PHYSICS PROJECT ON LED

(Light-emitting diode)

NAME: Naman Kalra

STD: XIIth – S
ROLL NUMBER: 17612239
TOPIC: LIGHT EMITTING DIODE (LED)
GUIDED BY: Mr. Ravish Kumar Mishra
SCHOOL: Rawal International School

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 Acknowledgement

In performing my investigatory project, we had to take the

help and guideline of some respected persons, who deserve
our greatest gratitude. The completion of this project gives
me much Pleasure. I would like to show our gratitude Mr.
Ravish Kumar Mishra, Physics teacher of Rawal International
School for giving me a good guideline for project throughout
numerous consultations. I would also like to expand our
deepest gratitude to all those who have directly and indirectly
guided me in making this project.
Many people, especially my classmates, have made valuable
comment suggestions on this proposal which gave us an
inspiration to improve my project. I would like to thank all the
peoples for their help directly and indirectly to complete my
investigatory project.
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 INDEX

• INTRODUCTION

• HISTORY

• TECHNOLOGY

• COLOURS AND MATERIALS

• TYPES

• CONSIDERATIONS FOR USE

• APPLICATIONS

• BIBLIOGRAPHY

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 INTRODUCTION
Light-emitting diode (LED) is a two-lead semiconductor light
source. It is a basic pn-junction diode, which emits light when
activated. When a fitting voltage is applied to the
leads, electrons are able to recombine with electron
holes within the device, releasing energy in the form
of photons. This effect is called electroluminescence, and the
color of the light (corresponding to the energy of the photon) is
determined by the energy band gap of the semiconductor. An
LED is often small in area
(less than 1 mm2) and
integrated optical
components may be used to
shape its radiation pattern.
Appearing as practical
electronic components in
1962, the earliest LEDs
emitted low-intensity
infrared light. Infrared LEDs
are still frequently used as transmitting elements in remote-
control circuits, such as those in remote controls for a wide
variety of consumer electronics. The first visible-light LEDs were
also of low intensity, and limited to red.

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 HISTORY
Discoveries and early devices

Green
electroluminescence
from a point contact
on a crystal of Sic
recreates H. J.
Round's original
experiment from
1907.
Electroluminescence as a phenomenon was discovered in 1907
by the British experimenter H. J. Round of Marconi Labs, using a
crystal of silicon carbide and a cat's-whisker detector. Russian
inventor Oleg Losev reported creation of the first LED in
1927. His research was distributed in Russian, German and
British scientific journals, but no practical use was made of the
discovery for several decades. Rubin Braunstein of the Radio
Corporation of America reported on infrared emission
from gallium arsenide (GaAs) and other semiconductor alloys in
1955. Braunstein observed infrared emission generated by
simple diode structures using gallium antimonite (GaSb),

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GaAs, indium phosphide (InP), and silicon-germanium (SiGe)
alloys at room temperature and at 77 Kelvin.
In 1957, Braunstein further demonstrated that the rudimentary
devices could be used for non-radio communication across a
short distance.
The first visible-spectrum (red) LED was developed in 1962 by
Jr., while working at General Electric Company. Holonyak first
reported this breakthrough in the journal Applied Physics
Letters on the December 1, 1962. M. George Craford a former
graduate student of Holonyak invented the first yellow LED and
improved the brightness of red and red-orange LEDs by a factor
of ten in 1972.In 1976; T. P. Pearsall created the first high-
brightness, high-efficiency LEDs for optical fiber
telecommunications by inventing new semiconductor materials
specifically adapted to optical fiber transmission wavelengths.

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 TECHNOLOGY

The inner workings of an LED, showing circuit (top) and band

diagram (bottom)

Physics
The LED consists of a chip of semiconducting
material doped with impurities to create a p-n junction. As in
other diodes, current flows easily from the p-side, or anode, to
the n-side, or cathode, but not in the reverse direction. Charge-
carriers—electrons and holes—flow into the junction

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from electrodes with different voltages. When an electron
meets a hole, it falls into a lower energy level and
releases energy in the form of a photon.
The wavelength of the light emitted, and thus its color,
depends on the band gap energy of the materials forming
the p-n junction. In silicon or germanium diodes, the electrons
and holes usually recombine by a non-radiative transition,
which produces no optical emission, because these are indirect
band gap materials. The materials used for the LED have
a direct band gap with energies corresponding to near-infrared,
visible, or near-ultraviolet light.
LED development began with infrared and red devices made
with gallium arsenide. Advances in materials science have
enabled making devices with ever-shorter wavelengths,
emitting light in a variety of colors.
LEDs are usually built on an n-type substrate, with an electrode
attached to the p-type layer deposited on its surface. P-type
substrates, while less common, occur as well. Many commercial
LEDs, especially GaN/InGaN, also use sapphire substrate.

Transition coatings
Many LED semiconductor chips are encapsulated or potted in
clear or colored molded plastic shells. The plastic shell has
three purposes:

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 • Mounting the semiconductor chip in devices is easier to
accomplish.
• The tiny fragile electrical wiring is physically supported and
protected from damage.
• The plastic acts as a refractive intermediary between the
relatively high-index semiconductor and low-index open
air.
The third feature helps to boost the light emission from the
semiconductor by acting as a diffusing lens, allowing light to be
emitted at a much higher angle of incidence from the light cone
than the bare chip is able to emit alone.

Efficiency and Operational Parameters

Typical indicator LEDs are designed to operate with no more
than 30–60 mill watts (mW) of electrical power. Around
1999,Philips Lumileds introduced power LEDs capable of
continuous use at one watt. These LEDs used much larger
semiconductor die sizes to handle the large power inputs. Also,
the semiconductor dies were mounted onto metal slugs to
allow for heat removal from the LED die.
One of the key advantages of LED-based lighting sources is
high luminous efficacy. White LEDs quickly matched and
overtook the efficacy of standard incandescent lighting
systems. In 2002, Lumileds made five-watt LEDs available with
aluminous efficacy of 18–22 lumens per watt (lm/W). For
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comparison, a conventional incandescent light bulb of 60–100
watts emits around 15 lm/W, and standard fluorescent
lights emit up to 100 lm/W.
As of 2012, the Lumiled catalog gives the following as the best
efficacy for each color. The watt-per-watt value is derived using
the luminosity function.

Lifetime and Failure

Solid-state devices such as LEDs are subject to very
limited wear and tear if operated at low currents and at low
temperatures. Many of the LEDs made in the 1970s and 1980s
are still in service in the early 21st century. Typical lifetimes
quoted are 25,000 to 100,000 hours, but heat and current
settings can extend or shorten this time significantly.
The most common symptom of LED failure is the gradual
lowering of light output and loss of efficiency. Sudden failures,
although rare, can occur as well. Early red LEDs were notable
for their short service life. With the development of high-power
LEDs the devices are subjected to higher junction
temperatures and higher current densities than traditional
devices. This causes stress on the material and may cause early
light-output degradation. To quantitatively classify useful
lifetime in a standardized manner it has been suggested to use
the terms L70 and L50, which is the time it will take a given LED
to reach 70% and 50% light output respectively.

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 COLORS AND MATERIALS
Ultraviolet and Blue LEDs

Current bright blue LEDs are based

on the wide band
gap semiconductors GaN (gallium
nitride) and InGaN (indium gallium
nitride). They can be added to
existing red and green LEDs to
produce the impression of white
light. Modules combining the
three colors are used in big video
screens and in adjustable-color
fixtures.
The first blue-violet LED using magnesium-doped gallium nitride
was made at Stanford University in 1972 by Herb Maruska and
Wally Rhines, doctoral students in materials science and
engineering. In August 1989, Cree Inc. introduced the first
commercially available blue LED based on the indirect band
gap semiconductor, silicon carbide. SiC LEDs had very low
efficiency, no more than about 0.03%, but did emit in the blue
portion of the visible light spectrum.

White light
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 There are two primary ways of
producing white light-emitting
diodes (WLEDs), LEDs that
generate high-intensity white
light. One is to use individual
LEDs that emit three primary
colors—red, green, and blue—
and then mix all the colors to
form white light. The other is to use a phosphor material to
convert monochromatic light from a blue or UV LED to broad-
spectrum white light, much in the same way a fluorescent light
bulb works.

RGB systems

White light can be

formed by mixing
differently colored
lights; the most
common method is to
use red, green, and
blue (RGB). Hence the
method is called
multi-color white LEDs

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(sometimes referred to as RGB LEDs). Because these need
electronic circuits to control the blending and diffusion of
different colors, and because the individual color LEDs typically
have slightly different emission patterns (leading to variation of
the color depending on direction) even if they are made as a
single unit, these are seldom used to produce white lighting.
There are several types of multi-color white LEDs: di-, tri-,
and tetra chromatic white LEDs. Several key factors that play
among these different methods include color stability, color
rendering capability, and luminous efficacy. For example, the
dichromatic white LEDs have the best luminous efficacy (120
lm/W), but the lowest color rendering capability. However,
although tetra chromatic white LEDs have excellent color
rendering capability, they often have poor luminous efficiency.
Trichromatic white LEDs are in between, having both good
luminous efficacy (>70 lm/W) and fair color rendering
capability.
One of the challenges is the development of more efficient
green LEDs. The theoretical maximum for green LEDs is 683
lumens per watt but as of 2010 few green LEDs exceed even
100 lumens per watt. The blue and red LEDs get closer to their
theoretical limits.

Organic Light-emitting Diodes (OLEDs)

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In an organic light-emitting diode (OLED),
the electroluminescent material comprising the emissive layer
of the diode is an organic compound. The organic material is
electrically conductive due to the delocalization of pi electrons
caused by conjugation over all or part of the molecule, and the
material therefore functions as an organic semiconductor. The
organic materials can be small organic molecules in
a crystalline phase, or polymers.

The potential advantages

of OLEDs include thin,
low-cost displays with a
low driving voltage, wide
viewing angle, and high
contrast and color
gamut. Polymer LEDs have the added benefit of
printable and flexible displays. OLEDs have been used to make
visual displays for portable electronic devices such as cell
phones, digital cameras, and MP3 players while possible future
uses include lighting and televisions.

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 TYPES
The main types of LEDs are miniature, high-power devices
and custom designs such as alphanumeric or multi-color.

Miniature
These are mostly single-die LEDs used as indicators, and they
come in various sizes from 2 mm to 8 mm, through-
hole and surface mount packages. They usually do not use a
separate heat sink. Typical current ratings range from around
1 mA to above 20 mA. The small size sets a natural upper
boundary on power consumption due to heat caused by the
high current density and need for a heat sink.
There are three main categories of miniature single die LEDs:

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• Low-current: typically rated for 2 mA at around 2 V
(approximately 4 mW consumption).
• Standard: 20 mA LEDs (ranging from approximately 40 mW
to 90 mW) :
• 1.9 to 2.1 V for red, orange and yellow,
• 3.0 to 3.4 V for green and blue,
• 2.9 to 4.2 V for violet, pink, purple and white
• Ultra-high-output: 20 mA at approximately 2 V or 4–5 V,
designed for viewing in direct sunlight.

Mid-range
Medium-power LEDs are often through-hole-mounted and
mostly utilized when outputs of just tens of lumens are
needed. They sometimes have the diode mounted to four
leads (two cathode leads, two anode leads) for better heat
conduction and carry an integrated lens. An example of this
is the Super flux package, from Philips Lumileds. These LEDs
are most commonly used in light panels, emergency lighting,
and automotive tail-lights. Due to the larger amount of metal
in the LED, they are able to handle higher currents (around
100 mA). The higher current allows for the higher light
output required for tail-lights and emergency lighting.

High-power

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 High-power LEDs
(HPLEDs) or high-output
LEDs (HO-LEDs) can be
driven at currents from
hundreds of mA to more
than an ampere,
compared with the tens
of mA for other LEDs.
Some can emit over a
thousand lumens. LED power densities up to 300 W/cm2have
been achieved. Since overheating is destructive, the HPLEDs
must be mounted on a heat sink to allow for heat
dissipation.

AC driven LED

LEDs have been developed

by Seoul Semiconductor
that can operate on AC
power without the need
for a DC converter. For
each half-cycle, part of the
LED emits light and part is
dark, and this is reversed

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during the next half-cycle. The efficacy of this type of HPLED
is typically 40 lm/W. A large number of LED elements in
series may be able to operate directly from line voltage. In
2009, Seoul Semiconductor released a high DC voltage LED,
named as 'Acrich MJT', capable of being driven from AC
power with a simple controlling circuit. The low-power
dissipation of these LEDs affords them more flexibility than
the original AC LED design.

Application-specific variations:
• FLASHING:
• BI-COLOR LED
• TRI-COLOR LED
• RGB
• DECORATIVE MULTICOLOR
• ALPHANUMERIC
• DIGITAL RGB

ADVANTAGES AND DISADVANTAGES

Advantages
• Efficiency: LEDs emit more lumens per watt
than incandescent light bulbs. The efficiency of LED

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 lighting fixtures is not affected by shape and size, unlike
fluorescent light bulbs or tubes.
• Color: LEDs can emit light of an intended color without
using any color filters as traditional lighting methods need.
This is more efficient and can lower initial costs.
• Size: LEDs can be very small (smaller than 2 mm2) and are
easily attached to 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.
• Slow failure: LEDs mostly fail by dimming over time, rather
than the abrupt failure of incandescent bulbs.
Disadvantages
• High initial price: LEDs are currently more expensive, price
per lumen, on an initial capital cost basis, than most
conventional lighting technologies.
• Temperature dependence: LED performance largely
depends on the ambient temperature of the operating
environment – or "thermal management" properties.
Over-driving an LED in high ambient temperatures may
result in overheating the LED package, eventually leading
to device failure. An adequate heat sink is needed to
maintain long life.

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• 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.
• 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 metameric, red surfaces
being rendered particularly badly by typical phosphor-
based cool-white 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 Photo biological Safety for
Lamp and Lamp Systems.
• Impact on insects: LEDs are much more attractive to
insects than sodium-vapor lights, so much so that there
has been speculative concern about the possibility of
disruption to food webs.

APPLICATIONS AND USES

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 LED uses fall into four major categories:
• Visual signals where light goes more or less directly from
the source to the human eye, to convey a message or
meaning.
• Illumination where light is reflected from objects to give
visual response of these objects.
• Measuring and interacting with processes involving no
human vision.
Indicators and signs
The low energy consumption, low
maintenance and small size of LEDs
has led to uses as status indicators
and displays on a variety of
equipment and installations. Large-
area LED displays are used as
stadium displays and as dynamic
decorative displays.
Thin, lightweight message displays are used at airports and
railway stations, and as destination displays for trains, buses,
trams, and ferries.

Lighting

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With the development of high-efficiency and high-power
LEDs, it has become possible to use LEDs in lighting and
illumination. Replacement light bulbs have been made, as
well as dedicated fixtures and LED lamps. LEDs are used
as street lights and in other architectural lighting where color
changing is used. LED street lights are employed on poles
and in parking garages.

LEDs are used in

aviation
lighting. Airbus has
used LED lighting in
their Airbus A320
Enhanced since
2007, and Boeing plans its use in the 787. LEDs are also being
used now in airport and heliport lighting. LED airport fixtures
currently include medium-intensity runway lights, runway
centerline lights, taxiway centerline and edge lights,
guidance signs, and obstruction lighting. LEDs are used
in mining operations, as cap lamps to provide light for
miners. Research has been done to improve LEDs for mining,
to reduce glare and to increase illumination, reducing risk of
injury for the miners.
LEDs are now used commonly in all market areas from
commercial to home use: standard lighting, AV, stage,
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theatrical, architectural, and public installations, and
wherever artificial light is used.

Electronic contact lenses

Researchers have come

up with a way to place a
light-emitting diode on
a contact lens. The
research team tested
these high tech lenses
on rabbits with positive
results. In the future, computer embedded contact lenses
could be developed to work in a similar manner as Google
Glass.

Data communication and other signaling

Light can be used to transmit data and analog signals.
Assistive listening devices in many theaters and similar
spaces use arrays of infrared LEDs to send sound to listeners'
receivers. Light-emitting diodes are used to send data over
many types of fiber optic cable, from digital audio
over TOSLINK cables to the very high bandwidth fiber links
that form the internet backbone. For some time, computers
were commonly equipped with IrDA interfaces, which

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allowed them to send and receive data to nearby machines
via infrared.

Sustainable lighting

Efficient lighting is needed

for sustainable architecture. In
2009, a typical 13-watt LED lamp
emitted 450 to 650 lumens, which
is equivalent to a standard 40-watt
incandescent bulb. In 2011, LEDs
have become more efficient, so
that a 6-watt LED can easily
achieve the same results. A
standard 40-watt incandescent
bulb has an expected lifespan of
1,000 hours, whereas an LED can continue to operate with
reduced efficiency for more than 50,000 hours, 50 times
longer than the incandescent bulb.

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BIBLIOGRAPHY
• WWW.WIKIPEDIA.COM

• NCERT TEXTBOOK CLASS 12

• WWW.ENCYCLOPEDIA.COM

• WWW.PHYSICSRESEARCH.IN

• WWW.ALLPROJECTS.IN

• WWW.GOOGLE.COM

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