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On 7th October 2014, Isamu Akasaki, Hiroshi Amano and Shuji Nakamura were awarded the Nobel Prize in Physics for the invention of efficient blue light-emitting diodes (LEDs). Red and green LEDs were created in a number of laboratories during the 1950s and 1960s but it took another three decades to finally produce efficient blue LEDs – why were they so hard to make?
How do LEDs Work?
Light-emitting diodes are electronic devices which are illuminated by the movement of electrons in a semiconductor material. LEDs are able to emit light with a range of wavelengths from the infrared to the ultraviolet.
A semiconductor is a material with an electrical conductivity which is somewhere between a conductor such as copper, and an insulator such as rubber. They are usually made from a poor conductor which is then ‘doped’ by adding atoms of another material to it.
LEDs are typically made from aluminum-gallium-arsenide (AlGaAs) which in its pure form does not contain any free electrons to conduct electrical current. As a result, AlGaAs is doped with either free electrons or ‘electron holes’ in order to change the materials balance and make it more conductive.
Semiconductors can be classified into two types of material; N-type and P-type:
- N-type semiconductors contain extra negatively charged electrons and as a result the free electrons flow from negatively charged areas to positively charged areas.
- P-type semiconductors have extra holes which allows free electrons to jump between the holes and moving from negatively charged areas to positively charged areas as a result.
A diode consists of a section of an N-type semiconductor attached to a section of a P-type semiconductor (known as a p-n junction) with two electrodes placed at either end of this arrangement. When current flows across a diode, the negatively charged electrons move in one direction in the material and the positively charged holes move in the opposite direction. As the holes exist in lower energy states, a free electron will lose energy when it falls to a hole and emit this energy in the form of a photon of light.
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The size of the fall in energy determines the energy the photon has when it is emitted, which in turns determines the colour of the light the diode emits. An emitted photon with a large amount of energy will have a shorter wavelength than light emitted with a lower amount of energy.
The History of Light-Emitting Diodes
The first diode which was able to emit electrically produced light was created in 1907 by H.J. Round whilst he was experimenting with a cat's-whisker detector. Round applied a potential difference across a silicon carbide (SiC) crystal. He found that the colour of light emitted varied depending on the voltage which was applied across the crystal.
During the 1920s and 1930s, the phenomena of electroluminescence was studied by the Soviet physicist who published several journal articles on the subject.
In 1947, the electronic transistor was invented at Bell Telephone Laboratories thanks in part to the advancement in understanding of semiconductors and p-n junctions.
Electroluminescence was widely studied in detail during the 1950s by scientists, with J.R. Haynes demonstrating that the emission of light from germanium and silicon diodes was due to the interaction of holes and electrons in a p-n junction in 1956 at Bell Telephone Laboratories.
Infrared LEDs were created in 1962 using p-n junctions made from GaAs. This semiconductor material has a direct bandgap of 1.4 eV, which directly corresponds to the wavelength of infrared light. By the end of the 1960s, red and green LEDs were being manufactured in different countries using p-n junctions made from GaP. The development of a blue LED however proved far more difficult to scientists.
Blue Light at the End of the Tunnel
The first attempts at the emission of blue light from a diode used ZnSe and SiC, which are semiconductors with high indirect bandgaps, but did not produce efficient light emission. The material which enabled the development of blue LEDs was gallium nitride (GaN). GanN is a semiconductor with a Wurtzite crystal structure and a direct bandgap of 3.4 eV, which directly corresponds to the wavelength of light in the ultraviolet range.
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At the end of the 1950s, researchers at Philips Research Laboratories had considered using GaN to create blue LEDs but instead chose to concentrate on devices made from GaP instead due to difficulties in growing GaN crystals. These crystals were more effectively produced in the late 1960s by using the Hydride Vapour Phase Epitaxy (HVPE) growing GaN on a substrate.
In 1974 Isamu Akasaki began studying gallium nitride and took up a professorship at Nagoya University to continue his research alongside Hiroshi Amano. In 1986 the MOVPE technique was used in order to produce GaN with high crystal quality and good optical properties. Shuji Nakamura later development a similar method in order to grown GaN at low temperatures.
Another major problem in producing blue LEDs was the difficulty in p-doping GaN with precision. In the late 1980s, Amano and Akasaki discovered that when GaN was doped with zinc atoms, it emitted more light and thus this gave better p-doping. This phenomena was later explained in an article by Nakamura. This was an important discovery as it paved the way for p-n junctions to be used in gallium nitride semiconductors.
A key step in the development of blue LEDs was the development of heterojunctions in the early 1990s by research groups led by Akasaki and Nakamura. In 1994, Nakamura used a double heterojunction InGaN/AlGaN to produce a device with a quantum efficiency of 2.7%, which opened the door for efficient blue LEDs to be easily produced.
The emission of blue light based on GaN was observed between 1995 and 1996 by both research groups. Modern efficient Ga-N based LEDs are the result of long series of breakthroughs in the scientific fields of materials physics, optics, electronics and chemistry.
The Future of LEDs
Illumination technology is currently undergoing a major revolution, with light bulbs and fluorescent tubes being replaced by LEDs. White LEDs currently have an energy efficiency of around 50% when converting electricity into light. This is a massive improvement on the 4% energy efficiency of conventional light bulbs which were first invented in 1879 by Thomas Edison.
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White LEDs have lifetimes are around 100,000 hours and are quickly becoming more affordable as demand is rapidly increasing in the market. Replacing conventional light bulbs with LEDs will drastically reduce the planet’s energy requirement for light as between 20% and 30% of the world’s electricity consumption is as a result of lighting.
Presently, LED technology is used in the back-lit screens of many mobile phones, laptops and television screens. Blue GaN diode lasers find applications in the technology which underpins the data storage on Blu-ray Discs which are predicted to supersede the DVDs.
In the future it is believed that AlGaN/GaN LEDs which emit ultraviolet light will find applications in water purification as UV light is able to destroy the DNA of bacteria and viruses. In countries with poor electrical infrastructure, it is believed that solar powered white LEDs will replace the use of kerosene lamps during the night.
References and Further Reading