Chemistry for Everyone

LEDs: New Lamps for Old and a Paradigm
for Ongoing Curriculum Modernization
S. Michael Condren*
Department of Chemistry, Christian Brothers University, Memphis, TN 38104; mcondren@cbu.edu
George C. Lisensky*
Department of Chemistry, Beloit College, Beloit, WI 53511; lisensky@beloit.edu
Arthur B. Ellis*
Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706; ellis@chem.wisc.edu
Karen J. Nordell
Department of Chemistry, Lawrence University, Appleton, WI 54909
Thomas F. Kuech
Department of Chemical Engineering, University of Wisconsin–Madison, Madison, WI 53706
Stephen A. Stockman
LumiLeds Lighting, San Jose, CA 95131

A Paradigm for Updating the Curriculum
There is increasing recognition that chemistry curricula caneffectively draw on modern research and technology to provide
compelling story lines for communicating core principles.
Modules, for example, are now available for introductory chemistry courses that convey fundamental concepts through suchthemes as air-bag design, preservation of the ozone layer, olestra
fat-substitute chemistry, and construction of semiconductor
light-emitting diodes (LEDs) (1). These nontraditional formsof content permit instructors to connect chemistry to emerging, often interdisciplinary, fields such as environmental science,materials science, and biotechnology (2).
Cutting-edge research and technological advances are
frequently identified in research- and applications-driven
white papers, “road maps”, and future-directions articles (3, 4).
We believe that these summaries of research and technicalopportunities are an underutilized rich source of up-to-date
instructional materials. In this paper we present a paradigm
for use of such materials. A recent white paper identifies LEDs
as potential replacements for a significant fraction of vehicle,
traffic, display, home, and workplace lighting, with substantial
safety and energy- and environment-conserving benefits (5).
One of our objectives is to summarize key results from thisstudy that will engage student interest.
Another objective is to identify examples from this technology that instructors can use to illustrate core chemistry
topics. Such correlations have been published for semiconducting materials using a topic matrix (6 ). These correlations,which are updated and expanded in this article, embrace avariety of periodic trends in a family of essentially isostructuralsolids. The stoichiometry of these solids can be routinely tuned
to include three and even four elements using the periodic
table as a design tool. From a bonding standpoint, these solids
illustrate a variety of trends in periodic properties, including
electronegativity, atomic radii, bond polarity, and isoelectronic
principles. From a spectroscopic perspective, colors of emitted
light can be correlated with physical and electronic structure.

Semiconductors provide an acid–base and concentration cellsystem that complements traditional presentations of aqueous
systems. For more advanced treatments, the quantum mechanics of spatially confined particles can be presented with thisfamily of solids, demonstrating the remarkable control thatexists in the growth of semiconducting materials.
Background to the LED White Paper
Lighting is such a routine part of our lives that it is easy
to overlook the profound changes in lighting technologies
being brought about by LEDs. As is reported in the white
paper, arrays of these small, bright light sources are being
introduced into a variety of traditional lighting venues, yielding
substantial energy and safety benefits (5). Worldwide, lighting
applications consume about 20% of all electricity generated.
The luminous efficiency of recently developed high-efficiency
LEDs exceeds that of incandescent bulbs (Fig. 1). Estimates
predict that the widespread replacement of incandescent andfluorescent lighting with LED-based lighting could decrease
this consumption by 50%, a global savings of more than $100
billion/year. As a consequence of this lower demand forelectricity, the carbon emissions created during the generationof electricity could be significantly reduced.
Traditional incandescent light bulbs are blackbody emitters. A filament, typically tungsten, is resistively heated to a sufficiently high temperature that a portion of the emitted electromagnetic radiation is in the visible part of the spectrum (Fig. 2).
Most of the energy is wasted as heat. At high temperatures, thefilament sublimes, leading to a thinner wire with yet higher
electrical resistance and enhanced susceptibility to sublimation.
As this process naturally accelerates, it leads to failure of thefilament. A standard 100-W incandescent bulb has an average
lifetime of 1,000 hours, or about 84 days if it is used 12 h per
day (5).
In contrast, electrons supplied to an LED enter excitedstate energy levels. As these electrons relax to the ground state,energy corresponding to the difference in energy levels is

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Chemistry for Everyone

Luminous Efficiency / (lumens/watt)

100

AlGaInP / GaAs AlGaInP / GaP
yellowred
yellow red
10

unfiltered incandescent lamp
yellow filtered incandescent lamp

AlGaAs / AlGaAs
red

InGaN
green

red filtered incandescent lamp
AlGaAs / GaAs
red

InGaN
blue

GaAsP:N
THOMAS EDISON'S
GaP:Nyellow
FIRST BULB
red
green
GaP:Zn,O
red

SiC
blue

GaAs0.6P0.4
red
0.1
1970

1975

1980

1985

1990

1995

2000

Figure 1. Plot of log (luminous efficiency) versus year for LEDs, showing that many LEDs are now more efficient than
incandescent lamps; efficiencies are expressed in lumens of light output per watt of electrical input. LED compositions
and their corresponding emission color are indicated in the graph; three- and four-element formulas like AlGaAs and
AlGaInP represent solid solution compositions, wherein the emission band maximum can be tuned with composition, as
described in the text and in refs 6–9 and 23. Elements following the colons in these chemical formulas represent dopants
employed to improve the LED efficiency, and compositions following a slash ( / ) are the substrate. Adapted from ref 25.
We are grateful to M.G. Craford of LumiLeds for this figure.

emitted in a narrow band of wavelengths. The periodic properties that control the energy levels in the solid—atomic size andelectronegativity—also control the wavelength of light emitted(2, 6–9). Current LEDs have an expected lifetime of 100,000 h(5). It is estimated that even when driven at their maximum
rated current, LEDs will be usable for on the order of 10 to 100
years, (out)lasting the lifetime of most products into which they
will be incorporated. Disposal should pose little environmental threat, as the chips (the block comprising layers ofsemiconducting material that are subjected to a voltage toyield current and light), which currently contain principally
Al, Ga, In, N, and P, are encased in a robust plastic housing.
The typical semiconductor LED chip (or die) has dimensions of a few tenths of a millimeter. It is encased in a polymer
lens that helps focus the light and holds the electrical connections in place. One electrical connection is typically made byattaching the die directly to a lead that also performs the functions of heat dissipation and reflecting light into the desiredradiation pattern. The other connection is made by a gold wire
connection from the die to a second lead. The leads provide theprincipal means of heat dissipation for the electrical energythat does not become light. The size of a semiconductor LED is
limited by the quantity of heat produced. Changing the LED
packaging to include a larger heat sink has recently permitted
construction of LEDs that are more than an order of magnitude brighter than those previously used in many applications.
The principal cause of LED failure is damage of the polymer
housing, through either passage of too large a heat-generating
current, exposure to ambient UV light, or damage to the LED
die resulting from mechanical stress within the housing.
Applications of LEDs

Figure 2. Comparison of spectral outputs of red traffic lights. The narrow peak with λmax at ~ 640 nm is the spectrum of the red LED traffic
light. The broader spectrum is the output of an unfiltered incandescentbulb used in a traditional traffic light. When this bulb is covered witha red plastic lens, wavelengths exceeding ~ 600 nm (vertical line in
the figure) are transmitted, accounting for the red color.

1034

In comparison to incandescent bulbs, LEDs are smaller,
longer lived, more energy efficient, more robust, less heat generating, and more directional, and they have faster switchingtimes. The relative monochromaticity of LEDs compared toenergy-inefficient filtered incandescent bulbs makes them advantageous in a variety of applications involving colored lights(5). Part of the apparent vividness of the color associated withan LED is due to this monochromaticity or high color saturation. In the remainder of this paper, we discuss the wide

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Chemistry for Everyone

Figure 3. Illustrations of LED vehicle applications. Clockwise frommiddle right: close-up view of truck brake lights, and their positionon the vehicle; bus brake lights and turn signals; automotive centerhigh mount stop lamp (CHMSL) and closer view.

Figure 4. LEDs are being used for a variety of monochromatic
applications, such as (clockwise from upper left) speed indicators,
traffic and pedestrian signals, exit signs, large digital clocks, andconstruction arrows.

range of LED applications related to these characteristics andsummarize some of the chemical and physical properties underpinning the replacement of incandescent lights with LEDs.

The fast illumination rate of an LED is a significant safety
benefit. The sudden, brighter illumination captures drivers’
attention more effectively than does the slower buildup of thelight from an incandescent bulb while the filament is being
heated. As a measure of this characteristic, called “conspicuity”(11), LED brake lights become fully illuminated as muchas 200 ms before incandescent bulbs (12). This gives afollowing driver more response time for braking; at 65 mph,
this can mean as much as one full car length. The use of LED
brake lights may be even more important for large trucks. A
significant voltage drop is caused by the longer wires required
to reach the rear of these vehicles. This voltage drop causes theincandescent bulb to light even more slowly and to a lower
full brightness. An additional safety feature is that the single
color emitted by LEDs is never ambiguous, unlike a filteredincandescent light source, in which a broken filter can cause
confusion about the meaning of the illuminated lamp.

Vehicle Lighting
LEDs are increasingly being used for exterior lights on
automobiles, trucks, and buses. LED brake lights and rearturn signals are installed as original equipment and retrofitted onto buses and trucks as incandescent bulb assemblies
burn out or are damaged (Fig. 3) (10). It is expected that as thecost of LEDs decreases, they will replace nearly all of the exterior and interior lighting of vehicles.
Four major reasons for the switch to LEDs are energy
savings, durability, design flexibility, and safety. The use of LEDs
in motor vehicles reduces the amount of electrical power
required by the vehicle. This allows more power for otherapplications or the use of a smaller generator or alternator.1
A drawback of incandescent bulbs is their susceptibility to
failure from the mechanical shocks and vibrations associatedwith moving motor vehicles. LED assemblies should last thelife of the vehicle. The ability to permanently solder LEDs
to printed circuit boards also reduces manufacturing costs.2
The fact that LEDs are small allows considerable design
flexibility. For example, tail light assemblies can be placed insuch spatially restricted locations as the roof of vans and trucks
and the spoilers on the backs of automobiles (Fig. 3).3 Theirwide range of colors allows LEDs to be used in instrument
displays for information about the operation of the vehicle. For
example, blue LEDs are used to indicate that the vehicle’s
bright headlights are in operation, green LEDs identify turnsignal usage, red LEDs alert the driver to conditions requiring
immediate attention, and orange LEDs provide less urgent
warnings related to low fuel and servicing needs. The reduced
size and heat generation of LEDs compared to incandescentbulbs allows greater information density on the instrument console. LEDs are also more compatible with the microprocessor
logic that supplies information to the instrument console (10).

Traffic Control Signals
For reasons of energy savings and safety, LED arrays are
finding increasing use in traffic lights, vehicle speed indicators,
pedestrian crossings, construction signals, and railroad crossing
flashers (Fig. 4). Although red LED traffic signals are most common, amber and green LEDs also now meet the color standardsof the Institute for Transportation Engineers, and energy
savings can be realized in both new LED traffic-control signals
and retrofitted units. A single-color incandescent traffic light
consumes 150 W, whereas a brighter LED traffic light consumes
about 20 W of power (13). Relative to red LED traffic signals,
the slower adoption of amber and green LED traffic signals
reflects their longer payback times, which are due to reduced
illumination periods (amber) and higher manufacturing costs(green) (5, 14). LED traffic arrows use only 8 W compared to
the 150 W required for turn arrows constructed from an incandescent bulb with a mask. The savings arise in part because
only enough LEDs to form the arrow are needed (10). Similarly, pedestrian Walk/Don’t Walk lights operate at 8–12 W,

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Chemistry for Everyone

compared to the normal 70 W used by incandescent bulbs(15). When scaled over the number of traffic control signals
in the world, use of LEDs offers significant savings in global
energy usage.4
The redundancy of the LED array is a particularly important safety feature: failure of a single element is not catastrophic,since there are typically more than 100 LEDs on a single traffic
light, depending on size. In fact, many of the LEDs can fail andthe light will still operate.5 New designs with only 12–18 highpower LEDs still provide significant redundancy (16 ).
LEDs also reduce maintenance costs. In addition to the costof emergency replacement of failed incandescent bulbs, routine
maintenance of traffic signals calls for the replacement of all red
and green incandescent bulbs annually and of amber traffic
signals every two years (10).6 LED traffic lights are expected tohave significantly longer lifetimes.

Message Boards and Large Array Displays
LEDs are providing a new generation of display technologies because of energy savings, color mixing, and brightness.
Illustrating energy savings are lighted interior EXIT signs (Fig.
4). A typical 100-LED sign will consume less than 1⁄3 theenergy of conventional compact fluorescent lamps. Newer
signs with as few as six high-power LEDs provide the sameenergy savings. This low power consumption also means that
the LED-based sign stays lit longer on battery backup power(5). Solar-powered LED displays are currently used on roadway
construction sites. The solar panels generate sufficient power
to run the display during daylight hours and to charge thebatteries for use during the night. In contrast, portable displayswith low-efficiency incandescent bulbs require use of a noisy
and polluting gasoline-powered generator.
Television monitors and computer screens use combinations of red, green, and blue to produce millions of different
colors. The development of the blue and green LED chips
using GaInN technology, in conjunction with red chips basedon AlGaInP technology (see below and Fig. 1), has permitted
creation of red, green, and blue pixels that can be used forlarge-area, full-color video displays (9, 17 ). Virtually the entire
visible spectrum can be generated by independently varying
the current supplied to each LED. The fast switching times of
the LEDs allow them to be used in full-motion video displays.
Enormous LED video displays, some of them portable, areappearing in sports and civic venues as large array replay screensand message boards. The high efficiency of LEDs creates ex-

Figure 5. Giant outdoor LED displays like this Sony JumboTron in
the center of the Liberty Bowl Stadium scoreboard in Memphis, TN,
are being used to show live video and instant replays.

1036

ceptional brightness, permitting effective viewing even in brightoutdoor lighting. To illustrate the density with which LEDs arepacked, the Liberty Bowl Stadium’s JumboTron display (Fig. 5)
has nearly 250,000 light-emitting elements. Sony JumboTrons
are at least 10 times brighter than ordinary color cathode ray
tube displays and consume only 5% as much energy (18). LED
displays also offer lower weight and significantly lower maintenance costs. A new display in New York City’s Times Square,currently the world’s largest, is eight stories tall, contains nearly
19 million LEDs, and wraps around the corner of the Nasdaq
MarketSite Tower (19).

White LEDs
An emerging, potentially vast application of LEDs is forlong-lasting home and workplace white lighting (5). White light
can be generated by LEDs with at least two techniques: atrio of red, green, and blue LEDs will produce this color, or a
blue LED chip that excites red and green or yellow phosphorswithin the LED package can provide a similar effect (9). To
date, white LEDs are being used in only a few low-light anddirectional applications such as stair and path lights, shelfand accent lights in merchandise displays, and flashlights (20).
Clusters of white LEDs are available in a screw mount thatallows them to replace conventional light bulbs. Figure 6
shows such a screw mount using red LEDs (21). Future whitelighting products will likely employ a very small number ofhigh-power LEDs to reduce the size, cost, and complexity ofthese bulb replacements.
Novelty Applications
LEDs are appearing in products ranging from model railroad lights to yo-yos to children’s athletic shoes to writing
pens to ornaments. Model railroads have experimented with
LEDs for years in miniature signals, and white LEDs are beginning to replace tiny incandescent bulbs as headlights inmodel railroad locomotives (22). Virtually all portable devices,from key-chain flashlights to cell phones, will benefit fromthe excellent reliability and low power consumption of LEDs.
The Chemistry of LEDs
Detailed descriptions of the materials used in LEDs andthe devices’ construction have appeared (2, 6–9, 23). Recent
developments and the key concepts underpinning these devices are summarized below.

Figure 6. LEDs of any color can be arranged in a cluster that will
fit into a standard incandescent bulb socket, such as the red cluster shown here.

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Chemistry for Everyone

Analogies to Acid–Base Chemistry and Concentration Cells
The crystalline semiconductors used in the construction ofLEDs, like all semiconductors, are characterized by a band gap
energy. In a simple bonding picture, the band gap energy canbe regarded as the energy required to ionize an electron froma bond in the solid, permitting it to become mobile and contribute to electrical conductivity. The remaining one-electron
bond is also mobile and can be thought of as a positively
charged particle called a hole. (An electron from a nearby twoelectron bond can “fill” the one-electron bond, creating a new
one-electron bond in the process and, in effect, moving the hole.)
When mobile electrons occasionally restore a two-electron bondby recombining with a hole, they liberate energy as heat and/or light. The latter process is called radiative recombination andthe energy released will be in the form of photons possessing
roughly the band gap energy and occurring at a corresponding
wavelength given by the Planck relationship, E = h ν (6 ).
This creation and recombination of electron–hole pairs
in a 1:1 ratio is reminiscent of autoionization in water and isgoverned by a similar temperature-dependent equilibrium
constant. In water, the product of the proton and hydroxide
concentrations is typically expressed as Kw = [H+] × [OH ᎑].
The analogous equilibrium in semiconductors is described interms of the concentration of mobile electrons n and mobile
holes (missing electrons) p, with K = n × p. Just as acids andbases can be used in water to increase the proton or hydroxide
concentration, respectively, at the expense of the other, impurities called dopants can be deliberately added to semicon-

positive electrodesupplies holespnegative electrodesupplies electronsnelectrons and holes
combine at p-n junction
and produce light

conduction band

electrons

Potential
EnergyEfEgholes
valence band

p-typen-typeFigure 7. Schematics of (top) a p-n junction under forward bias and(bottom) the corresponding band diagram in the junction region. The
symbols Eg and Ef in the lower diagram represent the semiconductor’sband gap energy and Fermi level (the chemical potential of electrons
in the solid), respectively. The indicated polarity of the applied voltage reduces the barrier to the flow of electrons and holes across theinterface, promoting their radiative recombination. This figure isadapted from ref 25.

ductors to adjust the relative values of n and p. In GaAs, forexample, substitution of Zn for Ga or of Se for As can increase p or n, respectively, because the substituting atom has
fewer or more valence electrons than the atom it replaces.
When the concentration of electrons exceeds that of holes
through doping, the material is described as n-type; and when
the concentration of holes exceeds that of electrons, thematerial is p-type (6 ).
By growing semiconductors in which the p-type layer isin contact with the n-type layer, a p–n junction is created thatis characterized by a built-in voltage. This voltage results fromthe difference in electrochemical potentials of the n- and ptype materials and is analogous to the voltage that can beobtained from an aqueous concentration cell having different
proton concentrations (pH) in the two electrolytes. When avoltage is applied to a p–n junction in one direction (forwardbias) (Fig. 7), mobile electrons on the n-type side can easily
cross to the p-type side where they have a high probability forrecombining with the large pool of holes in this region, leading
to efficient emission of band-gap-energy light. Similarly, mobile holes in the p-type region can cross to the n-type sideand have a high likelihood for radiatively recombining with electrons, the majority charge carriers in this region. With avoltage applied in the opposite direction (reverse bias),
there is a substantial barrier for electrons or holes to cross theinterface, and little current flows. This pronounced directionality of current flow with applied voltage is the essence ofdiode behavior and is a property of LEDs (2, 6–9).

The LED Palette and Periodic Properties
The color of light emitted by an LED can be tuned byvarying the semiconductor composition guided by periodic
trends that involve atomic radii and electronegativity. Descending a group in the periodic table leads to larger internuclear
distances (6, 10). As the internuclear distance increases, thebonding electrons are held less strongly, and their more facile
ionization corresponds to a smaller band gap energy. For
example, the band gap energies of the binary semiconductors
GaP, GaAs, and GaSb are 2.3, 1.4, and 0.7 eV, respectively(24). The internuclear separations in AlAs and GaAs arealmost identical, but the larger band gap energy of AlAs,
2.1 eV, reflects the greater polarity and strength of the Al–As
bond compared to the Ga–As bond (6 ).
Even greater tunability in band gap energies is achieved
through the use of solid solutions (alloy semiconductors). Twosemiconductors with the same crystal structure can often form
solid solutions in which one atom substitutes for the otherin any proportion if the substituting atoms are chemicallysimilar. For example, the band gap energies of the ternary
solids Alx Ga1᎑x As (0 ≤ x ≤ 1) can be continuously tuned from
1.4 eV to 2.1 eV as x varies from 0 to 1. With quaternary
solids additional design flexibility is available, because unit
cell size can be varied while retaining the band gap energy.
Alternatively, the band gap energy can be varied while maintaining the size of the unit cell (lattice constant) (6 ). The latteris extremely important in growing high-quality devices comprising layers of semiconductors, as it permits materials withdifferent band gap energies to be grown atop one another sothat the atoms of the growing layer are in alignment withthe substrate, a process called epitaxial growth. This technique
may minimize the number of defects at the interface of theJChemEd.chem.wisc.edu • Vol. 78 No. 8 August 2001 • Journal of Chemical Education

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Chemistry for Everyone

junction between the materials that might otherwise degrade
the performance of the LED (25).

Preparation of LEDs
The design and preparation of LEDs exemplifies exquisite
control both of chemical composition and materials growth,
resulting in devices with tailored electro-optical properties(26 ). Techniques such as organometallic vapor-phase epitaxy(OMVPE, also known as metalorganic chemical vapor deposition, or MOCVD) provide kinetically controlled growthof semiconductor layers, which can be used to localize andoptimize radiative recombination of electrons and holes inthe solid (27 ). As noted above, the chemical composition of
the layers controls their band gap energy and thus the colorof light associated with the emitted photons that result fromradiative recombination (28).
The group 13–15 semiconductors (called III–V semiconductors by researchers in the field) comprising the layers of LED
dies are produced through co-decomposition of group 13 andgroup 15 precursor gas molecules. For example, using highly
purified gases like trimethyl gallium and arsine, a layer ofsemiconducting GaAs can be formed by the decompositionof the gases on a heated substrate, according to eq 1.
(1)x(CH3)nAl(g) + y(CH3)nGa(g) + z(CH3)nIn(g) + PH3(g) →(2)
AlxGayInzP(s) + CH4(g)The product subscripts indicate that these materials form afamily of solid solutions, possessing, in this case, a common
cubic zinc blende crystal structure but variable composition
with (x + y + z) = 1 (6 ). Typically, chemical compositions thatare nearly lattice-matched are used so that the atomic layers
continue to grow epitaxially and defects are minimized.
The emergence of the LED as a versatile lighting source
reflects the use of new atomic compositions that embrace theentire visible spectrum and yield brightness and efficiency thatrival traditional incandescent lamps. A summary of these compositions and colors and a plot of the rise in LED efficiency
over the past several decades is presented in Figure 1. From thered to the yellow part of the spectrum, quaternary semiconductor compositions that can be denoted as (AlxGa1᎑x)0.5In0.5P(0 ≤ x ≤ 1) have been found to yield superior LED characteristics. Figure 9 presents a plot of band gap energy and associated emitted wavelength versus lattice constant for the
AlGaInP family of solid solutions. The lower and higher edgesof the shaded region correspond to the ternary compositions
GaxIn1᎑xP and AlxIn1᎑xP, respectively. The interior shaded
region comprises the quaternary AlxGay InzP compositions.
Note the vertical dashed line in the figure, which correspondsto (AlxGa1᎑x)0.5In0.5P compositions. This line illustrates how
band gap energy increases from ∼1.9 eV to 2.3 eV—from thered to the green part of the visible spectrum—with increasing
Al content while maintaining a lattice constant of 5.65 Å.

Figure 8. (a) Chemical composition can be controlled at the nanoscale.
This scanning electron microscope (SEM) image shows various thicknesses of GaAs layers sandwiched between thicker (50-nm)
Al0.3Ga0.7As layers, forming so-called quantum wells. (b) Emission
from a set of quantum wells like those shown in (a), illustrating thatemission energy increases as thickness, labeled on the spectrum,
decreases.

2.4
2.2

AlP

500

AlxIn1−xP
AlGaInP lattice
matched to GaAs

GaP

600
2.0

GaxIn1−xP
1.8

700

1.6

800

1.4

GaAs
5.4

5.6

InP
5.8

Wavelength / nm

Such growth techniques permit chemical composition to be altered virtually in an atomically abrupt manner by changes in
the concentration and composition of the gaseous precursors(Fig. 8a). A more general, unbalanced equation for tuning
chemical composition is eq 2, where the value of n can be 0, 1,
2, or 3, reflecting the composition of species derived from thetrialkyl precursors that are present under growth conditions:

1038

(b)

Energy Gap / eV(CH3)3Ga(g) + AsH3(g) → GaAs(s) + 3CH4(g)

(a)

1000
6.0

Lattice Constant / ÅFigure 9. Diagram of band gap energies and corresponding wavelengths as a function of atomic bond distances, as represented by
the cubic unit cell lattice constant for AlGaInP compositions having
the zinc blende structure. The three corners labeled with open circles
correspond to pure binary phosphide phases. Solid edges represent
ternary solid solutions derived from the corner compositions. The
shaded interior represents quaternary solid solutions of AlGaInP; thecompositions below the interior solid line are highly emissive andused to construct LEDs. Compositions lying on the vertical dashed line
have the same unit cell size as GaAs and can be grown epitaxiallyon this substrate. Adapted from ref 26.

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Chemistry for Everyone

Within the past few years another significant breakthrough has
been the development of robust, efficient GaxIn1᎑xN compositions to access the blue and green parts of the visible spectrum.
Interestingly, LEDs prepared from these materials operate
effectively despite having very high defect densities (25).
Future breakthroughs in the use of LEDs will hinge on newmaterials and improvements in synthesizing semiconductors
so as to achieve even better control of the nanoscale structure of
the resulting devices. An emerging technology is the use of organic compounds for construction of LEDs. These organic LEDs
(OLEDs) may offer substantial benefits in cost, size, weight,
processability, and environmental impact that could make themcompetitive in a variety of lighting applications (29, 30).
Further advances in controlling nanostructures of traditional LED materials can also be expected. The “particle in abox” that is a mainstay of physical chemistry courses can begrown with thin semiconductor layers known as quantum
wells (31, 32). Figure 8b shows the emission spectrum of amaterial similar to that described in Figure 8a. Emission occurs
at higher energy when electrons are confined to thinner layers(i.e., a particle in a smaller “box”). The increasing sophistication with which we can create customized structures on
the atomic scale suggests that industry goals of 45% efficiency
for LEDs throughout the visible spectrum will soon be within
our grasp.
Conclusion
As indicated by these many applications, LEDs are causing
profound changes in lighting technology. The ability to massproduce the semiconductor compositions comprising thesesmall devices using modern materials growth methods, coupledwith their desirable operating characteristics, are permitting
arrays of LEDs to replace traditional lighting sources withconsiderable safety and economic benefits. These developments
readily lend themselves to incorporation into the chemistry
curriculum as examples of the application of chemical principles
to cutting edge research and technology. Industry white papers
and “road maps” and “future directions” articles often provide excellent summaries of such research breakthroughs andtheir potential economic and technological impact. They canthus be used as resource materials for updating the curriculum
and connecting chemistry to other disciplines.
Images and more information about applications ofLEDs are available at http://mrsec.wisc.edu/edetc/LED andhttp://www.lumileds.com.
Acknowledgments
We wish to thank Mike Friedman of Electro-Tech,
Roland Haitz of Agilent Technologies, and Jeff Tsao of Sandia
National Laboratories for much of the technical information
about LEDs and their applications. We also thank Bill Wilson,
Manager of the Liberty Bowl Stadium, for assistance in obtaining pictures of the Sony JumboTron replay screen. We appreciate helpful comments from several reviewers and from Jonathan
Breitzer and Cynthia Widstrand, which substantially improved
the manuscript. We are grateful to the National Science Foundation Materials Research Science and Engineering Center for
Nanostructured Materials and Interfaces (DMR-9632527)
for generous support of this research.

Notes
1. The weight of the vehicle is reduced a small amount with LEDs
because the lower power requirements allow thinner, lighter wiring to
be used. This reduces the load on the motor and results in lower fuelconsumption, an industry objective. The use of LED turn signals can
extend by a factor of 2 to 3 the amount of time that the emergency
flashers can operate on battery power. The low operating temperaturesof LEDs permit them to be weather-sealed with inexpensive, lowmelting plastics.
2. A major LED manufacturer estimates that as much as $80
million in warranty repair costs could be saved by using LEDs forthe rear exterior signal lights on the 15 million cars and light trucks
produced each year in North America (33).
3. One of the first applications of LEDs was in the center highmount stop lamp (CHMSL), which became mandatory on U.S.
vehicles in 1982. By 1999, 30–40% of new cars had LED CHMSLs.
For some year 2000 models, the tail, brake, and turn signal lights
are all LEDs (5, 34).
4. In the United States alone, 10 million red/yellow/green
traffic lights consume about 400 MW (3.5 terawatt-hours annually)of electricity. At $0.07 per kilowatt-hour, the cost of operating U.S.
traffic lights is $240 million annually. Replacing only the red signals
and arrows would save about 250 MW, and the estimated payback
period for the red LED signals is significantly less than one year (5).
Conversion to LED traffic signals would save 70 MW (enough power
for 70,000 homes) in California (15, 35). Since the lower power
consumption would help avoid the expense of building additional
power plants, many utility companies (e.g., Pacific Gas and Electric in
California; Northern States Power in Minnesota, Wisconsin, North
Dakota, South Dakota, and Michigan; GPU Energy in New Jersey)
have offered rebates to cities that install LED traffic lights (36–38).
5. In a test by the state of Oregon, several shots from a 9-mm
weapon were fired at a 12-inch traffic signal module from a distanceof 25 feet. While 112 of the more than 600 LEDs failed, there was noeffect on color and the module was considered viable from 50 to
75 feet (10).
6. This operation requires at least two persons with a bucket
truck, costing about $250 per intersection.

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