What do you think of when you hear the phrase ‘green technology’? Do solar panels, wind turbines, and electric cars come to mind? What about light-emitting diodes (LEDs)? Unlike many costly green technologies, LEDs are accessible to the majority of Individuals who want to help the environment and save money. Using an LED for 50,000 hours of white-light home lighting (i.e. LED light bulbs for use in lamps, overhead fixtures, etc.) costs only about $86 compared to $352 for incandescent light bulbs []. Despite the fact that some LEDs have been commercially available since 1962, white light-producing LEDs have only been available since 2006 []. The most important of the many advances necessary to bring white light-producing LEDs to market was the invention of the first bright blue LED in 1993 []. For this invention Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura won the 2014 Nobel Prize in Physics on October 7th.

What is an LED and how does it work?

Figure 1 ~ The addition of electricity to a chip of semiconductor produces light through the binding of electrons to atoms.

Light emitting diodes (LEDs) are components of electrical circuits that produce light. LEDs are made from small chips of semiconductors, materials that are able to conduct electricity. While they conduct better than some materials, such as glass, semiconductors conduct less efficiently than metals like aluminum or tin. When electricity runs through LEDs, electrons, the small negatively-charged particles in atoms, within the semiconductor can gain enough energy to move between multiple atoms instead of being bound to a specific one (Figure 1). However, this state is unstable and eventually the electron will bind to one atom again. When this happens, energy is released as photons, the elementary particles of light. The amount of energy released is determined by the material properties of the semiconductor, and in turn determines the color of the light emitted [3, 4].

However, inventing an LED is not as simple as running electricity through a chunk of a specific semiconductor. For one, the atoms of semiconductor must be organized in a repeating arrangement, or crystal lattice. If the arrangement is broken anywhere in the piece of semiconductor, the LED will not function properly. The process of making thin pieces of semiconductor, also known as “chips”, such that a crystal lattice forms is termed “growing crystals”, and for each new semiconductor the process has to be determined from scratch. Once pure chips can be made, the next challenge is to fabricate some chips to have an excess of free, roaming electrons and others to have an excess of atoms that have lost an electron. To create these conditions, atoms of other elements are introduced to the semiconductor in a controlled manner [3, 4]. This process is called “doping”. Again, the appropriate conditions for successfully doping a semiconductor need to be newly determined for each material [3, 4, 5].

Why was the invention of the blue LED so significant?

The invention of the blue LED was important both because it was a technical triumph and because it made a large number of new applications possible. It was a huge technical achievement because the necessary properties for making blue light could not be achieved with a semiconductor similar to those already being used for LEDs. As early as the 1950s, Gallium nitride (GaN) was identified as a semiconductor with the appropriate properties for producing blue light, but it quickly became clear that making chips for use in an LED was challenging []. In fact, by the early 1970s most scientists had stopped work on making LEDs from GaN [].  However, during the early 1970s new methods for growing crystals were developed, and starting in 1974 Isamu Akasaki and later Hiroshi Amano as well as others conducted research to determine how to use these new methods to make crystals of GaN. The problem was not solved until 1986, and the scientists still had to determine how to successfully dope the GaN crystals for practical use []. This was finally achieved in the late 1980s [].

The invention of the first bright blue LED enabled the use of LEDs to make white light. Whereas blue and red light have wavelengths that are within very specific spectrums, those of white light range across a very wide spectrum, making it desirable for practical purposes. While there are multiple methods for producing white light with blue LEDs, the one used most commonly combines a blue LED and a material that is fluorescent []. Fluorescent materials emit light of a specific wavelength after they are illuminated with light of a different wavelength. The fluorescent material used to make a white LED emits various colors of light when it is illuminated with blue light from the blue LED []. White light is produced when the blue light from the LED combines with the other colors of light emitted from the fluorescent material.

 What does the future look like for LEDs and lighting?

By far the most important application of blue LEDs has been for the efficient production of white light. There is a market for efficient white lighting in countries like the United States, where 21% of electricity use in 2012 in the commercial sector was for lighting []. Similarly, in countries where many people are dependent on solar panels for electricity, efficient white lighting from blue LEDs is desirable since it allows them to take full advantage of their limited electricity []. However, white lighting is not the only application for blue LEDs. Blue LEDs are also in the screens of many mobile phones, TVs, and tablets.

Unfortunately, the use of LED light bulbs in residential settings remains rather low, likely because even though the savings from energy use for LEDs are large, the initial cost of an LED light bulb is 25 times that of an incandescent bulb []. However, these disappointing statistics are likely to change; in fact, the U.S. Department of Energy predicts that by 2020 37.6% of residential lighting will come from LEDs and that in 2030 72.3% of it will []. Thanks to continued research and consumer education, we can look forward to a brighter future, lit by LEDs.

Elizabeth “Evi” Van Itallie is a graduate student in the Systems Biology PhD program.

References

[] http://eartheasy.com/live_led_bulbs_comparison.html

[] http://www.osram.com/osram_com/news-and-knowledge/led-home/professional-knowledge/led-basics/led-history/index.jsp

[] Wikipedia page on LEDs: https://en.wikipedia.org/wiki/Light-emitting_diode

[] Wikipedia page on Semiconductors: https://en.wikipedia.org/wiki/Semiconductor

[]

[] Wikipedia page on Phosphor: http://en.wikipedia.org/wiki/Phosphor

[] http://www.eia.gov/tools/faqs/faq.cfm?id=99&t=3

[] U.S. Department of Energy Report on the Energy Savings Potential of Solid-Sate Lighting in General Illumination Applications: http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_energy-savings-report_jan-2012.pdf  

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