emiconductor light emitters have evolved
rapidly over the past few years. We are now on
the threshold of an era in which most ambient
lighting will be provided by solid-state devices.
Many academic, government and industrial labs
around the world are contributing to continued advances in
the technology of light-emitting diodes (LEDs)—the engines
of solid-state lighting.
LEDs have been used for illumination applications for more
than a decade now. The development of blue LEDs has set
the stage for devices that could become serious contenders for
replacing traditional incandescent and fluorescent lamps with
more eco-friendly alternatives.
One such candidate is LEDs
that have photonic crystals
etched on their light-emitting
face. Photonic crystals can be
used to diffract trapped light
from LEDs and tailor the
direction of light propagation.
Photonic crystal LEDs are
being explored for applications
in display technology, space
lighting and other areas.
[ Photonic crystal structure ]
The complete rainbow
Blue LEDs provided the last
remaining component in the
rainbow of colors required to
produce white light. At first,
scientists assumed that solid-state lighting systems would
be made with a combination
of red, green and blue LEDs.
Now, however, the availability of efficient blue LED-excited
phosphors has changed the rules of the game.
Luminescent conversion LEDs (LUCO LEDs) use a coating
of a suitable phosphor material on top of the LED die. This phosphor converts some of the blue light into down-converted yellow
light. The combination of this yellow light with the residual blue
appears as white light to our eyes. This method of generating
white light is easier and cheaper than combining red, green
and blue, although the emission spectrum is fixed by the characteristics of the driving LED and the phosphor material used.
LED phosphors have been much improved over the past
few years, so that we now have both “cold white” LEDs (which
have a bluish tint) and “warm white” LEDs (with a yellowish
shade). Regardless of whether one uses red-green-blue LEDs
or the LUCO LED approach to building solid-state lighting
systems, the efficiency of electric-to-light energy conversion
remains of paramount importance. Fortunately, researchers
continue to make great strides in this area. LEDs have experienced impressive gains in energy efficiency in recent years.
LEDs are bipolar charge injection devices that convert the
electrical energy that is spent in injecting electrons and holes
over an energy barrier into
light energy. They differ
from simple rectifying diodes
in that they are made from
direct bandgap semiconduc-
tor materials. Essentially,
this means that electron-
hole recombination events
require almost no change
in linear momentum. Thus,
these events can result in the
creation of photons.
In indirect bandgap
materials such as silicon,
the recombination energy is
entirely dissipated as heat.
Thus, such materials are
not used for making light-emitting devices. The basic
structure of LEDs remained
the same for more than two
decades after their commercial introduction.
This scanning electron micrograph shows plan and cross-sectional views of a photonic crystal structure. The holes are
relatively shallow blind holes.
Quantum wells and surface texturing
The first major change in the LED industry was the introduction of quantum wells that could confine charge carriers and
make them recombine more efficiently. The quantum wells
raise the device’s internal quantum efficiency, which is the
ratio of the number of photons generated and the number of
electron hole pairs that recombine. However, an increase in the
number of generated photons alone doesn’t necessarily make
an LED brighter. This is because internally generated light can
remain trapped inside the confines of the device because of the
The development of blue LEDs has set the stage for devices that
could become serious contenders for replacing traditional incan-
descent and fluorescent lamps with more eco-friendly alternatives.