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Written by: Tian Li Woon
Written on: October 10th, 2005
Tags: electrical engineering
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About the Author
Tian Li Woon was a junior majoring in Electrical Engineering with emphasis on integrated circuits at University of Southern California, Los Angeles, CA in the fall of 2005.
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Volume VII Issue III > The Little Plastic Bulb
Since being introduced in the 1960's the light emitting diode (LED) has found applications in many areas due to its power efficiency, low power consumption, long life and toughness. Applications include indicator lights on electronic devices, backlights for liquid crystal display screens on cell phones and laptops, traffic lights and even wireless communications and counterfeit detection methods. It has also started to replace conventional lighting sources such as incandescent and fluorescent lighting in some areas. Despite its widespread use, the LED is often overlooked and taken for granted. This article will introduce you to the physics behind the "little plastic bulb," its many advantages and some insight into its wide spectrum of applications.

Introduction

The 20th century saw the prominent rise of semiconductors. The ability to fabricate transistors from semiconductors has brought about the "Electronic Revolution" and the world has changed drastically since then. Astonishingly, it has only been about 60 years since the invention of the first solid state transistor. Almost anything around you from computers to cell phones to a simple digital alarm clock has a semiconductor chip in it. All the hype surrounding the complex semiconductor chips found in computers' central processing units (CPUs) or graphics cards (GPUs) have long obscured another simple semiconductor device that is just as widespread as, if not more, than its more complex counterparts. This simple semiconductor device is the light emitting diode, or better known as the LED. You have probably seen LEDs in many devices but have not given it a second thought. You see it on computers, optical mice, remote controls, microwave ovens, traffic lights, billboard signs and even your cell phone display and keypad are lit up by little LEDs. The list goes on and is set to continue growing indefinitely simply because the LED has yet to realize its full potential and much research is still going on to improve its already amazing energy efficiency and to emit photons across a wider spectrum of frequencies.
[image=649 file="800pxRBGLED.jp​g" placement="right"]Fi​gure 1: Red, green, and blue LED lights.[/image]The first LED made in the early 1960's emitted radiation in the infrared range, invisible to human eyes. Further research and discovery of new materials enabled the production of light at higher frequencies. The first LED that emitted light in the visible range was invented by Nick Holonyak Jr. at General Electric Corp. in Syracuse, N.Y. in the l962. It emitted very low intensity (in the millicandela range) light and could only be used as indicator lights and not for general illumination. Over time, LEDs moved up the color spectrum moving into red-orange, then green. It was only in the 1990's that the blue LED was created by Shuji Nakamura at Nichia Chemical Industries Ltd. and the primary colour triumvirate was finally completed. Developmentof LEDs did not stop there, but went on to the violet and even the ultraviolet frequencies. This in turn led to the production of white LEDs, which is often made using a blue LED coated with yellow phosphors. While the LEDs climbed the frequency ladder, the brightness of the emitted light also improved dramatically (see Fig. 1). This improvement in light intensity, combined with the creation of white LEDs, introduced LEDs into the competition for "the rights to general illumination" with rivals being the incandescent and fluorescent lighting.
In this article we will look at the physics behind the LED, some of its advantages and disadvantages over conventional lighting and some of its many applications in our daily life.

Semiconductors

LEDs work based on a principal that is radically different from that of an incandescent bulb or fluorescent light. The main component of an LED is a semiconductor device called the diode. Hence, to understand the workings of an LED, it is essential to first understand what makes a semiconductor. A semiconductor is typically a Group IV element (e.g. silicon, gallium) or its compound (e.g. gallium arsenide, gallium nitrite). As its name implies, a semiconductor has conductivity between that of a conductor and insulator. The energy band structure of a material is what determines its conductivity. The energy band structure essentially consists of the valence band and the conduction band. The valence band is fully filled with electrons and thus the electrons are not free to move and cannot contribute to conduction. Conversely, the conduction band consists of only of holes. The band gap is the energy range between the valence and conduction band in which electrons and holes are forbidden to exist.
For a material to conduct electricity, an electron has to gain enough energy to be excited from the valence band directly to the conduction band. When an electron is excited into the conduction band, it is free to move in response to a potential difference or electric field. By leaving the valence band, the excited electron also leaves behind a hole in the valence band which an adjacent electron can recombine with. The movement of electrons in the conduction band and recombination of electrons and holes in the valence band results in a net current in the direction of an applied electric field. It follows that the size of the energy band gap is crucial in determining the conductivity of a material. A conductor such as a metal has overlapping valence and conduction bands and thus does not have a band gap. Electrons do not have an energy barrier to overcome for conduction. An insulator on the other hand has a very large band gap and electrons require a very large amount of energy to get excited into the conduction band. Semiconductors have a relatively small band gap (e.g. 1.1eV for silicon compared to 5eV for diamond [1]) that makes it possible for electrons to get excited into the conduction band by application of external thermal or optical energy. To improve its conductivity, impurities such as elements from Group III or Group V are added to it. By adding Group III elements (e.g. boron), a p-type or electron poor semiconductor is created. Conversely, by adding Group V elements (e.g. phosphorous), an n-type or electron rich semiconductor is created.

The Dipole

A diode is formed by bringing together an n-type semiconductor and a p-type semiconductor. A diode is the most basic semiconductor device and it allows current to pass through it in only one direction. When the two pieces of semiconductor come in contact, electrons from the n-type diffuse into the p-type and recombine with the holes there. Similarly, holes in the p-type diffuse into the n-type and recombine with electrons there. This diffusion creates a thin insulating layer at the p-n junction called the depletion zone, void of free-moving electrons or holes.
By applying higher potential to the n-type and a lower potential to the p-type, electrons in the n-type and holes in the p-type will be pulled further away from the p-n junction and result in a wider depletion layer and no current will flow. If potential was applied the other way around, electrons in the n-type are thus attracted to the electrode of the p-type and holes in the p-type are attracted to the electrode of the n-type. A net current flowing from the p-type to the n-type results from this flow of electrons and holes. When electrons pass the p-n junction into the p-type region, they recombine with the holes in the valence band there and result in release of energy equal to the band gap. The physics behind a diode is what enables certain semiconductor materials to emit light when a current flows through them.

The Light Emitting Diode

An LED is no different from an ordinary diode except that the energy released from recombination of electrons and holes is mostly released in the form of photons instead of heat. Diode lasers such as those in CD-ROM or DVD-ROM drives essentially work on the same principle as LEDs. Different materials have different energy band gaps and so release different amounts of energy upon recombination. The frequency of the resultant photon is proportional to the energy released and the color of the light emitted depends on the band structure of the material used. For example, an LED made from gallium nitrite emits blue light and one made from gallium arsenide emits red light. Not all LEDs emit photons with frequencies that lie in the visible spectrum; in fact some emit infrared and ultraviolet photons. By experimenting with different compounds, engineers have been able to come up with LEDs of different colors and shades. Sometimes a heat sink is attached to the chip to improve heat dissipation, improving junction performance and prolonging the lifetime of the device. LEDs can also be soldered directly onto printed circuit boards (PCBs) of electronic devices.

White - Plain but not Simple

The physics behind light emission of the LED allow it only to produce monochromatic light, meaning it emits photons with a single frequency. What we perceive as white light is actually the collective effect of photons of multiple frequencies. One solution is to use blue LEDs and cover it with a layer of phosphors that give out yellow light on absorbing blue light. The combination of the yellow light from the phosphors and the residual blue light creates a bluish white light. The problem with using phosphors is that it decreases the power efficiency, but this method produces the brightest LEDs to date and is thus often used for illumination purposes such as flashlights. Another method used to produce white light is by combining three chips - producing red, green and blue light - into one LED. By controlling the current going into each of the chips, the three colors can be mixed in a proportion that appears as white light to human eyes. Mixing and matching of the three primary colors creates any hue. So far, typical applications for full-color LEDs are in video and other display devices, rather than in general illumination [2].

LEDs vs Light Bulbs

The three main advantages LEDs have over conventional incandescent light bulbs are their power efficiency, long lifespan and their resistance to shock and external elements.
Incandescent bulbs convert only about 5 percent of the power supplied into light, meaning 95 percent of electrical power is wasted as heat. Fluorescent lighting does much better, converting 20 to 30 percent of electricity to light. The best white LEDs currently have power conversion efficiency between that of an incandescent and fluorescent, but while conventional lighting has more or less maxed out its potential, LEDs have great potential for dramatic improvements. Theoretically, LEDs can achieve a close to 100 percent power efficiency.
According to a study commissioned by the U.S. Department of Energy, widespread adoption of next-generation white LEDs for lighting could, by 2025, slash electricity consumption by 10 percent worldwide, cutting $100 billion per year from electric bills and saving $50 billion in averted power-plant construction costs [3]. Furthermore, higher power efficiency also means less heat is produced to raise ambient temperature, reducing the load on air-conditioning systems and leading to further cost savings.
The low heat dissipation due to high power efficiency also leads to the long lifespan of the LED. A typical LED has a lifetime of 60,000 to 100,000 hours [4] which is a minimum of 7 years whereas a light bulb probably only lasts about a few months to a year. Another point worth mentioning is that LEDs do not just blow the way incandescent bulbs do, instead they just dim over time. This property of the LED together with its long lifespan and power efficiency makes it an ideal candidate for traffic lights. Traffic lights lit by incandescent bulbs behind red, amber and green filters are gradually being replaced by LED traffic lights. The fact that LEDs do not burn out like bulbs mean less disruption to traffic and less scrambling around by the contractors to replace burnt out bulbs to restore traffic control. LED traffic lights only have to be replaced before they get too dim, which is probably many years down the road, and replacement can be done at a time when it causes less disruption to traffic.
LEDs are solid state devices meaning they do not have moving parts and they are thus more resistant to shock. Also an LED does not shatter like incandescent bulbs and fluorescent tubes do when you drop them. Protected by an external epoxy cover, an LED is also resistant to water and other elements. This resillience led to the use of LEDs on billboards and jumbo screens. The brightness of LEDs also ensures that these billboards and jumbo screens can be viewed under strong sunlight.
While the use of LEDs are becoming more and more widespread and is slowly replacing the incandescent bulbs in many areas of lighting, there are still some factors that are slowing down this process. First, an LED costs many times more than an incandescent bulb. Although the higher initial cost is made up for by lower operating costs, it still turns many people away.
Although LEDs are still relatively expensive, prices have already plummeted since they were introduced more than 40 years ago and with further improvements in manufacturing processes and device efficiency, the price of LEDs should continue to decrease. While LEDs are highly likely to replace incandescent bulbs, they face much stiffer competition against fluorescent lighting. The low price and incredible efficiency (70-80 lm/W) of fluorescent tubes will be very hard to beat, meaning that displacing the technology could prove impossible. "I can't say that I see a clear path for that ever happening," said Frank Steranka, vice-president of research and development at Lumileds [5].
Another hurdle the LED faces is that since white LEDs are inherently blue LEDs coated with yellow phosphors, the white light produced has a blue tint to it and is thus not as pleasing as the white light produced by incandescent bulbs. This however is a much smaller problem compared to reducing the cost of LEDs and should also be relatively easier to overcome.

LEDs LEDs Everywhere

The LED has been touted as the "light of the next generation" and has in fact begun to move into lighting areas traditionally dominated by incandescent bulbs and fluorescent lamps, but its progress has been hindered by factors such as cost. Although LEDs are still not commonly used for general illumination in places such as homes and offices, they have found their niche in many other applications.
As mentioned earlier in the preceding section, LED traffic lights are getting more and more widespread. According to Strategies Unlimited, a market research firm in Mountain View, California, as of 2002, 39 percent of red lights and 29 percent of green lights nationwide used LEDs [3].
The toughness of LEDs encourages their use in outdoor applications such as billboards and jumbo television screens, bike lights and even tail lights for cars. LEDs are also featured on many signboards replacing neon tubes as the diodes are tougher, last much longer and are easier to replace.
Flashlights made from white LEDs are also gaining popularity and becoming commonplace. Not only are LED flashlights more power efficient, they offer big safety and maintenance benefits. In tests conducted at two U.S. Air Force bases, military firefighters used smoke machines to fill a room. LED flashlights made it possible for firefighters to read the words on a compressor at the opposite end of the room, whereas other flashlights could not penetrate the smoke particles clearly enough [6].
The low power consumption of LEDs make them perfect candidates for indicator lights source of illumination in electronic devices such as computers, personal digital assistants (PDAs), televisions, microwaves and others. They are also the source of light for optical mice and the most common backlighting used in liquid crystal display (LCD) screens.
Recently, a nursing home experimented with LED illumination for nurses making their rounds at night. With conventional lighting, when nurses made their rounds at night, turning on the lights woke the patients up and disturbed their sleep. After switching to LED lights, survey results from the nurses and patients were very positive; nurses could make their rounds with sufficient lighting and patients were not woken up at night. "I was quite pleased by the positive response we received from the staff and residents about the new lighting. These subjective results demonstrate the need and opportunity for innovative lighting options in long-term care facilities," said Dr. Mariana Figueiro, a Lighting Research Center(LRC) light and health specialist who has researched lighting designs and treatments for the elderly and those with Alzheimer's disease [7].
Even LEDs emitting light in outside the visible spectrum are useful. LEDs emitting light in the infrared region are used in television remote controls, night vision goggles, infrared communication channels and infrared imaging and surveillance systems. LEDs in the ultraviolet (UV) spectrum are used for counterfeit detection, chemical detection and medical applications. UV LEDs are also utilized in certain air purifiers.

Conclusion

It is amazing how LEDs have developed since the 1960's and penetrated our lives without us realizing it. We have seen the extremely diverse applications of LEDs from something as basic as television remote controls and indicator lights to extremely advanced technology such as wireless communication. The technology covers such a broad spectrum of applications that we encounter and overlook multiple times per day.
During times like these when energy costs together with energy demands are rising, power efficient LEDs have a bright prospect and developments such as the large scale replacement of conventional traffic lights and the experimenting of LED lighting in nursing homes imply increasing awareness of the benefits of LEDs both in the general population and government agencies. This is good news and as long as continued efforts are made in research and development, LEDs might one day make its way into our homes and Thomas Edison's incandescent bulb will belong only in the museum as an artifact for our children and future generations to admire.

References

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