Colors and Materials
Conventional LEDs are made from a variety of inorganic semiconductor materials. The following table shows the available colors with
wavelength range, voltage drop and material:
Ultraviolet and Blue LEDs
Current bright blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They
can be added to existing red and green LEDs to produce the impression of white light. Modules combining the three colours are used in
big video screens and in adjustable-colour fixtures.
The first blue LEDs using gallium nitride were made in 1971 by Jacques Pankove at RCA Laboratories. These devices had too little light
output to be of practical use and research into gallium nitride devices slowed. In August 1989, Cree Inc. introduced the first commercially
available blue LED based on the indirect bandgap semiconductor, silicon carbide. SiC LEDs had very low efficiency, no more than about
0.03%, but did emit in the blue portion of the visible light spectrum.
In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping ushered in the modern era of GaN-based optoelectronic
devices. Building upon this foundation, in 1993 high-brightness blue LEDs were demonstrated. High-brightness blue LEDs invented by
Shuji Nakamura ofNichia Corporation using gallium nitride revolutionized LED lighting, making high-power light sources practical.
By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN quantum wells
sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the
light emission can in theory be varied from violet to amber. Aluminium gallium nitride (AlGaN) of varying Al/Ga fraction can be used to
manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and
technological maturity of InGaN/GaN blue/green devices. If un-alloyed GaN is used in this case to form the active quantum well layers,
the device will emit near-ultraviolet light with a peak wavelength centred around 365 nm. Green LEDs manufactured from the InGaN/GaN
system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit
efficiency too low for high-brightness applications.
With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range
of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often
encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and
paper currencies. Shorter-wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 247
nm. As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV
LED emitting at 250–270 nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that
commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices.
Deep-UV wavelengths were obtained in laboratories using aluminium nitride (210 nm), boron nitride (215 nm) and diamond (235 nm).
There are two primary ways of producing white light-emitting diodes (WLEDs), LEDs that generate high-intensity white light. One is to use
individual LEDs that emit three primary colors—red, green, and blue—and then mix all the colors to form white light. The other is to use
a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a
fluorescent light bulb works.
Due to metamerism, it is possible to have quite different spectra that appear white.
Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. FWHM spectral bandwidth is
approximately 24–27 nm for all three colors.
White light can be formed by mixing differently colored lights; the most common method is to use red, green, and blue (RGB). Hence the
method is called multi-color white LEDs (sometimes referred to as RGB LEDs). Because these need electronic circuits to control the
blending and diffusion of different colors, and because the individual color LEDs typically have slightly different emission patterns (leading
to variation of the color depending on direction) even if they are made as a single unit, these are seldom used to produce white lighting.
Nevertheless, this method is particularly interesting in many uses because of the flexibility of mixing different colors, and, in principle,
this mechanism also has higher quantum efficiency in producing white light.
There are several types of multi-color white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these
different methods, include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency will mean lower color
rendering, presenting a trade-off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the
best luminous efficacy (120 lm/W), but the lowest color rendering capability. However, althoughtetrachromatic white LEDs have excellent
color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous
efficacy (>70 lm/W) and fair color rendering capability.
One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt
but today few green LEDs exceed even 100 lumens per watt. The blue and red LEDs get closer to their theoretical limits.
Multi-color LEDs offer not merely another means to form white light but a new means to form light of different colors. Mostperceivable
colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. As more effort is
devoted to investigating this method, multi-color LEDs should have profound influence on the fundamental method that we use to produce
and control light color. However, before this type of LED can play a role on the market, several technical problems must be solved. These
include that this type of LED's emission power decays exponentially with rising temperature, resulting in a substantial change in color
stability. Such problems inhibit and may preclude industrial use. Thus, many new package designs aimed at solving this problem have
been proposed and their results are now being reproduced by researchers and scientists.
Spectrum of a “white” LED clearly showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more
broadbandStokes-shifted light emitted by the Ce3+:YAG phosphor, which emits at roughly 500–700 nm.
This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light;
the resultant LEDs are called phosphor-based white LEDs. A fraction of the blue light undergoes the Stokes shift being transformed from
shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several
phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively raising the color rendering index (CRI) value
of a given LED.
Phosphor-based LED efficiency losses are due to the heat loss from the Stokes shift and also other phosphor-related degradation issues.
Their efficiencies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of
the LED itself. For example, the efficiency of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the efficiency of the
original blue LED because of the greater luminous efficacy of yellow compared to blue light. Due to the simplicity of manufacturing the
phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or
light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority
of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.
Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient
phosphors. Today the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stoke shift loss. Losses attributable to
internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of
efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light
output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a
more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.
The phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor coated epoxy. A common yellow phosphor material is
cerium-doped yttrium aluminium garnet (Ce3+:YAG).
White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that
emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way
fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is
converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the
ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a
malfunctioning light source and cause harm to human eyes or skin.
Other White LEDs
Another method used to produce experimental white light LEDs used no phosphors at all and was based on homoepitaxially grown zinc
selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.
Organic light-emitting diodes (OLEDs)
Organic light-emitting diode
In an organic light-emitting diode (OLED), the electroluminescent material comprising the emissive layer of the diode is an organic
compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of
the molecule, and the material therefore functions as an organic semiconductor. The organic materials can be small organic molecules in a
crystalline phase, or polymers.
The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and
color gamut. Polymer LEDs have the added benefit of printable and flexible displays. OLEDs have been used to make visual displays for
portable electronic devices such as cellphones, digital cameras, and MP3 players while possible future uses include lighting and
Quantum dot LEDs (experimental)
Quantum dots (QD) are semiconductor nanocrystals that possess unique optical properties.Their emission color can be tuned from the
visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on the CIE diagram. This provides
more color options and better color rendering than white LEDs. Quantum dot LEDs are available in the same package types as traditional
phosphor-based LEDs.There are two types of schemes for QD excitation. One uses photo excitation with a primary light source LED
(typically blue or UV LEDs are used). The other is direct electrical excitation first demonstrated by Alivisatos et al.
One example of the photo-excitation scheme is a method developed by Michael Bowers, at Vanderbilt University in Nashville, involving
coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This method emits a warm, yellowish-
white light similar to that made by incandescent bulbs. Quantum dots are also being considered for use in white light-emitting diodes in
liquid crystal display (LCD) televisions.
The major difficulty in using quantum dots-based LEDs is the insufficient stability of QDs under prolonged irradiation. In February 2011
scientists at PlasmaChem GmbH could synthesize quantum dots for LED applications and build a light converter on their basis, which could
efficiently convert light from blue to any other color for many hundred hours. Such QDs can be used to emit visible or near infrared light
of any wavelength being excited by light with a shorter wavelength.
The structure of QD-LEDs used for the electrical-excitation scheme is similar to basic design of OLED. A layer of quantum dots is
sandwiched between layers of electron-transporting and hole-transporting materials. An applied electric field causes electrons and holes to
move into the quantum dot layer and recombine forming an exciton that excites a QD. This scheme is commonly studied for quantum dot
display. The tunability of emission wavelengths and narrow bandwidth is also beneficial as excitation sources for fluorescence imaging.
Fluorescence near-field scanning optical microscopy (NSOM) utilizing an integrated QD-LED has been demonstrated.
In February 2008, a luminous efficacy of 300 lumens of visible light per watt of radiation (not per electrical watt) and warm-light emission
was achieved by using nanocrystals.
LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but
not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colourless housings. Modern high
power LEDs such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages, (not
The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.
These are mostly single-die LEDs used as indicators, and they come in various sizes from 2 mm to 8 mm, through-hole and surface mount
packages. They usually do not use a separate heat sink.Typical current ratings ranges from around 1 mA to above 20 mA. The small size
sets a natural upper boundary on power consumption due to heat caused by the high current density and need for a heat sink.
Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and
triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle.
There are three main categories of miniature single die LEDs:
Low-current: typically rated for 2 mA at around 2 V (approximately 4 mW consumption).
Standard: 20 mA LEDs (ranging from approximately 40 mW to 90 mW) at around
1.9 to 2.1 V for red, orange and yellow. 3.0 to 3.4 V for green and blue. 2.9 to 4.2 V for violet, pink, purple and white.
Ultra-high-output: 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.
Five- and twelve-volt LEDs are ordinary miniature LEDs that incorporate a suitable series resistor for direct connection to a 5 V or 12 V
Medium-power LEDs are often through-hole-mounted and mostly utilized when an output of just a few lumen is needed. They sometimes
have the diode mounted to four leads (two cathode leads, two anode leads) for better heat conduction and carry an integrated lens. An
example of this is the Superflux package, from Philips Lumileds. These LEDs are most commonly used in light panels, emergency lighting,
and automotive tail-lights. Due to the larger amount of metal in the LED, they are able to handle higher currents (around 100 mA). The
higher current allows for the higher light output required for tail-lights and emergency lighting.
This high powered A19 sized LED light bulb thermal animation, was created using high resolution CFD analysis, and shows temperature
contoured LED heat sink and flow trajectories, predicted using a CFD analysis package, courtesy of NCI.
High-power LEDs (HPLED) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for
other LEDs. Some can emit over a thousand lumens. LED power densities up to 300W/cm2 have been achieved. Since overheating is
destructive, the HPLEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the
device will fail in seconds. One HPLED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED
Some well-known HPLEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and
Cree X-lamp. As of September 2009, some HPLEDs manufactured by Cree Inc. now exceed 105 lm/W (e.g. the XLamp XP-G LED chip
emitting Cool White light) and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent lights, as LEDs
grow more cost competitive.
The impact of Haltz's law governing the light output of LEDs over time can be readily seen in year over year increases in lumen output and
efficiency. For example, the CREE XP-G series LED achieved 105 lm/W in 2009, while Nichia released the 19 series with a typical efficiency
of 140 lm/W in 2010.
LEDs have been developed by Seoul Semiconductor that can operate on AC power without the need for a DC converter. For each half-
cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HPLED is
typically 40 lm/W. A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul
Semiconductor released a high DC voltage LED capable of being driven from AC power with a simple controlling circuit. The low-power
dissipation of these LEDs affords them more flexibility than the original AC LED design.
Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs
but they contain an integrated multivibrator circuit that causes the LED to flash with a typical period of one second. In diffused lens
LEDs this is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between
multiple colors and even fade through a color sequence using RGB color mixing.
Bi-color LEDs are two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the
same two leads antiparallel to each other. Current flow in one direction emits one color, and current in the opposite direction emits
the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode,
so that they can be controlled independently.
Tri-color LEDs are three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled
independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color.
RGB LEDs are Tri-color LEDs with red, green, and blue emitters, in general using a four-wire connection with one common lead
(anode or cathode). These LEDs can have either common positive or common negative leads. Others however, have only two leads
(positive and negative) and have a built in tiny electronic control unit.
Alphanumeric LED displays are available in seven-segment and starburst format. Seven-segment displays handle all numbers and a
limited set of letters. Starburst displays can display all letters. Seven-segment LED displays were in widespread use in the 1970s and
1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity
of numeric and alphanumeric LED displays.
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