LED technology
Physical function
Like a normal diode, an LED consists of a chip of semiconducting material
impregnated, or doped, with impurities to create a p-n junction. As in other
diodes, current flows easily from the p-side, or cathode, to the n-side, or
anode, but not in the reverse direction. Charge-carriers ¡ª electrons and
electron holes ¡ª flow into the junction from electrodes with different voltages.
When an electron meets a hole, it falls into a lower energy level, and releases
energy in the form of a photon.
The wavelength of the light emitted, and therefore its colour, depends on the
band gap energy of the materials forming the p-n junction. In silicon or
germanium diodes, the electrons and holes recombine by a non-radiative
transition which produces no optical emission, because these are indirect band
gap materials. The materials used for an LED have a direct band gap with
energies corresponding to near-infrared, visible or near-ultraviolet light.
LED development began with infrared and red devices made with gallium arsenide.
Advances in materials science have made possible the production of devices with
ever-shorter wavelengths, producing light in a variety of colors.
LEDs are usually built on an n-type substrate, with electrode attached to the
p-type layer deposited on its surface. P-type substrates, while less common,
occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire
substrate. Substrates that are transparent to the emitted wavelength, and backed
by a reflective layer, increase the LED efficiency. The refractive index of the
package material should match the index of the semiconductor, otherwise the
produced light gets partially reflected back into the semiconductor, where it
gets absorbed and turns into additional heat lowering the efficiency. In 2007
experiments tried to avoid multiple internal reflection by roughening the chip.
Again at the surface from the package to a low refractive index medium like a
glass fiber or air total internal reflection is avoided by using a sphere shaped
package, with the diode in the center, so that the light rays hit the surface
quite perpendicular, and anti-reflection coating may be added. The package may
be cheap plastic, which may be colored, but this is only for cosmetic reasons or
to improve the contrast ratio; the color of the packaging does not substantially
affect the color of the light emitted.
Ultraviolet and blue LEDs
An ultraviolet GaN LED.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, though
white LEDs today rarely use this principle.
The first blue LEDs were made in 1971 by Jacques Pankove (inventor of the
gallium nitride LED) at RCA Laboratories.However, these devices were too feeble
to be of much practical use. In the late 1980s, key breakthroughs in GaN
epitaxial growth and p-type doping by Akasaki and Amano (Nagoya, Japan) ushered
in the modern era of GaN-based optoelectronic devices. Building upon this
foundation, in 1993 high brightness blue LEDs were demonstrated through the work
of Shuji Nakamura at Nichia Corporation.
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 InN-GaN fraction
in the InGaN quantum wells, the light emission can be varied from violet to
amber. AlGaN aluminum gallium nitride of varying AlN 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 the InGaN-GaN blue/green devices. If the active quantum well layers are GaN,
as opposed to alloyed InGaN or AlGaN, the device will emit near-ultraviolet
light with wavelengths around 350-370 nm. Green LEDs manufactured from the
InGaN-GaN system are far more efficient and brighter than green LEDs produced
with non-nitride material systems.
With aluminium containing nitrides, most often AlGaN and AlGaInN, even shorter
wavelengths are achievable. Ultraviolet LEDs are becoming available on the
market, in a range of wavelengths. Near-UV emitters at wavelengths around
375-395 nm are already cheap, common to encounter eg. 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 peak at about 260 nm, UV LEDs emitting at 250-270 nm are
perspective for disinfecting devices.
White LEDs
Blue LEDs can be added to existing red and green LEDs to produce the
impression of white light, though white LEDs today rarely use this principle.
Most "white" LEDs in production today are based on an InGaN-GaN structure, and
emit blue light of wavelengths between 450 nm ¨C 470 nm blue GaN. These GaN-based,
InGaN-active-layer LEDs are covered by a yellowish phosphor coating usually made
of cerium-doped yttrium aluminum garnet (Ce3+:YAG) crystals which have been
powdered and bound in a type of viscous adhesive. The LED chip emits blue light,
part of which is efficiently converted to a broad spectrum centered at about 580
nm (yellow) by the Ce3+:YAG. The single crystal form of Ce3+:YAG is actually
considered a scintillator rather than a phosphor. Since yellow light stimulates
the red and green receptors of the eye, the resulting mix of blue and yellow
light gives the appearance of white, the resulting shade often called "lunar
white". This approach was developed by Nichia and was used by them from 1996 for
manufacturing of white LEDs.
The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium
with other rare earth elements such as terbium and gadolinium and can even be
further adjusted by substituting some or all of the aluminum in the YAG with
gallium. Due to the spectral characteristics of the diode, the red and green
colors of objects in its blue yellow light are not as vivid as in broad-spectrum
light. Manufacturing variations and varying thicknesses in the phosphor make the
LEDs produce light with different color temperatures, from warm yellowish to
cold bluish; the LEDs have to be sorted during manufacture by their actual
characteristics. Philips Lumileds patented conformal coating process addresses
the issue of varying phosphor thickness, giving the white LEDs a more consistent
spectrum of white light.
Spectrum of a "white" LED clearly showing blue light which is directly emitted
by the GaN-based LED (peak at about 465 nanometers) and the more broadband
stokes shifted light emitted by the Ce3+:YAG phosphor which extends from around
500 to 700 nanometers.White LEDs can also be made by coating near ultraviolet (NUV)
emitting LEDs with a mixture of high efficiency europium-based red and blue
emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu,
Al). This is a method analogous to the way fluorescent lamps work. However the
ultraviolet light causes photodegradation to the epoxy resin and many other
materials used in LED packaging, causing manufacturing challenges and shorter
lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor,
as the Stokes shift is larger and more energy is therefore 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 approaches offer comparable brightness.
The newest method used to produce white light LEDs uses no phosphors at all and
is based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which
simultaneously emits blue light from its active region and yellow light from the
substrate.
A new technique developed by Michael Bowers, a graduate student at Vanderbilt
University in Nashville, involves coating a blue LED with quantum dots that glow
white in response to the blue light from the LED. This technique produces a
warm, yellowish-white light similar to that produced by incandescent bulbs.
Organic light-emitting diodes (OLEDs)
If the emitting layer material of an LED is an organic compound, it is known as
an Organic Light Emitting Diode (OLED). To function as a semiconductor, the
organic emitting material must have conjugated pi bonds. The emitting material
can be a small organic molecule in a crystalline phase, or a polymer. Polymer
materials can be flexible; such LEDs are known as PLEDs or FLEDs.
Compared with regular LEDs, OLEDs are lighter, and polymer LEDs can have the
added benefit of being flexible. Some possible future applications of OLEDs
could be:
OLEDs have been used to produce visual displays for portable electronic
devices such as cellphones, digital cameras, and MP3 players. Larger displays
have been demonstrated, but their life expectancy is still far too short (<1,000
hours) to be practical.
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Operational parameters and efficiency
Most typical LEDs are designed to operate with no more than 30-60 milliwatts
of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable
of continuous use at one watt. These LEDs used much larger semiconductor die
sizes to handle the large power input. Also, the semiconductor dies were mounted
to metal slugs to allow for heat removal from the LED die.
One of the key advantages of LED-based led lighting is its high
efficiency, as measured by its light output per unit power input. White LEDs
quickly matched and overtook the efficiency of standard incandescent lighting
systems. In 2002, Lumileds made 5-watt LEDs available with efficacy of 18¨C22
lumens per watt. For comparison, a conventional 60-100 watt incandescent
lightbulb produces around 15 lumens/watt. However, note that standard
fluorescent lights produce up to 100 lumens/watt. (The Luminous efficiency page
discusses this in more detail.)
In September 2003 a new type of blue LED was demonstrated by the company Cree,
Inc. to give 24 mW at 20 mA. This produced a commercially packaged white light
giving 65 lumens per watt at 20 mA, becoming the brightest white LED
commercially available at the time, and over four times more efficient than
standard incandescents. In 2006 they demonstrated a prototype with a record
white LED efficacy of 131 lm/W at 20 mA. Also Seoul Semiconductor has plans for
135 lm/W by 2007 and 145 lm/W by 2008, which would be approaching an order of
magnitude improvement over standard incandescents and better even than standard
fluorescents.[12]. Nichia Corp. has developed a white light LED with efficacy of
150 lm/W at a forward current of 20 mA [13].
It should be noted that high-power (¡Ý 1 Watt) LEDs are necessary for practical
general led lighting applications. Typical operating currents
for these devices begin at 350 mA. The highest efficiency high-power white LED
is claimed by Philips Lumileds Lighting Co. with a luminous efficacy of 115 lm/W
(350 mA).
Today, OLEDs operate at substantially lower efficiency than inorganic
(crystalline) LEDs. The best efficacy of an OLED so far is about 10% of the
theoretical maximum of 683, so about 68 lm/W. These claim to be much cheaper to
fabricate than inorganic LEDs, and large arrays of them can be deposited on a
screen using simple printing methods to create a color graphic display.
Failure modes
The most common way for LEDs (and diode lasers) to fail is the gradual
lowering of light output and loss of efficiency. However, sudden failures can
occur as well.
The mechanism of degradation of the active region, where the radiative
recombination occurs, involves nucleation and growth of dislocations; this
requires a presence of an existing defect in the crystal and is accelerated by
heat, high current density, and emitted light. Gallium arsenide and aluminum
gallium arsenide are more susceptible to this mechanism than gallium arsenide
phosphide and indium phosphide. Due to different properties of the active
regions, gallium nitride and indium gallium nitride are virtually insensitive to
this kind of defect; however, high current density can cause electromigration of
atoms out of the active regions, leading to emergence of dislocations and point
defects, acting as nonradiative recombination centers and producing heat instead
of light. Ionizing radiation can lead to the creation of such defects as well,
which leads to issues with radiation hardening of circuits containing LEDs (eg.
in optoisolators). Early red LEDs were notable for their short lifetime.
White LEDs often use one or more phosphors. The phosphors tend to degrade with
heat and age, losing efficiency and causing changes in the produced light color.
Pink LEDs often use an organic phosphor formulation which may degrade after just
a few hours of operation causing a major shift in output color.
High electrical currents at elevated temperatures can cause diffusion of metal
atoms from the electrodes into the active region. Some materials, notably indium
tin oxide and silver, are subject to electromigration. In some cases, especially
with GaN/InGaN diodes, a barrier metal layer is used to hinder the
electromigration effects. Mechanical stresses, high currents, and corrosive
environment can lead to formation of whiskers, causing short circuits.
High-power LEDs are susceptible to current crowding, nonhomogenous distribution
of the current density over the junction. This may lead to creation of localized
hot spots, which poses risk of thermal runaway. Nonhomogenities in the
substrate, causing localized loss of thermal conductivity, aggravate the
situation; most common ones are voids caused by incomplete soldering, or by
electromigration effects and Kirkendall voiding. Thermal runaway is a common
cause of LED failures.
Laser diodes may be subject to catastrophic optical damage, when the light
output exceeds a critical level and causes melting of the facet.
Some materials of the plastic package tend to yellow when subjected to heat,
causing partial absorption (and therefore loss of efficiency) of the affected
wavelengths.
Sudden failures are most often caused by thermal stresses. When the epoxy resin
used in packaging reaches its glass transition temperature, it starts rapidly
expanding, causing mechanical stresses on the semiconductor and the bonded
contact, weakening it or even tearing it off. Conversely, very low temperatures
can cause cracking of the packaging.
Electrostatic discharge (ESD) may cause immediate failure of the semiconductor
junction, a permanent shift of its parameters, or latent damage causing
increased rate of degradation. LEDs and lasers grown on sapphire substrate are
more susceptible to ESD damage.
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