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What Is The State Of Solid State LED Lighting?
By Chris Rollins
Created Dec 29 2008 - 3:12pm
It seems like they're already everywhere - if you look
down at your keyboard, one is providing the little green
indicator for your Num Lock key, while another may be
frantically blinking to inform you of your waiting voice mail.
On your next drive, chances are you will stop at a stoplight
lit purely by LED's and perhaps may notice a blinker made up of
small, bright lights instead of one bulb. LED's certainly are
everywhere, faithfully providing indications of all kinds. But,
with a few exceptions, the primary purpose of an LED is to
indicate rather than illuminate, despite the fact that LED's do
nothing useful besides produce light. A few new developments,
however, may bring solid-state lighting into our homes very
soon.
Little, but Bright: LED's may be poised to replace incandescent
and compact fluorescent bulbs as our home or office light
source. Photo Credit: MIT
An LED, or Light Emitting Diode, is a type of circuit element
that emits light when electrical current flows through it in
the appropriate direction. Like a normal diode, current flow is
blocked when it travels against the direction of the diode.
When a positive voltage is applied to the positive side of an
LED, the electrons that flow are required to jump down in
energy as they cross the diode - emitting the lost energy as
light. This light is generated at one specific frequency, based
on the characteristics of the materials used. When creating
white light that may be useful for lighting, however, a
blending of many colors is needed - something the LED does not
seemed easily poised to accomplish, despite numerous advances
in LED technology.
Red light was the first visible light LED to be produced, in
1962, followed by yellow and green. Much later, in the early
1990's, blue-spectrum LED's were developed - a huge advance in
usefulness, since blue light can be combined with red and green
to make white light. However, this approach was inefficient,
and companies were scrambling to make a single diode that could
produce white light on its own. Then, in 1993, a company named
Nichia created the first white LED using a blue LED with a
phosphor coating. The coating was the trick - it provided
enough shifting of the light wavelength coming from the diode
itself to create white colored light.
Why, then, aren't we using LED's - which are much more
efficient than either incandescent or fluorescent bulbs - to
light up our homes and offices? The major problem, of course,
is cost. Current white LED's require a substrate made of
sapphire and an additional mirroring layer to reflect light
that would otherwise be lost. As a result, LED lights already
on the market cost approximately $100, far too much for the
average consumer.
Researchers at Purdue University have found one method of
significantly reducing the cost of a white LED by eliminating
the expensive layer of sapphire. Instead, they used silicon as
the substrate (the material the diode is printed on) and
zirconium nitride as the reflector. This had never been done
before, mainly because silicon reacts with zirconium nitride
and changes its properties. The researchers solved this by
putting a layer of aluminum nitride between the silicon and
zirconium nitride.
"One of the main achievements in this work was placing a
barrier on the silicon substrate to keep the zirconium nitride
from reacting," said Timothy D. Sands, the Mary Jo and Robert
L. Kirk Director of the Birck Nanotechnology Center in Purdue's
Discovery Park.
Reactor: Timothy D. Sands, left, watches over a reactor that
deposits gallium nitride on silicon at a temperature of 1000°
C. Photo Credit: University of Purdue
Silicon also provides a crystalline structure that the other
materials conform to when deposited on the substrate.
"We call this epitaxial growth, or the ordered arrangement of
atoms on top of the substrate," Sands said. "The atoms travel
to the substrate, and they move around on the silicon until
they find the right spot."
Crystalline structure is very important to the efficient
working of an LED - if the materials used to create the LED
were sprayed on glass, for instance, the LED would operate very
inefficiently. Using silicon also reduces cost by allowing
industry to scale up, or create large batches of LED's on large
wafers of silicon, something the semiconductor industry is
already good at.
LED lighting holds many advantages over traditional lighting,
most notably in efficiency. "If you replaced existing lighting
with solid-state lighting, following some reasonable estimates
for the penetration of that technology based on economics and
other factors, it could reduce the amount of energy we consume
for lighting by about one-third," Sands said. "That represents
a 10 percent reduction of electricity consumption and a
comparable reduction of related carbon emissions." LED's are
also more durable than incandescent or compact fluorescent
bulbs, and have the added bonus of being devoid of mercury - a
chemical found in compact fluorescent bulbs that makes them
difficult to dispose of.
E. Fred Schubert, a professor of electrical engineering and
physics at Rensselaer Polytechnic Institute in Troy, NY,
recently published a paper where he describes the coming
revolution of solid-state lighting. "Replacement is fine," he
says. "But we must look beyond the replacement paradigm to see
the true benefits of LED lights." Schubert imagines large
panels that are able to control all aspects of lighting for a
room, far more than simply "on" or "off." With cheap LED's
available, many different wavelengths could be created in a
room and blended to produce specific effects, like an accurate
representation of sunlight at different periods of the day.
This is important, since current white LED's produce a harsh
bluish light that people generally don't like to use when doing
normal indoor activities, like reading. The ability to control
the hue and saturation of light is something many consumers may
find appealing.
The way the industry is booming, Sands expects that we will
have LED lights within 2 years. There are still technical
hurdles, but he expects industry to clear them easily. "These
are engineering issues," he said. "Not major show
stoppers."
ION Publications LLC
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