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Definition:
LED technology Physical
function
An LED is a special type of
semiconductor diode. Like a normal diode, it consists of a chip of
semiconducting material impregnated, or doped, with impurities to
create a structure called a p-n junction. As in other diodes,
current flows easily from the p-side, or anode to the n-side, or
cathode, but not in the reverse direction. Charge-carriers -
electrons and electron holes 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 as it does so.
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 a 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.
Conventional LEDs
are made from a variety of inorganic semiconductor materials,
producing the following colors:
aluminum gallium arsenide
(AlGaAs) - red and infrared
aluminum gallium phosphide (AlGaP) -
green
aluminum gallium indium phosphide (AlGaInP) -
high-brightness orange-red, orange, yellow, and green
gallium
arsenide phosphide (GaAsP) - red, orange-red, orange, and yellow
gallium phosphide (GaP) - red, yellow and green
gallium
nitride (GaN) - green, pure green (or emerald green), and blue
indium gallium nitride (InGaN) - near ultraviolet, bluish-green
and blue
silicon carbide (SiC) as substrate - blue
silicon
(Si) as substrate - blue (under development)
sapphire (Al2O3) as
substrate - blue
zinc selenide (ZnSe) - blue
diamond (C) -
ultraviolet
aluminum nitride (AlN), aluminum gallium nitride
(AlGaN) - near to far ultraviolet
 
Features and Benefits of
LED
The inherent features of LED decide
it to be the best alternative to conventional light source, and
provide a wide range of application.
Small size:
A LED
is essentially a tiny chip ,about 4.5mm, encapsulated in an epoxy
resin enclosure, so it
can be extremely small and
light.
Low Power Consumption:
Generally a LED is
designed to operated at 2-3.6v,0.02-0.03A current, that mean, it
needs no
More than 0.1w to operate.
Long
lifetime:
Operating at a desired rated voltage, current and
environment, LEDS enjoys a long life of up to
100,000hours.
High luminous efficiency and low
heat emitting :
Due to the special materials that used to
make LEDs, during electrons transition, LEDs mainly emit
electromagnetic energy in the visible parts of spectrum, unlike the
incandescent filaments heated, which emit a lot of electromagnetic
energy in the infrared pat of the spectrum which can’t be seen but
is felt as heat. That is to say .LEDs are of high Luminous
efficiency and low heat emitting .The best LED in the market now
emits 150LM/w, which is almost tenfold as efficient as an equivalent
tungsten-filament light bulb.
Environmental
protection:
LEDs are made from non-toxic material, unlike the
fluorescent lights with mercury that will pose a pollution danger.
LEDs can be recycled too.
Unbreakable:
The light-emitting
device of a LED is completely embedded in an epoxy resin enclosure
which is much more sturdy than the bulb and fluorescent tube;there
are no loose and moving parts within the solid epoxy enclosure. This
makes LEDs virtually indestructible.
The application of
LED
LED’s proprietary characteristics above mentioned
determines their ideal suitability in a wide range of
application.
Decorative lighting
Due to the rich variety
in colors, small size, durability, energy savings, LEDs are perfect
light source for decorative application. Attached on a PCB, flexible
cable, or other desired materials; LEDs can be used for light source
of channel letter, signs, border light, light tube and so
on.
Illumination
With the advance of the white light LED
technology, an array of white light LEDs assembled in a certain way
are now bright enough to be used for domestic illumination, such as
light, table lamp, and garden lamp etc.
Display
A typical
application of LEDs is for message display or large-scale display
screen, which are popularity used in sport fields, airports,
business centers and so on. The largest LED display in the world is
36mm high, at Times Square,Manhattan.
LCD
backlight
Because of LED’s small size and low power consumption,
they are perfect light source for LCD
backlight.
Color
Monochromaticity is the inherent
property of LEDs. They emit a pure color in a narrow frequency
range. The color emitted from a LED is identified by peak
wavelength(Ipk) and measured in nanometers(nm).Peak wavelength is a
function of the LED chip material, the composition of the different
materials determines the wavelength and therefore the color of light
emitted.
As we know, white color is a combination of all
colors. But eye does not require a mixture of all the colors of the
spectrum to perceive white light.A mixture of three primary
colors(red, green, blue) can be well enough.
Based on this
principle, many solutions have been proposed to fabricate white
color LEDs. The typical one is to bury a GaN type blue LED having an
InGaN active layer into a YAG pond emitting yellow
fluorescene.
Intensity an View Angle
LED light output
varies with the type of chip, encapsulation, and other variables
.But there is no consistent international criterion for LED
brightness measurement. Generally, the amount of light emitted from
a LED is quantified by a single point, on-axis luminous intensity
value(IV), and is specified in millicandela(mcd).
A LED with
higher luminous intensity value does not mean that it has higher
total light output. To measure the total light output, the viewing
angle must be taken into account too. LED viewing angle is also a
function of the LED chip type an the epoxy lens that distributes the
light, LEDs with different chips and epoxy lens will have different
viewing angle. If two LEDs have the same luminious intensity value,
the lamp with the larger viewing angle will have the higher total
light output.
Parameter of typical LEDs: |
