Light communication, medical etc. LEDs can be utilised as

Light Emitting Diode (LED): The Best Alternate
for Illumination

Dr Manoj Kumar Srivastava, Department of
Physics, Army Cadet College,IMA,Dehradun-248007

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Abstract:
It was day of celebration to all in general and
to Thomas Alva Edison in particular, when he succeeded to produce light without
burning in 1879 by passing current in a carbon filament. Though incandescence
was demonstrated even before Edison but credit to invent electric bulb goes to Edison
only. Since then, lighting sources have history of long journey with continuous
evolution in the field with entries of Discharge Lamps, Compact Fluorescent
Lamps (CFL) and Light Emitting Diodes LEDs. LED, after its birth has
continuously evolved and finally won the race of lighting sources today 1.
Now, we have LEDs of different colors including white light LED available. As
predicted by Roland Haitz (called Haitz’s law), LEDs since the birth in 1962
reduced their cost to 10 times with increase in efficiency 20 times in every
decade. LED technology has again gave the impetus to electronics which has
slowed down in the last couple of years.

Keywords: LED; alloy semiconductor;
electroluminescence; band gap

 

Introduction

It was Oct 1879 when
Thomas Alva Edison invented an incandescent bulb and light could be produced
without burning very first time. These light sources continued with the entries
of other sources such as discharge lamps in 1950’s and LEDs in 1960’s. LED
technology was quite expensive in the beginning but with advancements, its
price is decreasing day by day with more and more efficiency. Since electrical
consumption in illumination is about 20% of the total consumption worldwide,
LEDs have accepted this challenge to reduce the power consumption in
illumination. Now LED’s are well ahead to their rival, incandescent and
discharge lamps.

 

Figure-1                   

Figure-2

Courtesy: Google
images

 

 

Figure-3 Courtesy: Google images

 

Light emitting diode is
a solid state electronic device which has numerous applications in the fields such
as an efficient lighting 3, backlighting of display, communication, medical etc.
LEDs can be utilised as a point source (zero dimension), line source (one
dimension) and surface or areal source (implying LEDs in two dimensions). A
properly designed LED has extremely low power consumption with high energy efficiency
in which voltage requirement is as low as 4 volts and current upto 0.7A.  LEDs have a wider temperature tolerance range
2 from -200C to 85 0C, hence, can be generally used
anywhere on the surface of the earth. These are the sources of light which
converts electrical energy directly to light unlike the other conventional
sources of light. LEDs are foreseen as the best lighting source of the future
5.

 Principle of operation

LED is simply a P-N
junction diode (symbolised as Fig.1) made up of such semiconducting material
whose energy band gap falls approximately between 3.09 eV to 1.58 eV to produce
light in the visible region of electromagnetic spectrum. When LED is forward
biased (i.e. anode of battery is connected to p-side and cathode to n-side of
the diode), majority charge carriers (holes) in p-side and electrons in n-side
cross the junction to meet each other i.e. an electron from n-side moves to
p-side. As per the band theory of semiconductors, an electron in the conduction
band conduction band falls to valence band to occupy the vacancy of electron
i.e. hole and an energy equal to the gap of both these bands is released. This
phenomenon is called electro luminescence. If photon of electromagnetic energy
emitted has wavelength in visible region of electromagnetic spectrum i.e.
between 400 nm to 780 nm, it gives out light. Therefore, color of light
released by LED depends upon the band gap of semiconducting material used to
fabricate the diode.

Wavelength of light
released (?)

History
of LED development

Electrical
luminescence was discovered in 1907 by the British scientist H J Round at
Marconi lab in Britain using a crystal of silicon carbide. Ovlosev, a Russian
scientist first time proposed the correct theory of LED with its practical
applications such as electroluminescence in 1927. It took more than three
decades to commercialise this phenomena, when Biard and Pittman made
semiconductor radiant diode using a zinc diffused P-N junction diode under
forward bias condition for emission of infrared light4. Nick Holoyank
developed the first visible LED of red colour in 1962 using GaAs1-xPx.

In 1972, Holoyank’s
student M George Craford invented the first yellow LED. In the beginning, these
LEDs were extremely costly approximately 200 US$ per unit. The prices reduced
drastically to 5 cent per unit in mid 70s. The brightness of red LED in the
beginning was so poor that it can only be used in indicators not to illuminate
space. The first commercial use of LED was in seven segment display. The blue
LED was discovered by Herbert Paul Marushaka in 1972 using Gallium Nitride
(GaN) on sapphire substrate but brightness of both gallium nitride and silicon
carbide blue LED were not good. The first high brightness blue LED was made
using indium gallium nitride (InGaN) 7,8,9 in 90’s by a Japan born American
scientist Shuji Nakamura 7 with  Akashaki and H Amano grabbed Nobel Prize for
his work in 2014.

With the discovery of
blue LED, a new way to produce white light LED source was set up. Hence, blue
LED paved the path for its illuminating applications replacing incandescent
lamps of 20th century. This white light LED has revolutionized the field of
illuminating sources.

 

Fabrication
of LED

Generally, a flat
surface LED chip emits light in a conical shape towards semiconductor surface.
The cone of light emission is called Escape Cone 11. The cone angle, when
exceeds the critical angle of semiconductor – air interface, light gets total
internally reflected inside the semiconductor and does not come out, when LED
chip is in cuboidal shape i.e. all the surfaces of semiconductor is at 90 degrees
angled surfaces.

A LED as a package
(Fig.2) include, LED wafer (which can be made up of different semiconducting
materials such as Sapphire, GaAs, Si, SiC, GaN etc.) and many epitaxial layers grown
on this bare wafer. Different colors of LED are obtained by using different
types of these epilayers. Different types of these epilayers are InGaN and
AlGaN for producing green, blue and UV light LEDs. For red and yellow light
LEDs, these epilayers are of InAlGaP. AlGaAs produce red and Infrared LEDs 6.
Implanting electrical contacts in these epilayers and cutting these into LED
dies is called LED packaging to finally grow a LED chip as shown in figure.

A convoluted chip
surface with inclined faces can reduce TIR effect which will increase light
output. A perfect shape of semiconductor chip in a LED for maximum light output
would be a hemisphere with flat bottom and the point of emission of light is
exactly at the centre. Now, all the rays reaching on the semiconductor-air
interface will be normal and TIR is entirely avoided. At the same time, flat
bottom will cause TIR and light coming towards bottom of the sphere, will be
reflected back to increase the efficiency of LED.

A LED package, other
than active components as discussed above, also composed of metal heat
radiator, housing, bond wires, die attaches, lead frames and solder joints
(Fig.3). Housing to protect LED’s die is made up of Liquid Crystal Polymers.
Encapsulation on the housing is done using a resin (epoxy or Si) in the shape
of hemispherical dome. The die is a compound semiconductor as told earlier, is
responsible for the color of the LED. Die attaches are to thermally connect the
LED die to metal heat radiators for cooling. In white light LEDs, phosphors are
dispersed inside the encapsulating material which emits white light after
absorbing the light from different color LED dies.

Materials used to fabricate LED

LED can be made out
of direct band gap semiconductors (such as III-V alloy semiconductors e.g.
GaAs, InP, GaAsP etc) only. In direct band gap semiconductors, both electrons
in conduction band and holes in valence band are same state. Hence, an electron
can jump directly into the valence band releasing a photon. However, in
indirect band gap semiconductors such as Si, Ge, AlSb etc. a transiting
electron jumps to valence band via an intermediate state and transfer momentum
to crystal lattice by a non-radiative transition. These transitions do not
produce any optical emission. Though, indirect band gap semiconductors can be
also be used as LED material with certain impurity addition such as addition of
Nitrogen in GaP produce red LED. LED development started with IR and red
devices made up of gallium arsenide and with advancements in the material
sciences, LEDs of even shorter wavelength with variety of colours are realised
today.

Gallium Arsenide (GaAs)
is a direct band-gap semiconductor with energy band-gap of 1.44eV and produces
Infrared LED of wavelength 860nm. Gallium Phosphide (GaP), though an indirect
band-gap semiconductor with poor radiation recombination produces green LED
with wavelength 565nm by adding nitrogen to both p and n-side of the GaP. GaP
doped with Zn and O produce the emission of light with red color. Dopant Zn
replaces Ga atoms and O replaces P atoms.

Ga As 1-xPx
13 has band- gap between 1.44eV when x=0 and 2.26eV when x=1. This
emits light from IR(?=860nm) to green (?= 548nm). It is a direct band gap alloy
semiconductor with 00.44. Indirect band-gap semiconducting materials can be used for LED after
addition of nitrogen as impurity. GaAs 0.35P0.65 N
and  Ga As 0.15P0.85 Nare
used to produce orange and yellow LEDs respectively. A new material GaxAl
1-x As is also in use to fabricate LED and different colors are
emitted by changing the x from 0.7 to 1. Flexible light-emitting
diodes made from soluble conducting polymers were also reported 14.

Classification
of LEDs

According to their
color outputs, LEDs are of two types, white and RGB LEDs. Now LEDs of different
colors such as red, blue, green and yellow are available. According to mixing
of different colors and design of its cover, LED is also available in wider
spectral bandwidth of visible spectrum with different luminous intensities. LED
in its original concept emits single color only, depending upon the material
used to fabricate it. But appropriate mixing of different colors with envelope
design, leads to different shades of LEDs including white light LED, which is
most widely used as an illuminating source.

Another way of
classifying LEDs is on the basis of their power output. Low power LEDs have a
power rating of less than 1W and current is approximately 20mA. Medium power
LED has power rating between 1W to 3W with current range from 30mA to 150mA.
These medium power LEDs are also categorized as high brightness LEDs. Ultrahigh
brightness LEDs or high power LEDs, power consumption range is more than 3 W
with current equivalent in the range 350mA to 1A. By observing the graph
between time span and lumen output, one can estimate the life span of a LED
which may go upto 80000 hrs or even more.

Different color LEDs available
along with material used is as under

Figure-4 Courtesy: Google images

 

 

White
light LED

Suitable LED light to
illuminate the space should be white light. However, light display panels use green,
red, blue or yellow LEDs but light sources with illuminating capacity are of
highest demand. With the discovery of blue LED in 1995, path for bright white
light LED sources paved. White light LED is fabricated using the mixture of
three primary colour i.e. blue green and red in appropriate ratio with suitable
envelope design. An ultra high efficiency white light LED was reported in 2006
10.  There are three ways in which
white light LED can be produced

1. Mixing of light emitted from red, green and
blue LEDs. This mixture produces white light but intensity of this light is
poor hence, primarily used for backlighting the displays.

2. Mixing of near UV or are UV LED with RGB
phosphor encapsulation. Here UV LED is used to excite RGB phosphors.

3. Mixing of blue LED with yellow LED. Since
these are complementary colours, hence produce white light.  This method of producing white LED is most
commonly used because of its high efficiency.

Conclusion

Optoelectronics with
development of LED technology has widely covered the fields of mobile phones,
computers, TV screens and display panels etc. other than its primary use in
lighting 12. LEDs with their luminous efficacy at 120-200 lm/W are winning
the race amongst incandescent, halogens and discharge lamps as lighting source.
As predicted by Roland Haitz (called Haitz’s law), LEDs since the birth in 1962
reduced their cost to 10 times with increase in efficiency 20 times in every
decade. LED technology has again gave the impetus to electronics which has slowed
down in the last couple of years.

References:

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11.  Mueller, Gerd (2000) Electroluminescence
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