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How Light Emitting Diodes Work

发布时间:2014-05-16

浏览次数:3066

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 dont have a filament that will burn out, and they dont 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, well 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 (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 -- all of the holes are filled, so there are no free electrons or empty spaces for electrons, and charge cant flow.

At the junction, free electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle of the diode called the depletion zone.

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.

When 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 and the depletion zone disappears.

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. (See How Semiconductors Work for more information on the entire process.)

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, free electrons collect on one end of the diode and holes collect on the other. The depletion zone gets bigger.


The interaction between electrons and holes in this setup has an interesting side effect -- it generates light! In the next section, well 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 and momentum but no mass. 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. (Check out How Light Works for a full explanation.)
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 photons frequency is so low that it is invisible to the human eye -- it is in the infrared portion of the light spectrum. This isnt necessarily a bad thing, of course: Infrared LEDs are ideal for remote controls, among other things.
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 dont 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.

LEDs have several advantages over conventional incandescent lamps. For one thing, they dont 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 (the filament must be warmed). This is completely wasted energy, unless youre using the lamp as a heater, because a huge portion of the available electricity isnt going toward producing visible light. 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 because theyre built around advanced semiconductor material. 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.

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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



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

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