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LEDs
LEDs
(Light Emitting Diodes)
(Light Emitting Diodes)
Supervisor: Dr. Khorasani
Supervisor: Dr. Khorasani
Prepared by: Shirzad Malekpour
Prepared by: Shirzad Malekpour
Winter 1384
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Introduction
Introduction

Introduction to LEDs
Introduction to LEDs

How LEDs work + some points
How LEDs work + some points

Comparison with other sources of light
Comparison with other sources of light

LED in communication
LED in communication

Blue &White LED technologies
Blue &White LED technologies

How they are made

How they are made

Their application
Their application

Brief about blue laser
Brief about blue laser
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Light Sources in Electronics
Light Sources in Electronics
SOURCES
LEDs Laser diodes LCD
NEW
Digital light processors
(plasma)
By TI
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LED
LED

Stands for light emitting diode.
Stands for light emitting diode.

Semiconductor device:
Semiconductor device:



p-n junction
p-n junction




forward-biased.current
forward-biased.current

emits incoherent narrow spectrum light
emits incoherent narrow spectrum light
(due to recombination in transition region near the junction.)
(due to recombination in transition region near the junction.)

Color of the emitted light depends on the chemical of the
Color of the emitted light depends on the chemical of the
semiconducting material used.
semiconducting material used.


(Near-ultraviolet, visible or infrared.)
(Near-ultraviolet, visible or infrared.)
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LED
LED

Normally constructed of (Direct Gap):
Normally constructed of (Direct Gap):
GaAs, GaAsP , GaP :
GaAs, GaAsP , GaP :
Recombination
Recombination



light
light

Si and Ge are not suitable because of indirect
Si and Ge are not suitable because of indirect
band.
band.


recombination result heat
recombination result heat
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Structure and
Structure and
electroluminescence
electroluminescence
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Band Gap
Band Gap

Various band gaps
Various band gaps


different photon energies
different photon energies
Ultra violet :GaN 3.4 ev –infra-red: InSb 0.18 ev
Ultra violet :GaN 3.4 ev –infra-red: InSb 0.18 ev


Ternary&quarternary
Ternary&quarternary


increasing number of available
increasing number of available
energies
energies
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Example: 0<X<1
Example: 0<X<1


0<X<0.45
0<X<0.45


Direct usually 0.4 used for LEDs
Direct usually 0.4 used for LEDs


0.45<X<1
0.45<X<1


Indirect
Indirect
xx
PGaAs
−1

•
Heisenberg uncertainty
Heisenberg uncertainty
principle
principle
•
Indirect can emit
Indirect can emit
light if we add nitogen.
light if we add nitogen.
π
2
h
px ≥∆∆
xx
PGaAs
−1
xx
PGaAs
−1
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LED Radiation Patterns
LED Radiation Patterns



LED:Directional light source, maximum emitted power in
LED:Directional light source, maximum emitted power in
the direction perpendicular to the emitting surface.
the direction perpendicular to the emitting surface.


typical radiation pattern shows that most of the energy is
typical radiation pattern shows that most of the energy is
emitted within 20° of the direction of maximum light.
emitted within 20° of the direction of maximum light.

Some packages for LEDs include plastic lenses to spread the
Some packages for LEDs include plastic lenses to spread the
light for a greater angle of visibility.
light for a greater angle of visibility.
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Colors
Colors

III-V materials
III-V materials

Before II-VI (hard to have p-n junction)
Before II-VI (hard to have p-n junction)

Solution: Nitrogen ZnSe (MBE grown)
Solution: Nitrogen ZnSe (MBE grown)

Progress: using multilayer hetero structures by MBE(Mulecular Beam
Progress: using multilayer hetero structures by MBE(Mulecular Beam
Epitaxy) & OMVPE
Epitaxy) & OMVPE
(organometalic vapor-phase epiaxy)
(organometalic vapor-phase epiaxy)


(AlGaAs) - red and
(AlGaAs) - red and
infrared
infrared



(AlGaP) - green
(AlGaP) - green

(AlGaInP) - high-brightness orange-red, orange, yellow, and green
(AlGaInP) - high-brightness orange-red, orange, yellow, and green

(GaAsP) - red, orange-red,
(GaAsP) - red, orange-red,
orange
orange
, and
, and
yellow
yellow



(GaP) - red, yellow and green
(GaP) - red, yellow and green

(GaN) - green, pure green (or emerald green), and
(GaN) - green, pure green (or emerald green), and
blue

blue



(InGaN) - near ultraviolet, bluish-green and blue
(InGaN) - near ultraviolet, bluish-green and blue

(SiC) as substrate - blue
(SiC) as substrate - blue

(Si) as substrate - blue (under development)
(Si) as substrate - blue (under development)



(Al2O3) as substrate - blue
(Al2O3) as substrate - blue

(ZnSe),(GaN) - blue
(ZnSe),(GaN) - blue



(C) - ultraviolet
(C) - ultraviolet

(AlN), (AlGaN) - near to far
(AlN), (AlGaN) - near to far
ultraviolet
ultraviolet




New colors: pink and purple :2 layers of phosphors on Blue LED chip
New colors: pink and purple :2 layers of phosphors on Blue LED chip
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Definition
Definition

Luminous performance:
Luminous performance:



The efficiency of a device in converting electrical power to visible
The efficiency of a device in converting electrical power to visible
light. (
light. (
lumens/watt
lumens/watt
)
)



Low pressure sodium lights
Low pressure sodium lights


very high efficiency

very high efficiency


(because of the dominance of the
(because of the dominance of the
sodium d-lines
sodium d-lines
in the response of
in the response of
sodium vapor.)
sodium vapor.)



one type of red LED, the inverted pyramid type developed by
one type of red LED, the inverted pyramid type developed by
Hewlett-Packard has exceeded the efficiency of "old yellow", the
Hewlett-Packard has exceeded the efficiency of "old yellow", the
sodium light.
sodium light.
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Advantages of LEDs
Advantages of LEDs

Great stride in power and efficiency
Great stride in power and efficiency



100,000 hours of work compared to 1000 hours of life time for incandescent bulbs.
100,000 hours of work compared to 1000 hours of life time for incandescent bulbs.

LEDs :capable of emitting light of an intended color without the
LEDs :capable of emitting light of an intended color without the
use of color filters that traditional lighting methods require.
use of color filters that traditional lighting methods require.

The shape of the LED package allows light to be focused.
The shape of the LED package allows light to be focused.
Incandescent and fluorescent sources often require an external
Incandescent and fluorescent sources often require an external
reflector to collect light and direct it in a useable manner.
reflector to collect light and direct it in a useable manner.

LEDs are insensitive to vibration and shocks, unlike
LEDs are insensitive to vibration and shocks, unlike
incandescent and discharge sources.
incandescent and discharge sources.

LEDs are built inside solid cases that protect them, making them
LEDs are built inside solid cases that protect them, making them
hard to break and extremely durable
hard to break and extremely durable
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Advantages continued
Advantages continued

LEDs have an extremely long life span: typically ten years, twice
LEDs have an extremely long life span: typically ten years, twice

as long as the best fluorescent bulbs and twenty times longer
as long as the best fluorescent bulbs and twenty times longer
than the best incandescent bulbs.
than the best incandescent bulbs.

Further, LEDs fail by dimming over time, rather than the abrupt
Further, LEDs fail by dimming over time, rather than the abrupt
burn-out of incandescent bulbs.
burn-out of incandescent bulbs.

LEDs give off less heat than incandescent
LEDs give off less heat than incandescent
light bulbs
light bulbs
with
with
similar light output.
similar light output.

LEDs light up very quickly. A LED will achieve full brightness in
LEDs light up very quickly. A LED will achieve full brightness in
approximately 0.01 seconds, 10 times faster than an incandescent
approximately 0.01 seconds, 10 times faster than an incandescent
light bulb(0.1 seconds), and many times faster than a compact
light bulb(0.1 seconds), and many times faster than a compact
fluorescent lamp, which starts to come on after 0.5 seconds or 1
fluorescent lamp, which starts to come on after 0.5 seconds or 1
second, but does not achieve full brightness for 30 seconds or
second, but does not achieve full brightness for 30 seconds or
more.

more.

Only disadvantage: needs positive voltage(forward bias)
Only disadvantage: needs positive voltage(forward bias)
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LED Characteristics
LED Characteristics

Forward biased fast increase in current
Forward biased fast increase in current
(control needed)
(control needed)
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Overview on optoelectronics
Overview on optoelectronics
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LED Modulation
LED Modulation

LED light output is linearly proportional to the
LED light output is linearly proportional to the
current:
current:


Usage in sending a signal, that signal can then be
Usage in sending a signal, that signal can then be
send through a fiber optic cable and detected on
send through a fiber optic cable and detected on
the other end.

the other end.
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LED Modulation Circuit
LED Modulation Circuit
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Fiber Optic Communication
Fiber Optic Communication

Enhancement of optical communication by fiber
Enhancement of optical communication by fiber
between source and receiver
between source and receiver

Fiber : Light pipe or wave guide for optical
Fiber : Light pipe or wave guide for optical
frequencies
frequencies

Made of: Outer layer of pure fused silica, core of
Made of: Outer layer of pure fused silica, core of
germanium
germanium
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Different Types
Different Types
x
eIxI
α
−
=

0
)(
Step Index
Graded index
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)(
λα
f=
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Reyleigh Scattering
Reyleigh Scattering

Reyleigh Scattering
Reyleigh Scattering


reduced scattering from small random
reduced scattering from small random
inhomogeneties.
inhomogeneties.
Fluctuations of n1
Fluctuations of n1


Decrease with fourth power of
Decrease with fourth power of
wavelenghth
wavelenghth
You can see the effect while sun rise and sunset
You can see the effect while sun rise and sunset

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Infrared Absorption
Infrared Absorption

Dominates for wavelength longer than about 1.7
Dominates for wavelength longer than about 1.7
μ
μ
m
m

Due to vibrational excitation of atoms making up the
Due to vibrational excitation of atoms making up the
glass
glass


Pulse Dispersion
Pulse Dispersion

Spreading the data propagating the fiber
Spreading the data propagating the fiber

Reason: n=f(
Reason: n=f(
λ
λ
)
)



different frequencies travel with
different frequencies travel with
different velocity
different velocity


less for 1.3
less for 1.3
μ
μ
m window
m window

Another reason: different modes propaget in different
Another reason: different modes propaget in different
path lengths
path lengths