junction nearest the surface, since then the
chances of re-absorption are lessened.
Such a structure is shown in the next figure and it is known as
planar LED
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This is the simplest LED structure. It is fabricated by epitaxially
growing doped semiconductor layers on a suitable substrate (e.g.
GaAs or GaP) as depicted in the following figure. The substrate is an
essential mechanical support and can be of different material
a
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b
The p-side is on the surface from which the light is emitted and is
therefore made narrow (a few microns) to allow the photons to escape
without being reabsorbed.
The photons that are emitted towards the n-side become either
absorbed or reflected back at the substrate interface depending on the
substrate thickness and the exact structure of the LED.
The use of a segment electrode will encourage reflections from the
semiconductor-air interface. (fig a)
It is also possible to form the p-side by diffusing dopants into the
epitaxial n+ - layer which is a diffuse junction planar LED as illustrated
in figure b.
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Not all light rays reaching the semiconductor –air interface can escape
due to the total internal reflection. Those rays with angles of incidence
grater than the critical value θC become reflected as shown in the
next figure
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Light output
p
n+
Total
reflection
reabsorbed
substrate
To reduce the internal total reflection we construct LED with a
special structure with the same of a dom for the p-side like in the
next figure a.
As this type of construction is rather expensive and the p-side is
extended to the air we prefer to encapsulate the p-side with a
plastic transparent material like in figure b
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Burrus LEDs or Surface Emitter LEDs
It was design to be suitable to fiber optics communication
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The edge emitting LED
The edge emitting LED use an active area having stripe geometry.
Because the layers above and below the stripe have different
refractive indices acting as a waveguide for the emitted light.
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An heterostructure (heterojunction) diode is formatting when we have
two different semiconductors in contact of different type anisotype
heterojunction) or of the same type but with different concentrations
(isotype heterojunction).
The band diagram of an heterojunction compose a waveguide and
this is very useful for LED construction as the light emitted in the
limited area of this “wave guide”
p-type
p-type
EC2
n-type
EC2
n-type ΔEc
ΔEc
EC1
EC1
EF
EF1
Eg2
Eg1
Eg2
Eg1
ΔEV
EV1
EF2
ΔEV
EV1
EV2
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WD
EV2
Stripe geometry DH edge emitter LED (ELED).
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Materials for LED construction
The main requirements for a suitable LED material are:
 The semiconductor material must have an energy gap of
appropriate width;
 The semiconductor may exist in both p and n types, preferably
with low resistivity
 Efficient radiative pathways must be present.
 They must have energy gaps greater than 2 eV for visible
radiation.
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To chose the appropriate material for LED we have
also to remember:
 The semiconductor bandgap energy defines the energy of the
emitted photons in a LED.
 To fabricate LEDs that can emit photons from the infrared to the
ultraviolet parts of the e.m. spectrum, we need to user several
different material systems.
 No single system can span this energy band, at present,
although the III-V nitrides come close.
 The criterion of luminous efficacy limits display diodes to p-n
junctions in single-crystalline, zinc blended-structured III-V
semiconductors.
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There are various direct gap semiconductor materials that can be
readily doped to make commercial p-n junction LEDs.
An important class of commercial semiconductor materials which
cover the visible spectrum is the III-V ternary alloys based on GaAs
and GaP which are denoted as GaAs1-yPy.
When y<0,45 the alloy is a direct band gap semiconductor and the
radiative recombination process is direct as shown in the following
scheme.
The rate of recombination is proportional to the product of electron
and hole concentrations. The emitted wavelengths range from about
630 nm(red) for y=0,45 to 870 nm for y =0.
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GaAs1-yPy with y>0,45 are indirect gap semiconductor.
The recombination processes occur through recombination centers
and involve the creation of phonons rather than photons and the
radiative recombination is negligible.
If we add isoelectronic impurities, such as nitrogen (N) into the
semiconductor crystal we introduce localized energy levels (electron
traps). In this case the recombination take place through these centers
and the emitted photons have energies slightly different to Eg .
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The recombination process depends on the concentration of such
impurities and in general the efficiency of LEDs is lower.
However Nitrogen doped indirect band gap GaAs1-yPy alloys are
widely used in inexpensive green, yellow and orange LEDs.
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Another semiconductor used for LED construction is GaN. This is a
direct band gap semiconductor with Eg = 3.4 eV. (≈365 nm) That
means an emission spectrum in the ultraviolet region. In order to
construct LEDs emitting blue light we use the InGaN alloy which has
a band gap of Eg = 2,7 eV (≈ 460 nm, i.e. blue color).
As GaN is a defect rich material with typical dislocation densities
exceeding 108 cm−2. Light emission from InGaN layers grown on
such GaN buffers used in blue and green LEDs is expected to be low
because of non-radiative recombination at such defects.
In the indium-rich regions, with a lower band gap than the
surrounding material, most electron-hole pairs recombine and by the
lower potential energy of these clusters carriers are hindered to
diffuse and recombine non-radiatively at crystal defects.
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Recently considerable progress made towards more efficient blue
LEDs using direct band gap semiconductors of the II-VI group (i.e.
ZnSe). The main problem is the current technological difficulty in
appropriate doping to construct efficient pn-junctions.
Usually for LED fabrication we use ternary (3 elements) or
quaternary (4 elements) alloys based on III-V elements like,
GaAs1-yPy or Al1-xGaxAs or In1-xGaxAs1-yPy.
The composition can be varied to adjust the band gap and hence the
emitted radiation to cover wide rang od wavelengths.
In the following table are shown the materials used for LED‟s
construction
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Color
Wavelength (nm)
Voltage (V)
Infrared
λ > 760
ΔV < 1.9
Red
610 < λ < 760
Semiconductor Material
Gallium arsenide (GaAs)
Aluminium gallium arsenide (AlGaAs)
1.63 < ΔV < 2.03
Aluminium gallium arsenide (AlGaAs)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Orange
590 < λ < 610
2.03 < ΔV < 2.10
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Yellow
570 < λ < 590
2.10 < ΔV < 2.18
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
1.9[36] < ΔV < 4.0
Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
Gallium(III) phosphide (GaP)
Aluminium gallium indium phosphide (AlGaInP)
Aluminium gallium phosphide (AlGaP)
Green
500 < λ < 570
Blue
450 < λ < 500
2.48 < ΔV < 3.7
Zinc selenide (ZnSe)
Indium gallium nitride (InGaN)
Silicon carbide (SiC) as substrate
Silicon (Si) as substrate — (under development)
Violet
400 < λ < 450
2.76 < ΔV < 4.0
Indium gallium nitride (InGaN)
Purple
multiple types
2.48 < ΔV < 3.7
Dual blue/red LEDs,
blue with red phosphor,
or white with purple plastic
Diamond (235 nm)[37]
Boron nitride (215 nm)[38][39]
Aluminium nitride (AlN) (210 nm)[40]
Aluminium gallium nitride (AlGaN)
Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm
Ultraviolet
λ < 400
3.1 < ΔV < 4.4
White
Broad spectrum
ΔV = 3.5
Blue/UV diode with yellow phosphor
LED Construction
 Efficient light emitter is also an efficient absorbers of radiation
therefore, a shallow p-n junction required.
 Active materials (n and p) will be grown on a lattice matched
substrate.
 The p-n junction will be forward biased with contacts made by
metallisation to the upper and lower surfaces.
 Ought to leave the upper part „clear‟ so photon can escape.
 The silica provides passivation/device isolation and carrier
confinement
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Efficient LED

Need a p-n junction (preferably the same semiconductor
material only different dopants)

Recombination must occur  Radiative transmission to give
out the „right coloured LED‟

“Right coloured LED”  hc/ = Ec-Ev = Eg
 so choose material with the right Eg

Direct band gap semiconductors to allow efficient
recombination

All photons created must be able to leave the semiconductor

Little or no reabsorption of photons
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Correct band gap
Direct band gap
Material Requirements
Efficient radiative
pathways must exist
Material can be made
p and n-type
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LED spectrum
Because the statistical nature of the recombination process between
electrons and holes the emitted photons in spontaneous emission are
in random directions.
The spectrum of spontaneous emission has a threshold energy Eg, a
peak at Eg+ ½ kT and a half power width of 1.8 kT.
This translates into a spectrum width of
The theoretical emission spectrum of a LED is given on the following
figure
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LED Spectrum characteristics
Spectrum
of a blue
LED.
Experimental emission spectra
from various LEDs are shown in
the aside figures.
They can be described by the
use of a Gaussian function.
Therefore the spectral power
density function can be given by:
Spectrum
of a red
LED.
1 𝜆 − 𝜆𝑝𝑒𝑎𝑘
𝑃 𝜆 =𝑃
exp −
2
𝜎
𝜎 2𝜋
1
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2
Theoretical emission spectrum of a semiconductor exhibiting
substantial alloy broadening. The full width at half maximum (FWHM)
is related to the standard deviation by the equation shown in the
figure
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RGB System
White light can be produced by mixing differently colored light,
the most common method is to use red, green and blue (RGB).
Combined spectra of a common
blue LED, a yellow-green LED
and a high brightness red LED.
They took these spectra on an
Ocean Optics HR2000
spectrometer
This spectrum is not calibrated
for intensity.
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Temperature dependence of LED spectrum
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Temperature dependence of LED spectrum
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Light distribution from a LED
This radiation diagram shows the
output of a blue LED with a waterclear case (Photron PL-BA31).
Most of the light is shooting straight
out the front of the package.
This radiation diagram shows the
output of a green LED with a
diffused colored case (Photron PLGB574G).
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Efficiency of a LED
The internal quantum efficiency of a LED may approach unity but the
total efficiencies are much lower due to reabsorption through the
material and total reflections between the different materials (diodesubstrate, substrate – contacts, diode-air etc)
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External Quantum Efficiency (EQE)
It gives a measure to how efficiently the device coverts electrons to
photons and allows them to escape.
It is the ratio of the number of photons emitted from the LED to the
number of electrons passing through the device
EQE =
[Injection efficiency] x [Internal quantum efficiency] x [Extraction efficiency]
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Internal Quantum Efficiency (IQE) or Radiative Efficiency
Not all electron-hole recombination are radiative.
IQE is the proportion of all electron-hole recombination in the active
region that are radiative, producing photons.
Extraction Efficiency or Optical Efficiency
Once the photons are produced within the semiconductor device, they
have to escape from the crystal in order to produce a light-emitting
effect.
Extraction efficiency is the proportion of photons generated in the
active region that escape from the device.
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Wall-Plug Efficiency (also termed Radiant Efficiency)
Wall-plug efficiency is the ratio of the radiant flux, measured in watts,
and the electrical input power i.e the efficiency of converting electrical
to optical power.
Wall-Plug Efficiency = [EQE] x [Feeding efficiency]
Feeding Efficiency
Feeding efficiency is the ratio of the mean energy of the photons
emitted and the total energy that an electron-hole pair acquires from
the power source.
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Efficiency’s calculation for various LEDs structure
Injection Efficiency
It is due to electron-hole recombination
to produce photons. The electrons
passing through the device have to be
injected into the active region.
Injection efficiency is the proportion of electrons passing through the
device to those injected into the active region
In the case of a planar LED this is given by
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Dome LEDs
In a planar LED, due to the phenomenon of total reflection, the
fraction F of the total generated radiation that is actually transmitted
into the second medium is given by:
A method to reduce the losses from total reflection, is to give to the
LED a hemispherical or a parabolic scheme, like in the next figures
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Light Output versus Current Characteristic for a LED
The light output power against
current characteristics for a LED is
given in the figure.
LED is a very linear device and so it
is suitable for analog optical
transmission
where
severe
constraints are put on the linearity
of the optical source
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I-V characteristic of a LED
The I-V characteristic of a LED is similar to those of a forward bias p-n
diode
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ADVANTAGES OF LEDs
Efficiency:
LEDs produce more light per watt than incandescent bulbs. Their efficiency is
not affected by shape and size, unlike fluorescent light bulbs or tubes.
Color:
LEDs can emit light of an intended color without the use of the color filters that
traditional lighting methods require. This is more efficient and can lower initial
costs.
Size:
LEDs can be very small (smaller than 2 mm2) and are easily populated onto
printed circuit boards.
On/Off time:
LEDs light up very quickly. A typical red indicator LED will achieve full
brightness in under a microsecond. LEDs used in communications devices
can have even faster response times.
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ADVANTAGES OF LEDs
Cycling: LEDs are ideal for use in applications that are subject to frequent
on-off cycling, unlike fluorescent lamps that burn out more quickly when
cycled frequently, or HID lamps that require a long time before restarting.
Dimming: LEDs can very easily be dimmed either by pulse-width modulation
or lowering the forward current.
Cool light: In contrast to most light sources, LEDs radiate very little heat
in the form of IR that can cause damage to sensitive objects or fabrics.
Wasted energy is dispersed as heat through the base of the LED.
Slow failure: LEDs mostly fail by dimming over time, rather than the
abrupt burn-out of incandescent bulbs.
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ADVANTAGES OF LEDs
Lifetime: LEDs can have a relatively long useful life. One report
estimates 35,000 to 50,000 hours of useful life, though time to
complete failure may be longer. Fluorescent tubes typically are rated
at about 10,000 to 15,000 hours, depending partly on the conditions
of use, and incandescent light bulbs at 1,000–2,000 hours.
Shock resistance: LEDs, as solid state components, are difficult to
damage with external shock, unlike fluorescent and incandescent
bulbs which are fragile.
Focus: The solid package of the LED can be designed to focus its
light. Incandescent and fluorescent sources often require an external
reflector to collect light and direct it in a usable manner.
Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.
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DISADVANTAGES OF LEDs
Low Efficiency
Fluorescent lamps are usually can be more efficient
High initial price:
LEDs are currently more expensive, price per lumen, on an initial
capital cost basis, than most conventional lighting technologies.
The additional expense partially stems from the relatively low
lumen output and the drive circuitry and power supplies needed.
Temperature dependence:
LED performance largely depends on the ambient temperature of
the operating environment.
Over-driving the LED in high ambient temperatures may result in
overheating of the LED package, eventually leading to device
failure.
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DISADVANTAGES OF LEDs
Voltage sensitivity:
LEDs must be supplied with the voltage above the threshold and a
current below the rating.
This can involve series resistors or current-regulated power supplies.
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DISADVANTAGES OF LEDs
Light quality:
Most cool-white LEDs have spectra that
differ significantly from a black body
radiator like the sun or an incandescent
light.
The spike at 460 nm and dip at 500 nm
can cause the color of objects to be
perceived differently under cool-white
LED illumination than sunlight or
incandescent
sources,
due
to
metamerism, red surfaces being rendered
particularly badly by typical phosphor
based cool-white LEDs.
However, the color rendering properties of
common fluorescent lamps are often inferior to
what is now available in state-of-art white LEDs.
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DISADVANTAGES OF LEDs
Area light source:
Incandescent Bulb
radiation pattern
LEDs do not approximate a “point
source” of light, but rather a
lambertian distribution.
So LEDs are difficult to use in
applications requiring a spherical
light field.
LEDs are not capable of providing
divergence below a few degrees.
LED Bulb
radiation pattern
This is contrasted with lasers, which
can produce beams with
divergences of 0.2 degrees or less.
http://www.olino.org
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DISADVANTAGES OF LEDs
Blue pollution:
Because cool-white LEDs emit
proportionally more blue light than
conventional outdoor light sources such
as high-pressure sodium lamps, the
strong wavelength dependence of
Rayleigh scattering means that coolwhite LEDs can cause more light
pollution than other light sources.
The International Dark-Sky Association
discourages the use of white light
sources with correlated color
temperature above 3,000 K
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DISADVANTAGES OF LEDs
Blue hazard:
There is a concern that blue LEDs and
cool-white LEDs are now capable of
exceeding safe limits of the so-called
blue-light hazard as defined in eye
safety specifications such as ANSI/IESNA
RP-27.1-05: Recommended Practice for
Photobiological Safety for Lamp and
Lamp Systems.
Cool White LED, spectrum
Warm White LED, spectrum
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LED APPLICATIONS
The applications of LEDs are very wide but they can categorized into
four kinds:
1. In displays (Sign Applications)
2. In illumination
3. In telecommunication
4. In control systems
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Sign Applications With LEDs
•
•
•
•
Full Color Video
Monochrome Message Boards
Traffic/VMS
Transportation - Passenger Information
Light Emitting Diodes in VMS (Variable Message Signs) have been widely
used and accepted since the late 1980's. This "single color" signage is
visible in a myriad of applications including traffic management,
commercial advertising, shopping malls and public transportation to name
a few.
It wasn't until the mid 1990's with the advent and cost reduction of high
brightness InGaN blue LEDs that full color, RGB LED signs began their
major entry into the video display market. Sign builders often face a
variety of difficulties when integrating RGB technology into their video
systems
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Red lights in Taiwan will soon be much greener. By 2011, all traffic lights on the
small island republic will be fitted with efficient LED lights thanks to a NT $229
million (US $7 million) project set to begin next year. Almost half of all traffic
lights in Taiwan already use LEDs; the remaining 420,000 traffic lights will be
converted over three years, providing an estimated savings of 85% in power
consumption.
http://www.inhabitat.com/2007/07/10/taiwans-led-traffic-lights/
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Illumination With LEDs
With the recent advent of White LEDs as well as advancements in
ultra bright InGaAlP and GaN technologies, LEDs have begun to
replace conventional type lighting in a variety of illumination
applications.
LEDs not only consume far less electricity than traditional forms of
illumination, resulting in reduced energy costs, but require less
maintenance and repair.
Studies have shown that the use of LEDs in illumination
applications can offer:




Greater visual appeal
Reduced energy costs
Increased attention capture
Savings in maintenance and lighting replacements
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Organic Light Emitting Diodes (OLED)
Organic light emitting diodes (OLEDs) are optoelectronic devices based
on small molecules or polymers that emit light when an electric current
flows through them.
Simple OLED consists of a fluorescent
organic layer sandwiched between two
metal electrodes. Under application of
an electric field, electrons and holes are
injected from the two electrodes into the
organic layer, where they meet and
recombine to produce light.
They have been developed for applications in flat panel displays that
provide visual imagery that is easy to read, vibrant in colors and less
consuming of power.
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Origin of band gap on an
organic semiconductor
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OLED’s Advantages
 are light weight, durable, power efficient and ideal for portable
applications.
 have fewer process steps and also use both fewer and low-cost
materials than LCD displays.
 can replace the current technology in many applications due to
following performance advantages over LCDs.








Greater brightness
Faster response time for full motion video
Fuller viewing angles
Lighter weight
Greater environmental durability
More power efficiency
Broader operating temperature ranges
Greater cost-effectiveness
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OLED Dispay’s Advantages
 Self-luminousThe efficiency of OLEDs is better than that of other display
technologies without the use of backlight, diffusers, and polarizers.
 Low cost and easy fabricationRoll-to-roll manufacturing process, such as, inkjet printing and screen
printing, are possible for polymer OLEDs.
 Color selectivityThere are abundant organic materials to produce blue to red light.
 Lightweight, compact and thin devicesOLEDs are generally very thin, measuring only ~100 nm
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OLED Dispay’s Advantages
 FlexibilityOLEDs can be easily fabricated on plastic substrates paving the way
for flexible electronics.
 High brightness and high resolutionOLEDs are very bright at low operating voltage (White OLEDs can be
as bright as 150,000 cd/m2)
 Wide viewing angle –
OLED emission is lambertian and so the viewing angle is as high as
160 degrees
 Fast response –
OLEDs EL decay time is < 1μs.
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OLED’s Disadvantages
 Highly susceptible to degradation by oxygen and water
molecules.
So the main disadvantage of an OLED is the lifetime.
With proper encapsulation, lifetimes exceeding 60,000 hours have
been demonstrated.
 Low glass transition temperature
Tg for small molecular devices (>700 C).
So the operating temperature cannot exceed the glass transition
temperature.
 Low mobility
due to amorphous nature of the organic molecules.
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Some OLED Applications






TVs
Cell Phone screens
Computer Screens
Keyboards (Optimus Maximus)
Lights
Portable Device displays
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Future OLED Applications
OLEDs can be used in High-Resolution Holography (Volumetric
Display). Professor Orbit showed on May 12, 2007, EXPO Lisbon
the potential application of these materials to reproduce threedimensional video.
OLEDs could also be used as solid-state light sources. OLED
efficacies and lifetime already exceed those of incandescent light
bulbs, and OLEDs are investigated worldwide as source for general
illumination;
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References for further reading
http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/
http://en.wikipedia.org/wiki/Light-emitting_diode
http://www.materials.uoc.gr/el/undergrad/courses/ETY580/notes/optodevices.pdf
http://www.phy.iitkgp.ernet.in/ptaccd2/Speakers_manuscript/ADhar.pdf
(ORGANIC OPTOELECTRONICS : A FUTURE PROMISE)
www.ece.rochester.edu/courses/ECE423/ECE223.../Lu_06.pdf
http://www.madehow.com/Volume-1/Light-Emitting-Diode-LED.html
http://www.springerlink.com/content/1302581718g54q38/
(Materials for light emitting diodes , R.J. Archer)
www.riksutstallningar.se/upload/Semiarier/pdf/led.pdf
(LED Characteristics. Erik Swennen & Patrick van der Meulen)
www.pshk.org.hk/.../Green%20Automotive/.../20090508-08DrAlfredFELDEROSRAM_public.pdf
(A Future Driver for Green. Applications in Lighting,. Automotive and Display. Dr.
Alfred Felder )
http://www.solutionsforledlights.com/2008/08/applications-for-led-lighting.html
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Acknowledgments
I would like to express my sincere
thanks to:
•
•
•
•
•
E.U. LLP-ERASMUS
IKY
T.E.I. of Crete
Thomas Anthopoulos from IC
Costas Petridis and Popi Tsitou
from T.E.I.
• Our sponsors
For their invaluable support
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Thank you for your attention
We are waiting you to come back soon
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