Learn About... LEDs!

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Have you ever looked up at a crossing light while walking a few blocks in the city? The first thing you probably notice is whether the sign is showing that it’s safe to cross the road. But have you taken a closer look? If you’re very observant, you’ll notice that the sign contains mosaic-like patterning, pieces of which make up both the "stop" light and the "walk" light. Have you ever wondered how these types of lights work? Like many other wonderful light-up signs and devices that we see every day, the pedestrian crossing lights on street corners shine brightly thanks to the use of light emitting diodes.

Light emitting diodes, commonly called LEDs, have come to be a staple in the world of electronics. They do dozens of different jobs and are found in all kinds of devices. Some of the jobs that LEDs are used for include forming the numbers on digital clocks, transmitting information from remote controls, and letting you know when your appliances are turned on. More recently, LEDs have been utilized to provide the light needed for liquid crystal displays (LCDs) to shine in television displays and traffic lights.

LEDs are very small light bulbs that fit easily into an electrical circuit. Most smaller multi-color LEDs (like the ones we have at Science is Fun, shown in the animation at the top of the page) are actually a set of three bulbs, arranged as pictured at the left. Together, these small lights can be turned on in different combinations to make a variety of colors shine through! LEDs are illuminated by the movement of electrons in a semiconductor material. Unlike ordinary incandescent bulbs, LEDs do not have a filament that can burn out, and don't get hot when left on for a long time. The lifespan of an LED surpasses that of an incandescent bulb by thousands of hours! These properties are some of the reasons why LEDs are already replacing the tubes that light up LCD HDTVs, making dramatically thinner televisions.

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WHAT IS A DIODE?

A diode is the simplest type of a semiconductor device. 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 these impurities is called doping.

In the case of LEDs, the conductor material is typically aluminum-gallium-arsenide (AlGaAs). In pure aluminum-gallium-arsenide, all the atoms bond perfectly to any neighboring atoms, leaving no free electrons (negatively charged particles) to conduct electric current. When material is doped, the new atoms cause changes in the bonding between the semiconductor atoms, either adding free electrons or creating positively charged holes that electrons can fill. Either of these alterations make the material more conductive.

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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. When charges move across a material, we refer to this motion as current.

A diode is made by putting a section of N-type material adjacent to a section of P-type material and attaching 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 can fill holes from the P-type material in the area where the materials meet, forming a depletion zone. In a depletion zone, the semiconductor material is returned to its original insulating state, where all the holes are filled. With no free electrons or empty spaces for electrons, charge can't flow through the material. Diodes are used in many devices besides LEDs, including solar panels.

To remove the depletion zone and allow for current to flow, electrons need to move from the N-type area to the P-type area and holes need to move in the opposite direction. To do this, the N-type side of the diode must be connected 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 then repelled by the negative electrode and drawn to the positive electrode. Similarly, holes in the P-type material move away from the positive electrode towards the negative electrode. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone can be forced from their holes and begin moving freely again. The depletion zone then disappears, and charge can move 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. This happens because the negative electrons in the N-type material are attracted to the positive electrode, and the positive holes in the P-type material are attracted to the negative electrode. This causes the depletion zone at the junction of the materials to increase, making a better insulating material.

The interaction between electrons and holes in an LED 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. Light is made up of many small particles that have energy and momentum but no mass. These particles, called photons, are the units of light, which means they can’t be broken down into smaller pieces.

Photons are released due to the movement of electrons. In an atom, electrons move in orbitals around the nucleus, the positively-charged center of the atom. Electrons in different orbitals have different amounts of energy. Generally speaking, electrons with more energy move in orbitals that are farther away from the nucleus of the atom.

For an electron to move from a lower orbital to a higher orbital, it needs to gain energy. Conversely, an electron releases energy when it moves from a higher orbital to a lower one. This energy is sometimes released in the form of a photon. A greater energy loss from the electron gives a higher-energy photon, which is characterized by a higher frequency, resulting in a bluer wavelength, i.e. “bluer” light.

In a diode, free electrons moving across the material can fall into empty holes from the P-type layer. This involves the electrons moving from the higher energy conduction band to a lower energy valence band, releasing the difference of energy in the form of photons. The distance of this movement of electrons is called a voltage drop. This happens in all diodes, but if the material of the diode allows for it, you can see these photons being released in the form of light with your own eyes! The chart below shows how each LED color can be made with different diode materials.

Table compiled from: www.electronics-tutorials.ws/diode/diode_8.html

Visible light-emitting diodes (VLEDs), such as the ones that light up pedestrian crossing signs, are made of materials characterized by a larger gap between the conduction band and the lower orbitals. The size of this gap controls the energy of photons released, which is linked to the frequency—in other words, it determines the color of the light emitted. While both visible and infra-red (a non-visible type) LEDs are used in everything from remote controls to the displays on some electronics, visible LEDs are growing in popularity and use thanks to their long lifetimes and compact size. With the correct variation of materials, LEDs can be built to shine in infrared, ultraviolet, and all the colors of the visible spectrum in between!

LED ADVANTAGES

While most diodes release light, not all do it very efficiently. In an ordinary diode, the semiconductor material absorbs a lot of the light energy produced by the diode. LEDs are specially constructed to release most of the photons produced from the diode outward. To bolster the light that is being directed outward, LEDs are then encased in a plastic bulb that concentrates the light in a specific direction. As you can see in the diagram to the right, most of the light from the diode bounces off the sides of the case, moving up and concentrating at the rounded end of the casing.

LEDs have multiple advantages over conventional incandescent lights. A large advantage is that LEDs don't have a filament that will burn out, so they have longer lifetimes than incandescent bulbs. Additionally, their small plastic bulb makes them a lot more durable than the glass bulb that a traditional light uses. They also fit more easily into modern electronic circuits due to their compact size.

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But the main advantage is efficiency. In conventional incandescent bulbs, the light-production is accompanied by a lot of heat (the filament must be warmed before light is emitted). This energy is completely wasted, unless you're using the lamp as a heater, since a huge portion of the available electricity goes towards this heating. LEDs generate very little heat, relatively speaking. A much higher percentage of the electrical power is going directly to generating light, which means that more light can be generated for the amount of energy used by the LED. Per-watt, LEDs output more lumens of light than regular incandescent bulbs. This means LEDs have a higher luminous efficacy (how efficiently electricity is converted to visible light) than incandescent lights.

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If LEDs are so wonderful, why aren’t we using them in all devices and applications that require lights? Up until recently, LEDs were too expensive to use for most lighting applications because they're built around advanced semiconductor materials. However, the price of LEDs has been on the decline since 2000 due to advances in the processes used to make the materials, meaning LEDs are becoming 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 due to their luminous efficiency and long lifetime can make them a better buy.

LED LIGHT BULBS vs. INCANDESCENT AND FLUORESCENT
For decades, 100-watt incandescent light bulbs have lit up our houses, and 60-watt incandescent bulbs have been used in reading lamps. But incandescent lights have some problems: they’re inefficient, wasting a lot of energy in the form of heat, and have shorter lifespans than fluorescent lights. Recently, compact fluorescent (CFL) bulbs have become popular alternatives to incandescent bulbs due to their lower power consumption. Where incandescent lights last an average of around 750 hours, CFLs can last 10,000 hours. Unfortunately, CFLs contain toxic mercury that makes them potentially hazardous and difficult to dispose of once they burn out.
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LEDs offer the advantages of CFLs (lower power consumption and longer lifetime) without the downside of containing mercury. For example, a 60-watt incandescent light bulb draws about $13 worth of electricity per year and provides about 800 lumens of light; an equivalently bright CFL uses less than 15 watts and costs only about $3 of electricity per year. LED bulbs are even better, drawing less than 12 watts of power to output 800 lumens, costing about $2.50 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 your home!
Photo credit: DHgate.com	Why doesn’t everyone have LED bulbs in their homes? Although advancements have been made in the process of building LEDs, they still have a high up-front cost compared to other bulbs. As of 2018, incandescent bulbs are about half as expensive as comparable LED lights. However, because of their longer life spans and dramatically lower power usage, LED bulbs make up for the higher pricing over time. And since LEDs can be built to light up in a variety of colors, they don't need color filters like other bulbs. 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 getting used to at first, but LEDs offer numerous advantages over incandescent bulbs. LEDs are easy to dim and are perfect for encouraging plant growth, since they efficiently put out lots of light without producing heat that could potentially be damaging to plant life.

LED TVs AND THE FUTURE OF LEDs

In the 2000s, LCD TVs took over the high definition market, making bounds and leaps over standard televisions. Now LEDs are poised to make a similar leap. While LCDs are much 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 of these problems.

Have you ever seen a gigantic flat screen 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 made up of liquid crystals. Technically, they're LED-backlit LCD televisions. Instead of the fluorescent tubes, LEDs provide the light behind the screen, illuminating the LC pixels to create an image. Due to the small size and low power consumption of LEDs, LED-backlit TVs are far thinner and more energy efficient than purely LCD televisions. They can also provide a wider variety of colors, producing a more vivid picture.

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, which allows scientists to create bendable lights and displays. Someday, OLEDs will pave the way for the next generation of TVs and smart phones. Even now, some electronics companies have begun to use OLEDs to make components of smart phones and television displays. Can you imagine rolling your TV up like a poster and carrying it with you on the go?

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REFERENCES

Schubert, E. Fred (2003). "Light-Emitting Diodes”. Cambridge University Press. ISBN 0-8194-3956-8.
Schubert, E. Fred (2006) Light-emitting diodes, Cambridge University Press, ISBN 0-521-86538-7 p. 97, "Epoxy Encapsulants", "The light extraction efficiency can be enhanced by using dome-shaped encapsulants with a large refractive index."
"Cleaning Up a Broken CFL". www.epa.gov. United States Environmental Protection Agency.
Bausch, Jeffrey (December 2011). "The Long History of Light Emitting Diodes". Hearst Business Communications.

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