Blue light emitting diodes (LEDs) are widely used in the displays of smartphones and PCs and TV screens, and they form the basis for energy efficient white LED lighting. But whereas red and green LEDs have been around since the 1960s, the blue LED, which has made all these advances possible, took another three decades to come.
Now the three scientists responsible – Isamu Akasaki and Hiroshi Amano then working at the University of Nagoya and Shuji Nakamura at the Tokushima company Nichia Chemicals – have been recognized for this breakthrough, with the award of the 2014 Noble Prize in Physics.
Blue LED development
LEDs are narrow-band light sources based on semiconductor components, with wavelengths ranging from the infrared to the ultraviolet.
The basic building block of the semiconductor is the p-n junction, which is created by “doping” a material with other elements (such as silicon and magnesium) such that when a voltage is applied, light is emitted when the electrons injected from the negatively-doped (n-type) to the positively doped (p-type) side recombine with holes injected in the opposite direction.
The first LEDs were based on gallium arsenide and gallium phosphate, and mixed Ga, As and P crystals. However, blue light LEDs, with their shorter wavelength, were more challenging, requiring a different material. This was found in gallium nitride, but GaN itself is also very sensitive to the doping process. The breakthrough achieved by Akasaki, Amano and Nakamura in the mid-1990s, after more than two decades of work on GaN, was to find the way to create high quality GaN crystals by incorporating indium and aluminium (InGaN/AlGaN) on a sapphire substrate.
Subsequent developments took place very quickly, and were also soon followed with commercialization in lighting and storage applications.
21st century LED lighting
Illumination technology is presently going through a revolution in the transition from light bulbs and fluorescent tubes to LEDs. The light bulb, invented by Thomas Edison in 1879, has a low efficiency around 16lm/W representing approximately 4% energy efficiency from electricity into light. The fluorescent tube, invented by P. Cooper Hewitt in 1900, reaches an efficiency of 70lm/W. White LEDs currently reach more than 300lm/W, representing more than 50% wallplug efficiency.
White LEDs used for lighting are often based on efficient blue LEDs that excite a phosphor so that the blue light is converted to white light. These high-quality LEDs are getting cheaper, and the market is currently exploding. A little further on in the future, three-colour (red, green, blue) LEDs may replace the combination of blue LED and phosphor for efficient lighting. This technology will allow for dynamic control of colour composition.
With 20-30% of the electricity consumed in industrial economies used for lighting, replacement of light bulbs and fluorescent tubes with LEDs will lead to a drastic reduction of electricity requirements for this purpose. Materials consumption is also reduced with the 100,000 hour lifetime of LEDs comparing with 1,000 hours for incandescent bulbs and 10,000 hours for fluorescent lights.
Blue LED applications
In addition to the widespread application of GaN-based LEDs in liquid crystal displays, blue and UV-emitting GaN diode lasers are used in high-density (Blu-ray) DVDs, which has advanced the technology for storing music, pictures and movies. Future application may include the use of UV-emitting AlGaN/GaN LEDs for water purification, as UV light destroys the DNA of bacteria, viruses and microorganisms.