1:00 AMHow Light Emitting Diodes Work
Introduction to 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 numbers on digital clocks, transmit information fromremote 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.
In this article, we'll examine the technology 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 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. (See How Semiconductors Work for more information on the entire process.)
The interaction between electrons and holes in this setup has an interesting side 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 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 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.
In the next section we'll look at the advantages of LEDs.
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 [source: Sewell]. And they last: LEDs can have lifetimes of 50,000 hours or more [source: Design Recycle Inc].
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.
Over the next couple of pages we'll take a look at the future of LEDs in our homes. One day they may be plugged into our light bulb sockets, lighting up our digital readouts and illuminating the millions of pixels that make up our high-definition televisions.
LED Light Bulbs vs. Incandescents and Fluorescents
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 [source: Design Recyle Inc].
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 [source: Design Recyle Inc]. 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 [source: Fun-LED-Light].
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 [source: LED Tele]. 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.
In the future, some of the most incredible uses of LEDs will actually come from organic light emitting diodes, or OLEDs. The organic materials used to create these semiconductors are flexible, allowing scientists to create bendable lights and displays. Someday, OLEDs will pave the way for the next generation of TVs and smart phones -- can you imagine rolling your TV up like a poster and carrying it with you anywhere?
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