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Group Assignment

Prepared by: 1. Wilson Teng Yisheng (1104947 )

2. Bong Wilson (1104987 )

3. Teoh Kheng Hwa (1103790)

4. Lee Le Yu (1104890)

5. Ng Wei Zhang (1105014)

6. Cheok Kah Guan (1105017)

Programme Bachelor of Engineering (Hons) Mechanical Engineering

Course UEME 1122 Material Science

Year One

Semester One

Prepared for: Dr. Yap Yeow Hong

Date August 18, 2012

UNIVERSITI TUNKU ABDUL RAHMAN (UTAR)

FACULTY OF ENGINEERING AND SCIENCE (FES)

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Introduction

A light-emitting diode (LED) is a semiconductor light source made from a combination

of chemically polarized semiconductors. The chemical composition is chosen to define

the energy of the electrons that pass across the boundary between the two types of

semiconductor. This electron energy is converted to light as electrons flow though the

device. The electron energy defines the wavelength of the resultant coloured light.

Early development:

1907:

first experimenter H. J. Round (British)

1927:

first researcher to publish journal Oleg Vladimirovich Losev (Russian)

1955:

Rubin Braunstein reported on infrared emission from gallium arsenide (GaAs) in

simple diode structures.

1961:

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Robert Biard and Gary Pittman (American experimenter) found that GaAs emitted

infrared radiation when electric current was applied and received the patent for

the infrared LED.

First visible spectrum LED:

1962:

develop by Nick Holonyak (father of the light-emitting diode)

1960’s:

red LEDs became commercially available.

1970’s – 2000’s:

1972:

Nick Holonyak invented yellow LED and improved the brightness of red and red-

orange LEDs by a factor of ten.

1976:

T. P. Pearsall created high-brightness, high-efficiency LEDs for optical fiber

telecommunications.

1993:

Shuji Nakamura (Japanese) demonstrated first high-brightness blue LED.

1995:

Alberto Barbieri investigated the efficiency and reliability of high-brightness LED.

These developments produce white LED.

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Chemical Structure of LED

There are several types of semiconductor of LED. The chemical structure includes

molecular geometry, electronic structure and crystal structure of molecules.

1. Gallium Arsenide (GaAs) - has molecular shape of linear shape.

-has crystal structure of zincblende.

-has formula weight of 144.64.

-has lattice constant of 0.56533nm.

-produce red and infrared light.

2. silicon carbide (SiC) –has electron mobility of 900cm2/V.s(all polytypes)

-can doped n-type by nitrogen or phosphorus

-can doped p-type y aluminium, boron or beryllium.

-produce blue light.

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Structure of major SiC polytypes.

(β)3C-SiC

crystal structure :Zinc

blende(cubic)

4H-SiC

crystal structure:hexagonal

(α)6H-SiC

crystal structure:hexagonal

3. Gallium Nitride-has coordination geometry of tetrahedral.

-has band gap of 3.4 eV.

-has electron mobility of 440 cm2/ (V·s) at temperature 300 K.

-has crystal structure of Wurtzite.

-emitting green, pure green or emerald green and blue light.

Gallium Nitride

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Properties that make LED unique or desirable for the applications

1. Ability to generate photons, which can be seen as light due to the interaction

between electrons and holes. This is caused by the property of N-type material

bonded to a section of P-type material.

2. The size of gap between the conduction band and lower orbitals also contribute to

the uniqueness of LED. This is because the size of the gap between the conduction

band and lower orbitals can determine frequency of the photon. In simple words, it

determines the color of the light. It can also produce infrared when the frequency of

photon is so low that it is not visible to the human eye. This is to produce infrared

LED used in remote controls.

3. LEDs are also specially constructed to release a large number of photons outward.

The light are concentrated by the plastic house in a particular direction. As you can

see in the diagram, most of the light from the diode bounces off the sides of the

bulb, traveling on through the rounded end. Hence, allowing LED to be brighter

than incandescent bulbs and fluorescent lamps.

Diagram of LED

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4. LEDs output more lumens of light. For example, LED bulb produces 76.9 lumens

per watt compared to an incandescent bulb's 17 lumen per watt.

5. LED are also very durable as they can last up to 50 000 hours or more.

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Current and Potential Applications of LED

1. LED can be used to replace incandescent lamps as indicator as eco friendly

lighting.

2. LEDs are being commonly used for traffic signals, vehicle brake lighting and exit

signs.

3. Most of the LED lights in outdoor places are used as decoration lights, because

LED lights have different colors and shapes,

4. Another task for control circuitry is failure recognition. Modules consist of

individual LEDs which can be tested for total failure of a circuit or application.

5. LED displays are packages of many LEDs arranged in a pattern, the most familiar

pattern being the 7-segment displays for showing numbers (digits 0-9).

6. LEDs are used as movement sensors, as in optic computer mouse.

7. Remote controls, such as for TVs, VCRs, and LED Computers, infrared LEDs

are used.

8. LEDs are small, durable and need little power, so they are used in hand held

devices such as flashlights.

9. Infrared LEDs are also apply in night vision uses including security cameras.

10. LEDs can also use for Touch sensing.

11. LED can function as photodiodes, as they can be used for both photo emission

and detection.

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Processing of Light-emitting diode

Diodes are made of very thin layers of semiconductor material which allow

electrons to move from one layer to another, thereby generating light. The particular

semiconductors used for LED manufacture are gallium arsenide (GaAs), gallium

phosphide (GaP), or gallium arsenide phosphide (GaAsP). The processing of light-

emitting diode are summarized as follow.

(A) Making semiconductor wafers

1. Gallium, arsenic, and/or phosphor are purified, mixed, heated and pressure liquefy

and press together in the chamber to force into a solution. A rod is dipped into the

solution and pulled out slowly. The solution cools and crystallizes on the end of

the rod, forming ingot of GaAs, GaP, or GaAsP.

2. The ingot is then sliced into very thin wafers of semiconductor and polished.

3. The wafers are cleaned through a rigorous chemical and ultrasonic process using

various solvents.

(B) Adding epitaxial layers

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4. Impurities, or dopants, are added to the crystal by a process called Liquid Phase

Epitaxy (LPE). In this technique, epitaxial layers—semiconductor layers that have

the same crystalline orientation as the substrate below—are deposited on a wafer

while it is drawn under reservoirs of molten GaAsP.

5. The wafer is then immersed in a gaseous atmosphere containing the dopants—

nitrogen or zinc ammonium in high temperature furnace tube.

(C) Adding metal contacts

6. Contact patterns are reproduced in photoresist, a light-sensitive compound. The

liquid resist is deposited in drops while the wafer spins, distributing it over the

surface.

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7. A chunk of metal is heated to temperatures that cause it to vaporize. It condenses

and sticks to the exposed semiconductor wafer.

8. A single 2 inch-diameter wafer produced which gives an indication of the size of

the finished diodes. The diodes are cut apart into "die" either by cleaving or by

sawing with a diamond saw.

(D) Mounting and packaging

9. Individual dies are mounted on the appropriate package. The back of the wafer is

coated with metal and forms an electrical contact with the lead it rests on. A tiny

gold wire is soldered to the other lead and wire-bonded to the patterned contacts

on the surface of the die.

10. The entire assembly is sealed in plastic. The wires and die are suspended inside a

mold (liquid plastic or epoxy).

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Disadvantages of LED

1. The cost of LED is more expensive compared to others conventional lighting.

Besides, some semi-conductor materials used cost more than others.

2. LED is a temperature dependence diode. It depends on the ambient temperature of

the operating environment. The ambient temperature affects the emission

spectrum of LED. The LED will overheat if over-driving in high temperature and

will lead to failure.

3. LED have small area light source. LED not approximates a "point source" of light.

It does not radiate light from all angles.

4. The light produce by the LED is intense low light which is very dangerous for our

eyes. The blue LED harms our eyes because it exceeds the safe limit which is

called the blue hazard. Besides, LED also lead to headaches, disturbed sleep and

eye strain.

5. LED will cause blue pollution. It is because the cool-white LED emits more blue

light and more light pollution than others light sources.

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Disposal Method of LED

LED are recyclable. LED does not contain mercury, it will not affect the disposal process

and it can be safely recycled.

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Reference

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