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The Bright Present
and Brighter Future
of LED Technology
Photo: clalighting.com.au
Light-emitting diodes (LEDs) are fast becoming the devices of choice
for all artificial lighting applications. Their many benefits, such as
high efficiency, small size, long life and wide availability have made
them very popular over recent years.
T
he first LEDs appeared during the 1960s but attracted
little attention until the 1970s when they gradually
became more widespread as signalling devices in
electronic equipment.
In this role they supplanted the previously ubiquitous
miniature filament lamps.
During the 1980s work was undertaken to increase the
brightness of LEDs and by the 1990s illumination-class
LEDs had appeared on the market. This marked the advent
of a rapid upswing in LED consumption as they began to
be used for mainstream lighting applications.
With steady increase in the brightness level of individual
LEDs and multiple LED modules, applications for LEDs
have proliferated.
This has gone to such an extent
by Dr Faiz
that these devices are now found in
14 Silicon Chip
everything from domestic light bulbs and torches to high
power luminaires for architectural lighting and even street
lighting.
Manufacturers are now combining energy-efficient
LEDs with solar cells to produce lighting systems that are
especially popular in developing countries.
Light-emitting diodes generate light in a fundamentally
different way than sources such as incandescent lamps or
gas discharge tubes. LEDs utilise semiconductor materials to generate light with a small range of wavelengths.
In these materials, bands of very closely-spaced energy
levels separated by energy gaps can be used to create visible light photons.
Electrical charge carriers of opposite
signs (electrons and holes) are injected
Rahman *
into a LED from an external circuit. Insiliconchip.com.au
Extra close-up of a 1W balanced white LED from
Electrospell.
side the diode these carriers recombine when electrons fall
down the energy gap from the upper energy band (conduction band) to the lower energy band (valence band). This
leads to an effective reduction in the electrons’ energy, with
this energy being released by the recombining electronhole pairs. In ordinary silicon diodes, this energy appears
as heat whereas in light-emitting diodes the energy comes
out as light photons.
Coloured LEDs
The colour of emitted light depends on the energy gap
that separates the conduction and valence bands. The
wider the band gap the larger the photon energy, ie, the
shorter the wavelength of emitted light.
The band gap, in turn, depends on the material the LED
Fig.1: electron microscope view of cadmium sulphide
quantum dots inside pits on a GaN LED chip.
siliconchip.com.au
January 2013 15
Fig.2a (above) shows the spectrum of a flat-white LED, while Fig2b (right) shows the spectrum of a LED that mimics the
light from a tungsten-halogen lamp.
is made from. A wide range of semiconductor materials
such as gallium arsenide (GaAs), gallium indium arsenide
phosphide (GaInAsP) and gallium nitride (GaN) are used to
make LEDs that emit light from infrared to the ultraviolet.
The light coming out of single chip LEDs covers a narrow
band of colours and is thus of a more-or-less single colour.
This light can be modified in colour by using wavelength
conversion materials. Several approaches are possible but
two are most prevalent.
Quantum dots and phosphors
Materials called quantum dots, consisting of extremely
small particles similar in size to viruses, can be used to
convert light of one colour into another.
For this purpose, quantum dots are typically made from
materials such as cadmium sulphide or cadmium selenide
and consist of spherical particles, a few tens of nanometres
across. These particles behave somewhat like very large
atoms in that they can absorb light at one wavelength and
emit it, a short time later, at another wavelength.
The emitted light is almost always of a longer wavelength than the absorbed light; ie, the light is ‘red shifted’
so the emitted photons have less energy than the absorbed
photons. The difference in energy is simply converted into
heat. Quantum dots can be used to produce sharp colours
with a narrow distribution of wavelengths.
A remarkable feature of quantum dots lies in the ability to make quantum dots emitting light at any desired
wavelength simply by changing their size.
Blue LEDs can excite red or green emitting quantum
dots to produce highly saturated colour sources. The efficiency of this process can be further enhanced by making
microscopic pits on the surface of LED chips and filling
them up with quantum dot material, as seen in Fig.1. This
configuration increases the absorption of light by quantum
dots and leads to a marked increase in the brightness of
such LEDs.
While quantum dot LEDs are speciality devices, a different type of colour conversion material is widely used to
make white LEDs. No LED chips by themselves emit white
light. In order to obtain white emission LED manufacturers
use the same approach as is used with fluorescent lights.
The blue light emitted by a GaN LED is passed through
a coating of a suitable phosphor material which converts
some of the blue radiation to yellow light.
Typical phosphors consist of crystalline oxides or sulphides doped with rare-earth elements, such as cerium,
europium or gadolinium. The combination of yellow and
blue light appears white to our eyes. It is easy to guess
from this description that ordinary white LEDs produce a
very poor quality of white light which is severely deficient
in red and green.
In recent years, manufacturers have introduced better phosphors that generate warm white light that has a
distinctive yellowish tint. Most LED manufacturers now
offer warm white LEDs which are used for making LED
light bulbs and other luminaires.
Balanced white LEDs
For even better performance, companies such as Citizen,
Fig.3: spectrafill broadband red, green and blue LEDs. The
red and green LEDs use special phosphors whereas the
blue LED uses a ‘stressed’ chip.
16 Silicon Chip
Fig.4: spectra from red, green and blue Spectrafill LEDs.
siliconchip.com.au
B0
B = B0Cos Θ
Θ
Fig.5: Lambertian light intensity distribution pattern. The
angular emission from most LEDs takes this form.
Electrospell and Bridgelux now offer full-spectrum white
LEDs that feature a balanced white spectrum where all
colours are present in roughly equal proportions.
Fig.2a shows the spectrum of a flat-white LED from
Electrospell whereas Fig.2b shows the spectrum of a LED
that emits a close approximation to light from tungsten
halogen lamps. Innovations in phosphors and other optical
materials have made such high performance LEDs possible.
Flat spectrum white LEDs are rapidly penetrating high
colour fidelity lighting markets. Museums, art galleries
and retail outlets are increasingly turning towards lamps
MR16 7W LED GU5.3
siliconchip.com.au
Fig.6: a narrow emission angle LED. This device emits light
in a forward cone which is only 5° wide.
based on such colour-rich LEDs to display their exhibits
in full splendour.
Innovative phosphor technology has also enabled a new
generation of wide spectrum primary colour LEDs. These
so-called ‘broadband’ LEDs emit light with a much wider
spectrum than ordinary LEDs.
Whereas usual colour LEDs display a spectrum which
is typically 30nm wide, their broadband counterparts
emit light with 60 to 90nm wide spectra. By combining
PAR 38 19W E27
Outdoor LED
Classic A 10W E27/B22
January 2013 17
Fig.7a (left): electron microscope view of the patterned surface of a photonic crystal LED. Fig.7b (right): the pattern of light
emitted by a photonic crystal LED when observed from very close to the surface of the chip.
broadband red, green and blue LEDs it is possible to make
colour-tuneable white light luminaires.
By controlling the intensities of the red, green and blue
channels with pulse width modulation (PWM) waveforms
from a microcontroller it is possible to generate millions
of distinct hues. The wide spectra from individual LEDs
enable subtle variations of colour shades in illuminated
objects to be readily distinguished. The exceptionally
high colour rendering capability of wide spectrum LEDs
is creating new lighting markets.
Spectrafill LEDs
Fig.3 shows the spectra of red, green and blue Spectrafill
LEDs from Electrospell. This LED family is aptly named as
the LEDs each fill up their assigned slots in the red, green
and blue regions of the visible spectrum. Broadband LEDs
are also being used for indoor plant growth and for various
skin therapy applications.
Most LEDs on the market emit light in a characteristic
fan-like pattern seen in Fig.4 where the light appears
brightest when seen head on. As one moves away from the
vertical the light intensity falls as the cosine of the angle
away from the vertical. This is known as the Lambertian
intensity distribution. Typical commercial LEDs emit most
of their light in a 120° wide Lambertian fan.
By changing the chip geometry and the way it is mounted
in the LED package it is possible to reduce the emission
angle to as small as 15°. By combining surface texturing
with internal reflective optics LEDs with emission angles
as small as 5° can be obtained. Fig.5 shows a 5° narrow
emission angle green LED. Such LEDs are useful for applications such as back illumination of instrument clusters.
Even more interesting angular emission profiles can
be obtained by etching the top surface of LED chips with
shallow depressions in various regular patterns. Dimples
arranged in square or hexagonal patterns, called photonic
crystals, are often employed for this purpose. Additionally,
the relief causes light to come out of the LED chip with
higher efficiency, making the device appear considerably
brighter.
Textured surface photonic crystal LEDs emit light in a
collimated beam. Fig.6(a) shows the surface of a photonic
crystal LED whereas Fig.6(b) shows the near-field pattern
18 Silicon Chip
of light emitted by this device. The separate emissions
combine into a well-collimated beam that is suitable for
use in projectors and for back-lighting of LCD televisions.
GaN LEDs
GaN LEDs form the basis of not only blue and UV light
emitters but also of all phosphor-based LEDs.
As these LEDs power all LED bulbs and TV back-lights,
there is much research directed at improving them further
and reducing their prices. The most prominent development in this direction is the emergence of silicon-based
GaN LEDs. Conventional GaN LEDs are made by depositing
the active device material on a sapphire substrate.
This is now a well-established process but the LED
chips made in this way are both expensive (because of
the high cost of sapphire) and poor at getting rid of the
heat produced as the LED operates (because the heat has
to pass through nearly half a millimetre of sapphire which
is a poor heat conductor).
The next generation of GaN LEDs will be made by a very
different process using silicon wafers in place of sapphire
as the substrate material. This reduces the cost of LED chips
because, thanks to the silicon chip industry, silicon wafers
are much cheaper than sapphire wafers.
Silicon wafers are also available in sizes larger than
12 inches in diameter. In contrast, commercial GaN-onsapphire LEDs are made on 4-inch diameter wafers. The
larger wafer diameter will mean many more LED chips can
be obtained from each processed wafer, again contributing
to a reduction in LED costs.
GaN-on-Si LEDs will also perform better than sapphirebased LEDs because silicon has a higher thermal conductivity and thus LEDs will be able to run cooler, producing
more light and achieving longer lifetimes. LEDs made on
silicon substrates should be widely available within two
years once their specialised fabs come on line.
Zinc Oxide LEDs
Even more exciting LEDs are currently under development in various academic and industrial labs around the
world. A significant amount of effort is being directed to
develop LEDs from zinc oxide (ZnO).
This material can produce cheap and highly efficient
siliconchip.com.au
LEDs that emit blue and UV radiation. There have been
persistent problems in creating high quality p-type zinc
oxide which has so far held back the realization of a commercial ZnO LED.
With continuing research, however, it is possible that one
day we will see these devices becoming as commonplace
as GaN LEDs are today. Yet another interesting breed of
LEDs under development is based on very thin filaments
of semiconductor materials called quantum wires. Created
by carefully etching long strings of the base semiconductor
material and then topping them with suitable electrode
materials, quantum wire LEDs are substantially more efficient than LEDs made from bulk material.
Furthermore, there is evidence that quantum wire LEDs
might enable electrical tuning of the colour emitted by a
single LED chip, without the use of any colour conversion
material. Such a device will be a true breakthrough in LED
technology and thus several large LED companies, such as
Philips and Osram are working on this technology.
Massive industry
LEDs are now a major industry worth several tens of
billions of dollars a year and growing at an astounding
25% per annum. As their prices fall further and as new
types of devices come on the market their usage will grow
even more. Eventually, all light bulbs will be replaced by
LED-based lamps for better energy efficiency and longer
lifetimes. This transition has already started and will only
be spurred on by ongoing developments in universities
SC
and companies around the world.
Bigger - Brighter - Wider Angle
Outdoor LED Displays
Here are two economical, high performance, JUMBO
displays for wide angled outdoor applications such as race
timing, lap counting and sports scoreboards
Featuring state-of-the-art
Fully
super-bright elliptical LED
Assembled
technology, the NEW
D8-HB 300mm and 400mm
7 Segment Displays are
visible over long distances and
at an incredible 75 degrees
either side of normal.(actually
300mm
400mm
more than 150o in total)
Other features include:
Black Background for higher
contrast
On Board Segment Drivers
On-Board Serial Interface
User-accessible segment connections for custom interfaces
Compatible Modules are available for Counting, deMultiplexing,
BCD to 7 Segment Decoding and Driving
For further details and to buy on-line see us at:
www.kitstop.com.au
P.O. Box 5422 Clayton Vic.3168
Tel:0432 502 755
* Dr Faiz Rahman is from Electrospell Ltd, Glasgow, UK
LED Lighting Specialist
Huge range of stocked LED replacement lamps
Visit our
showrooms
Ceiling Lights
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GLS Lamps
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Brisbane
24-26 Campbell St.
Bowen Hills QLD 4006
Ph: (07) 3252 7466
Fax: (07) 3252 2862
Southport
Unit 11
The Brickworks Centre
Warehouse Rd,
Southport QLD 4215
Ph: (07) 5531 2599
Fax: (07) 5571 0543
Miniature Lamps
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Floodlights
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185 Parramatta Rd
Homebush NSW 2140
Ph: (02) 9704 9000
Fax: (02) 9746 1197
Est.1978
prime-electronics.com.au
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January 2013 19
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