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WHAT IS LED Light
emitting diodes, commonly called LEDs, are real unsung heroes in the
electronics world. They do dozens of different jobs and are found in all
kinds of devices. Basically,
LEDs are just tiny light bulbs that fit easily into an electrical circuit.
But unlike ordinary incandescent bulbs, they don't have a filament
that will burn out, and they don't get hot. They are illuminated solely by
the movement of electrons in a semiconductor material, and they last just as long as a
standard transistor. Solid state
LED light sources are known as p-n semiconductor devices. By doping a
substrate material with different materials, a p-n junction is formed within
the semiconductor crystal. The dopant in the n region provides mobile
negative charge carriers (electrons), while the dopant in the p region
provides mobile positive charge carriers (holes). Within a semiconductor
crystal, when a forward voltage is applied to the p-n junction from the p
region to the n region, the charge carriers inject across the junction into a
zone where they recombine and convert their excess energy into light. The
materials used at the junction determine the wavelength of the emitted light.
A clear or diffuse epoxy lens covers the semiconductor chip and seals the
LED. It also provides some optical control to the emitted light. LEDs are low-voltage,
low-current devices and come in a variety of angular distributions. Materials
used in early LEDs to generate specific colors are listed as follows, with
the peak spectral wavelength generated by the LED and the material used
(IESNA, 1993): §
Deep
blue; 470 nm; SiC §
Blue;
490 nm; GaN §
Green;
565 nm; GaP:N §
Yellow;
590 nm; GaAs.15P.85:N §
Amber;
610 nm; GaAs.5P.N §
Orange;
630 nm; GaAs.35P.65:N §
Red;
660 nm; GaAs.6P.4 §
Red
(super bright); 650 nm; AlGaAs §
Deep
red; 690 nm; GaP:Zn More
recently, a promising material for red, amber and yellow LEDs has been
identified. Aluminum indium gallium phosphide (AlInGaP: commonly pronounced
like "allen gap") is used to develop long visible wavelength -
yellow, amber and red - LEDs. This material results in a much lower
degradation in light output over the life of the LED. For shorter wavelength
LEDs - green and blue - a promising material is indium gallium nitride
(InGaN). Using these materials, LEDs that have a luminous efficacy (lumens
per watt) exceeding that of incandescent lamps have been developed. However,
the relatively small lumen package that is produced by a single LED still
means that dozens, if not hundreds, of LEDs must be used together to produce
even a modest amount of light. The
spectral power distribution of an LED is fairly narrow, with half-bandwidths
of around 20 to 50 nm, depending upon the substrate material. This means that
LEDs produce highly saturated, nearly monochromatic light. White LEDs are a
recent development, constructed by adding a phosphor to a blue LED. Some of
the blue light is converted by the phosphor into broader- spectrum yellow
which results in light that has a bluish-white appearance. White light can
also be generated by mixing the light generated by blue, green and red LEDs. What is a Diode? In
the case of LEDs, the conductor material is typically
aluminum-gallium-arsenide (AlGaAs). In pure aluminum-gallium-arsenide, all of
the atoms bond perfectly to their neighbors, leaving no free electrons
(negatively-charged particles) to conduct electric current. In doped material,
additional atoms change the balance, either adding free electrons or creating
holes where electrons can go. Either of these additions make the material
more conductive. A
semiconductor with extra electrons is called N-type material, since it has
extra negatively-charged particles. In N-type material, free electrons move
from a negatively-charged area to a positively charged area. A
semiconductor with extra holes is called P-type material, since it
effectively has extra positively-charged particles. Electrons can jump from
hole to hole, moving from a negatively-charged area to a positively-charged
area. As a result, the holes themselves appear to move from a
positively-charged area to a negatively-charged area. A
diode comprises a section of N-type material bonded to a section of P-type
material, with electrodes on each end. This arrangement conducts electricity
in only one direction. When no voltage is applied to the diode, electrons
from the N-type material fill holes from the P-type material along the
junction between the layers, forming a depletion zone. In a depletion zone,
the semiconductor material is returned to its original insulating state i.e.
all of the holes are filled, so there are no free electrons or empty spaces
for electrons, and charge can't flow.
To
get rid of the depletion zone, you have to get electrons moving from the
N-type area to the P-type area and holes moving in the reverse direction. To
do this, you connect the N-type side of the diode to the negative end of a
circuit and the P-type side to the positive end. The free electrons in the
N-type material are repelled by the negative electrode and drawn to the
positive electrode. The holes in the P-type material move the other way. When
the voltage difference between the electrodes is high enough, the electrons
in the depletion zone are boosted out of their holes and begin moving freely
again. The depletion zone disappears, and charge moves across the diode.
If
you try to run current the other way, with the P-type side connected to the
negative end of the circuit and the N-type side connected to the positive
end, current will not flow. The negative electrons in the N-type material are
attracted to the positive electrode. The positive holes in the P-type
material are attracted to the negative electrode. No current flows across the
junction because the holes and the electrons are each moving in the wrong
direction. The depletion zone increases.
The interaction between electrons
and holes in this setup has an interesting side effect -- it generates light!
How Can a Diode Produce
Light? Photons
are released as a result of moving electrons. In an atom, electrons move in
orbitals around the nucleus. Electrons in different orbitals have different
amounts of energy. Generally speaking, electrons with greater energy move in
orbitals farther away from the nucleus. For
an electron to jump from a lower orbital to a higher orbital, something has
to boost its energy level. Conversely, an electron releases energy when it
drops from a higher orbital to a lower one. This energy is released in the
form of a photon. A greater energy drop releases a higher-energy photon,
which is characterized by a higher frequency. As
we saw free electrons moving across a diode can fall into empty holes from
the P-type layer. This involves a drop from the conduction band to a lower
orbital, so the electrons release energy in the form of photons. This happens
in any diode, but you can only see the photons when the diode is composed of
certain material. The atoms in a standard silicon diode, for example, are
arranged in such a way that the electron drops a relatively short distance.
As a result, the photon's frequency is so low that it is invisible to the human
eye -- it is in the infrared portion of the light spectrum. This isn't necessarily a bad
thing, of course: Infrared LEDs are ideal for remote controls, among other things. Visible
light-emitting diodes (VLEDs), such as the ones that light up numbers in a digital clock, are made of materials
characterized by a wider gap between the conduction band and the lower
orbitals. The size of the gap determines the frequency of the photon -- in
other words, it determines the color of the light. While
all diodes release light, most don't do it very effectively. In an ordinary
diode, the semiconductor material itself ends up absorbing a lot of the light
energy. LEDs are specially constructed to release a large number of photons
outward. Additionally, they are housed in a plastic bulb that concentrates
the light in a particular direction. As you can see in the diagram, most of
the light from the diode bounces off the sides of the bulb, traveling on
through the rounded end.
LED
INTENSITY The unit of
measure commonly used to describe LED intensity is the millicandela (mcd) 1 candela =
1000 millicandela Acknowledgments: |
