The document is a physics investigatory project on light emitting diodes (LEDs) submitted by a 12th grade student. It includes an introduction to LEDs, their types, working mechanism, I-V characteristics, multi-colored LED structures, advantages, disadvantages and applications. The project contains contents, acknowledgments and bibliography sections and aims to increase the student's knowledge through research on LEDs according to their school curriculum.

1. PHYSICS INVESTIGATORY PROJECT
LIGHT EMITTINGDIODE (LED)
Roll No: - Class: - 12th
Submitted By: Submitted To:-

2. Certificate
This is to certify that ankit of class 12th
has completed
her project in the subject of Physics as required
according to the syllabus as prescribed by the central
board of secondary education(CBSE)for the academic
session 2017-2018.
---------------------------
Signature

3. Acknowledgement
I would like to express my special thanks of gratitude to my teacher Mr. RAM
Sir as well as our principal Mr. Anil SIRwho gave me the golden opportunity to
do this wonderfulprojecton the topic Light Emitting Diode(LED), which also
helped me in doing a lot of Research and I came to know about so many new
things I am really thankful to them.
Secondly, I would also like to thank my parents and friends who helped me a
lot in finalizing this projectwithin the limited time frame.
I am making this project not only for marks butto also increase my knowledge.

4. Contents
S.No. Topics
1 Introduction
2 Types of LED’s
3 Working of LED
4 LED I-V Characteristic
5 Multicoloured LED’s
6 LED Structure
7 Advantages of LED’s
8 Disadvantages of LED’s
9 Applications of LED’s
10 Bibliography

5. Introduction
A light-emitting diode (LED) is a semiconductor device that emits
visible light when an electric current passes through it. The light is not
particularly bright, but in most LEDs it is monochromatic, occurring at
a single wavelength. The output from an LED can range from red (at a
wavelength of approximately 700 nanometers) to blue-violet (about
400 nanometers). Some LEDs emit infrared (IR) energy (830
nanometers or longer); such a device is known as an infrared-emitting
diode (IRED).
An LED or IRED consists of two elements of processed material
called P-type semiconductors and N-type semiconductors. These two
elements are placed in direct contact, forming a region called the P-N
junction. In this respect, the LED or IRED resembles most
other diode types, but there are important differences. The LED or
IRED has a transparent package, allowing visible or IR energy to pass
through. Also, the LED or IRED has a large PN-junction area whose
shape is tailored to the application.
Benefits of LEDs and IREDs, compared with incandescent and
fluorescent illuminating devices, include:
Low power requirement: Most types can be operated with battery
power supplies.
High efficiency: Most of the power supplied to an LED or IRED is
converted into radiation in the desired form, with minimal heat
production.
Long life: When properly installed, an LED or IRED can function for
decades.

6. Types of LED’s
 Gallium Arsenide (GaAs) – infra-red
 Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange
 Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red,
orange-red, orange, and yellow
 Gallium Phosphide (GaP) – red, yellow and green
 Aluminium Gallium Phosphide (AlGaP) – green
 Gallium Nitride (GaN) – green, emerald green
 Gallium Indium Nitride (GaInN) – near ultraviolet, bluish-green and blue
 Silicon Carbide (SiC) – blue as a substrate
 Zinc Selenide (ZnSe) – blue
 Aluminium Gallium Nitride (AlGaN) – ultraviolet

7. Working Of LED
A Light emitting diode (LED) is essentially a pn junction diode. When carriers are
injected across a forward-biased junction, it emits incoherent light. Most of the
commercial LEDs are realized using a highly doped n and a p Junction.
Figure1: p-n+ Junction under Unbiased and biased conditions.
(pn Junction Devices and Light Emitting Diodes by Safa Kasap)
To understand the principle, let’s consider an unbiased pn+ junction (Figure1 shows the
pn+ energy band diagram). The depletion region extends mainly into the p-side. There is
a potential barrier from Ec on the n-side to the Ec on the p-side, called the built-in voltage,
V0. This potential barrier prevents the excess free electrons on the n+ side from diffusing
into the p side.
When a Voltage V is applied across the junction, the built-in potential is reduced from V0
to V0 – V. This allows the electrons from the n+ side to get injected into the p-side. Since
electrons are the minority carriers in the p-side, this process is called minority carrier
injection. But the hole injection from the p side to n+ side is very less and so the current
is primarily due to the flow of electrons into the p-side.
These electrons injected into the p-side recombine with the holes. This recombination
results in spontaneous emission of photons (light). This effect is called injection
electroluminescence. These photons should be allowed to escape from the device without
being reabsorbed.
The recombination can be classified into the following two kinds
• Direct recombination

8. • Indirect recombination
Direct Recombination:
In direct band gap materials, the minimum energy of the conduction band lies directly
above the maximum energy of the valence band in momentum space energy (Figure 2
shows the E-k plot of a direct band gap material). In this material, free
electrons at the bottom of the conduction band can recombine directly with free holes at
the top of the valence band, as the momentum of the two particles is the same. This
transition from conduction band to valence band involves photon emission (takes care of
the principle of energy conservation). This is known as direct recombination. Direct
recombination occurs spontaneously. GaAs is an example of a direct band-gap
material.
Figure2: Direct Bandgap and Direct Recombination
Indirect Recombination:
In the indirect band gap materials, the minimum energy in the conduction band is shifted
by a k-vector relative to the valence band. The k-vector difference represents a difference
in momentum. Due to this difference in momentum, the probability of direct electronhole
recombination is less.
In these materials, additional dopants(impurities) are added which form very shallow
donor states. These donor states capture the free electrons locally; provides the necessary
momentum shift for recombination. These donor states serve as the recombination
centers. This is called Indirect (non-radiative) Recombination.
Figure3 shows the E-k plot of an indirect band gap material and an example of how
Nitrogen serves as a recombination center in GaAsP. In this case it creates a donor state,
when SiC is doped with Al, it recombination takes place through an acceptor level.
The indirect recombination should satisfy both conservation energy, and momentum.
Thus besides a photon emission, phonon emission or absorption has to take
place.
GaP is an example of an indirect band-gap material.

9. Figure3: Indirect Bandgap and NonRadiative recombination
The wavelength of the light emitted, and hence the color, depends on the band gap energy
of the materials forming the p-n junction.
The emitted photon energy is approximately equal to the band gap energy of the
semiconductor. The following equation relates the wavelength and the energy band gap.
hν = Eg
hc/λ = Eg
λ = hc/ Eg
Where h is Plank’s constant, c is the speed of the light and Eg is the energy band gap
Thus, a semiconductor with a 2 eV band-gap emits light at about 620 nm, in the red. A 3
eV band-gap material would emit at 414 nm, in the violet.

10. LED I-V Characteristic
Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves
showing the different colours available.
Before a light emitting diode can “emit” any form of light it needs a current to flow through
it, as it is a current dependant device with their light output intensity being directly
proportional to the forward current flowing through the LED.
As the LED is to be connected in a forward bias condition across a power supply it should
be current limited using a series resistor to protect it from excessive current flow. Never
connect an LED directly to a battery or power supply as it will be destroyed almost instantly
because too much current will pass through and burn it out.
From the table above we can see that each LED has its own forward voltage drop across the
PN junction and this parameter which is determined by the semiconductor material used, is
the forward voltage drop for a specified amount of forward conduction current, typically for a
forward current of 20mA.
In most cases LEDs are operated from a low voltage DC supply, with a series resistor, RSused
to limit the forward current to a safe value from say 5mA for a simple LED indicator to 30mA
or more where a high brightness light output is needed.

11. Multi-coloured Light Emitting Diode’s
LEDs are available in a wide range of shapes, colours and various sizes with different light
output intensities available, with the most common (and cheapest to produce) being the
standard 5mm Red Gallium Arsenide Phosphide (GaAsP) LED.
LED’s are also available in various “packages” arranged to produce both letters and numbers
with the most common being that of the “seven segment display” arrangement.
Nowadays, full colour flat screen LED displays, hand held devices and TV’s are available which
use a vast number of multicoloured LED’s all been driven directly by their own dedicated IC.
Most light emitting diodes produce just a single output of coloured light however, multi-
coloured LEDs are now available that can produce a range of different colours from within a
single device. Most of these are actually two or three LEDs fabricated within a single package.
Bicolour Light Emitting Diodes
A bicolour light emitting diode has two LEDs chips connected together in “inverse parallel”
(one forwards, one backwards) combined in one single package. Bicolour LEDs can produce
any one of three colours for example, a red colour is emitted when the device is connected
with current flowing in one direction and a green colour is emitted when it is biased in the
other direction.
This type of bi-directional arrangement is useful for giving polarity indication, for example,
the correct connection of batteries or power supplies etc. Also, a bi-directional current
produces both colours mixed together as the two LEDs would take it in turn to illuminate if
the device was connected (via a suitable resistor) to a low voltage, low frequency AC supply.
A Bicolour LED

12. Tricoloured LightEmitting Diode
The most popular type of tricolour light emitting diode comprises of a single Red and a Green
LED combined in one package with their cathode terminals connected together producing a
three terminal device. They are called tricolour LEDs because they can give out a single red or
a green colour by turning “ON” only one LED at a time.
These tricoloured LED’s can also generate additional shades of their primary colours (the
third colour) such as Orange or Yellow by turning “ON” the two LEDs in different ratios of
forward current as shown in the table thereby generating 4 different colours from just two
diode junctions.
A Multi or Tricoloured LED

13. LED Structure
The LED structure plays a crucial role in emitting light from the LED surface. The LEDs
are structured to ensure most of the recombinations takes place on the surface by the
following two ways.
• By increasing the doping concentration of the substrate, so that additional free
minority charge carriers electrons move to the top, recombine and emit light at the
surface.
• By increasing the diffusion length L = √ Dτ, where D is the diffusion coefficient
and τ is the carrier life time. But when increased beyond a critical length there is a
chance of re-absorption of the photons into the device.
The LED has to be structured so that the photons generated from the device are emitted
without being reabsorbed. One solution is to make the p layer on the top thin, enough to
create a depletion layer. Following picture shows the layered structure. There are
different ways to structure the dome for efficient emitting.
Figure4: LEDstructure
(pn Junction Devices and Light Emitting Diodes by Safa Kasap)
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.
LED efficiency:
A very important metric of an LED is the external quantum efficiency ηext. It quantifies
the efficeincy of the conversion of electrical energy into emitted optical energy. It is
defined as the light output divided by the electrical input power. It is also defined as the
product of Internal radiative efficiency and Extraction efficiency.
ηext = Pout(optical) / IV
For indirect bandgap semiconductors ηext is generally less than 1%, where as for a direct

14. band gap material it could be substantial.
ηint = rate of radiation recombination/ Total recombination
The internal efficiency is a function of the quality of the material and the structure and
composition of the layer.

15. Advantages of LED’s
• LEDs produce more light per watt than incandescent bulbs; this is useful in
battery powered or energy-saving devices.
• LEDs can emit light of an intended color without the use of color filters that
traditional lighting methods require. This is more efficient and can lower initial
costs.
• 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.
• When used in applications where dimming is required, LEDs do not change their
color tint as the current passing through them is lowered, unlike incandescent
lamps, which turn yellow.
• 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
High Intensity Discharge (HID) lamps that require a long time before restarting.
• LEDs, being solid state components, are difficult to damage with external shock.
Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
• LEDs can have a relatively long useful life. A Philips LUXEON k2 LED has a
life time of about 50,000 hours, whereas Fluorescent tubes typically are rated at
about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.
• LEDs mostly fail by dimming over time, rather than the abrupt burn-out of
incandescent bulbs.
• LEDs light up very quickly. A typical red indicator LED will achieve full
brightness in microseconds; Philips Lumileds technical datasheet DS23 for the
Luxeon Star states "less than 100ns." LEDs used in communications devices can
have even faster response times.
• LEDs can be very small and are easily populated onto printed circuit boards.
• LEDs do not contain mercury, unlike compact fluorescent lamps.

16. Disadvantages of using LED’s
• LEDs are currently more expensive, price per lumen, on an initial capital cost
basis, than more conventional lighting technologies. The additional expense
partially stems from the relatively low lumen output and the drive circuitry and
power supplies needed. However, when considering the total cost of ownership
(including energy and maintenance costs), LEDs far surpass incandescent or
halogen sources and begin to threaten the future existence of compact fluorescent
lamps.
• 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. Adequate
heat-sinking is required to maintain long life.
• LEDs must be supplied with the correct current. This can involve series resistors
or current-regulated power supplies.
• LEDs do not approximate a "point source" of light, so they cannot be used in
applications needing a highly collimated beam. LEDs are not capable of providing
divergence below a few degrees. This is contrasted with commercial ruby lasers
with divergences of 0.2 degrees or less. However this can be corrected by using
lenses and other optical devices.
• There is increasing concern that blue LEDs and white LEDs are now capable of
exceeding safe limits of the so-called blue-light hazard as defined in the eye
safety specifications for example ANSI/IESNA RP-27.1-05: Recommended
Practice for Photobiological Safety for Lamp and Lamp Systems.

17. Applications of LED’s
 Indicator lights: These can be two-state (i.e., on/off), bar-graph, or alphabetic-numeric
readouts.
 LCD panel backlighting: Specialized white LEDs are used in flat-panel computer displays.
 Fiber opticdata transmission: Ease of modulation allows wide
communications bandwidth with minimal noise, resulting in high speed and accuracy.
 Remote control: Most home-entertainment "remotes" use IREDs to transmit data to the
main unit.
 Optoisolator: Stages in an electronic system can be connected together without
unwanted interaction.

18. Bibliography
The following books were used in completion of this project.
 NCERT Textbook of Physics for class 12th .
 Pradeep Fundamental of Physics for class 12th .
Also, the following websites were consulted for relevant materials.
 https://www.google.co.in/
 https://en.wikipedia.org
 http://www.electronics-tutorials.ws/diode/diode_8.html
 http://whatis.techtarget.com/definition/light-emitting-diode-LED