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英文原文
How
Light Emitting Diodes Work
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.
Among other
things, they form the numbers on digital clocks,
transmit information from
remote
controls, light up watches and tell you when your
appliances are turned on.
Collected
together, they can form images on a jumbo
television screen or illuminate a
traffic light. 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
especially hot. They are illuminated solely by the
movement of
electrons in a
semiconductor material, and they last just as long
as a standard transistor.
In this
article, we'll examine the simple principles
behind these ubiquitous blinkers,
illuminating some cool principles of
electricity and light in the process.
What is a Diode? A diode is the
simplest sort of semiconductor device. Broadly
speaking, a semiconductor is a material
with a varying ability to conduct electrical
current. Most semiconductors are made
of a poor conductor that has had impurities
(atoms of another material added to it.
The process of adding impurities is called doping.
In the case of LEDs, the conductor
material is typically aluminum-gallium-arsenide.
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 -- 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
a result, the
depletion zone the
negative end of the circuit is hooked up to the
N-
type layer and the positive end is
hooked up to P-type layer, electrons and holes
start
moving. If the P-type side is
connected to the negative end of the circuit and
the N-type
side is connected to the
positive end, current will not flow. No current
flows across the
junction because the
holes and the electrons are each moving in the
wrong direction.
When the positive end
of the circuit is hooked up to the N-type layer
and the negative end
is hooked up to
the P-type layer, the depletion zone gets bigger.
The interaction between
electrons and
holes has an interesting effect -- it generates
light! In the next section, we'll
find
out exactly why this is.
How Can a
Diode Produce Light? Light is a form of energy
that can be released by
an atom. It is
made up of many small particle-like packets that
have energy. These
particles, called photons, are the most
basic units of 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 in the
last section, 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 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.
LEDs have several
advantages over conventional incandescent lamps.
For one thing,
they don't have a
filament that will burn out, so they last much
longer. Additionally, their
small
plastic bulb makes them a lot more durable. They
also fit more easily into modern
electronic circuits. But
the main advantage is efficiency. In conventional
incandescent
bulbs, the light-
production process involves generating a lot of
heat. This is completely
wasted energy,
unless you're using the lamp as a heater. LEDs
generate very little heat,
relatively
speaking. A much higher percentage of the
electrical power is going directly to
generating light, which cuts down on
the electricity demands considerably. Up until
recently, LEDs were too expensive to
use for most lighting applications. The price of
semiconductor devices has plummeted
over the past decade, however, making LEDs a
more cost-effective lighting option for
a wide range of situations. While they may be
more expensive than incandescent lights
up front, their lower cost in the long run can
make them a better buy. In the future,
they will play an even bigger role in the world of
technology.
TRANSIENT
VOLTAGE SUPPRESSOR(TVS Diode PRESENTATION
? High protection on sensitive mobile
electronic devices
? Follow
strictly to the IEC 61000
-4-2 ESD test
standard
? Using the behavior of diode
P/N junction to achieve ESD protection
What are Transient Voltages?
? These are faults which caus e the
voltage to go outside normal limits for
a period
of time. Transient voltages
are characterized by three things:
VeryHigh Voltage, Occur For A Very
Short Period of time (in nanoseconds and
High Occurrence. Many transients cause
damage to micro-semiconductor chipsets by
degra ding their performance. This
damage is cumulative and eventually reaches apoint
where sudden and complete failure of
the component results. Moreover, some
transients are capable of causing immediate
equipment failures. Equipment failures
caused by transients are hard to detect and are