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 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. The lifespan of
an LED surpasses the short life of an incandescent bulb by thousands of hours.
Tiny LEDs are already replacing the tubes that light up
LCD HDTVs to make
dramatically thinner televisions.
What is a Diode?
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.
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.
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.
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
alterations 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 consists of 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 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?
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
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 LEDs are used in everything from remote controls to the digital
displays on electronics, visible LEDs are growing in popularity and use thanks
to their long lifetimes and miniature size. Depending on the materials used in
LEDs, they can be built to shine in infrared, ultraviolet, and all the colors
of the visible spectrum in between.
LED Advantages
The interior of a LED is actually quite
simple, which is one of the reasons this technology is so versatile.
While all diodes release light, most don't do it very effectively. In an
ordinary diode, the
semiconductormaterial 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 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 (the filament must
be warmed). This is completely wasted energy, unless you're using the lamp as a
heater, because a huge portion of the available electricity isn't 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.
Per-watt, LEDs output more lumens of light than regular incandescent bulbs.
Light emitting diodes have a higher
luminous efficacy (how
efficiently electricity is converted to visible light) than incandescents --
for example, Sewell's EvoLux LED bulb produces 76.9 lumens per watt compared to
an incandescent bulb's 17 lm/W]. And they last: LEDs can have lifetimes of
50,000
hours or more
Up until recently, LEDs were too expensive to use for most lighting
applications because they're built around advanced semiconductor material. The
price of semiconductor devices has plummeted since the year 2000, 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. Several companies
have begun selling LED light bulbs designed to compete with incandescent and
compact fluorescents that promise to deliver long lives of bright light and
amazing energy efficiency.
LED Light Bulbs vs. Incandescents and Fluorescents
LED lighting fixtures like this one may
eventually replace incandescent bulbs entirely, but their higher initial cost
is still a barrier for most households.
For decades, 100-watt incandescent
light bulbs have lit up
hallways and bedrooms; 60-watt incandescents have shone softer light from
reading lamps and closets. But incandescent lights have some problems. They're
inefficient, wasting lots of energy as heat, and have shorter lifespans than
fluorescent lamps. Recently,
compact
fluorescent (CFL) bulbs have become popular alternatives to incandescent
bulbs thanks to lower power consumption. Where incandescent lights last an
average of around 1,000 hours, CFLs can last 8,000 hours. Unfortunately,
CFLs
contain toxic mercury that makes them potentially hazardous and a pain
to dispose of
Enter the LED light bulb. LEDs offer the advantages of CFLs -- lower power
consumption and longer lifetimes -- without the downside of toxic mercury. For
example, a 60-watt incandescent light bulb draws more than $300 worth of
electricity per year and provides about 800 lumens of light; an equivalent
compact fluorescent uses less than 15 watts and costs only about $75 of
electricity per year. LED bulbs are even better, drawing less than 8 watts of
power, costing about $30 per year, and lasting 50,000 hours or longer . There
are only 8,760 hours in a whole year -- imagine how long an LED bulb would last
in the average home!
That makes LEDs sound pretty great -- and they are -- but there's a reason
incandescent and compact fluorescent bulbs are still around. LED bulbs present
a high up-front cost compared to other bulbs. Incandescent bulbs sell in
packages for only a few bucks. As of mid-2011, Sewell's EvoLux LED bulbs sold
for more than $70 apiece! However, because of their longer life spans and
dramatically lower power usage, LED bulbs make up for the high barrier of
entry. Since there's no toxic mercury in an LED, they're also easier and
cheaper to dispose of than CFLs. And since LEDs can be built to light up in a
variety of colors, they don't need filters like other bulbs.
LED lighting obviously isn't perfect yet. In addition to the high cost
barrier, LEDs are vulnerable to high temperatures. If LED circuitry gets too
hot, more current will pass through the
junction mentioned
earlier in this article. When too much current courses through the junction, it
will cause irreversible burn-out often called
LED meltdown
LEDs and fluorescents put off "cool" or bluish light compared to
the "warm," yellowish light typical of incandescent bulbs. The
difference in lighting types can take some adjustment, but LEDs obviously offer
numerous advantages over incandescents. LEDs are even easy to dim and are
perfect for encouraging plant growth, since they efficiently put off tons of
light without producing heat that could potentially be damaging to plant life.
LED TVs and the Future of Light Emitting Diodes
LEDs have come a long way since the early days of lighting up digital clock
faces. In the 2000s,
LCD
TVs took over the high definition market and represented a huge step over old
standard definition CRT televisions. LCD displays were even a major step above
HD rear-projection sets that weighed well over 100 pounds ( 45.4 kilos). Now
LEDs are poised to make a similar jump. While LCDs are far thinner and lighter
than massive rear-projection sets, they still use cold cathode fluorescent
tubes to project a white light onto the pixels that make up the screen. Those
add weight and thickness to the television set. LEDs solve both problems.
Have you ever seen a a gigantic flatscreen TV barely an inch thick? If you
have, you've seen an LED television. Here's where the acronyms get a bit
confusing: those LED TVs are still LCD TVs, because the screens themselves are
comprised of liquid crystals. Technically, they're
LED-backlit
LCD TVs. Instead of fluorescent tubes, LEDs shine light from behind the screen,
illuminating the pixels to create an image. Due to the small size and low power
consumption of LEDs, LED-backlit TVs are far thinner than regular LCD sets and
are also more energy efficient. They can also provide a wider color gamut,
producing more vivid pictures.
Because LED TVs are still in their infancy, several different types of
LED-blacklit sets are on the market -- and not all LED TVs are created equal.
Many sets use white
LED edge lighting to shine light across
the display.
The only real advantage afforded by these sets is thinness.
RGB
LED-backlit sets, on the other hand, provide improved color. Some
configurations even allow for a technique called
local dimming,
where LEDs in different parts of the display can be brightened or dimmed
independently to create a more dynamic picture
And that highlights one more great advantage of LEDs over compact
fluorescent lights:
Because the LEDs can actually be
instantly toggled on and off, they produce awesome black levels in dark scenes.
Since the white fluorescent lamps have to remain on during TV use, some light
tends to bleed through and lighten the picture in dark scenes.
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