The journey to LEDs
Nick Holonyak, George Craford, Russell Dupuis, Isamu Akasaki and Shuji Nakamura are recognised for their work in the development of today’s high brightness III-V LEDs. This means that the diode’s semiconductor is an alloy that contains elements from groups III (boron, aluminium, gallium, indium) and group V (nitrogen, phosphorus, arsenic and antimony) in the periodic table. The most common combination of these elements is gallium arsenide, or gallium nitride.
In 1962 Holonyak became the first person to create a visible LED. The LED colour was red due to its low frequency. He created the first group III-V compound semiconductors containing either three or four (ternary or quaternary) different elements and used these alloys to create semiconductors with specific properties. These can operate at extremely high frequencies or emit light with extraordinary efficiency. Modern day white LEDs are all based on his invention.
Craford invented a shorter wavelength yellow LED in 1972, and later the first high brightness yellow, amber and red devices in 1992. While at Hewlett-Packard (1979 onwards), his team pioneered the development of AlInGaP LEDs using Metal Organic Chemical Vapour Deposition or (MOCVD). He developed the technology in the late 1970s to increase the efficiency of these high brightness, alloy compound semiconductor red and yellow LEDs by a factor of ten. This ensured that the technology could be commercialised, manufactured reliably and cost effectively.
Dupuis developed the metal organic chemical vapor deposition (MOCVD) process to produce high quality semiconductors. The process combines the atomic elements from groups III and V with molecules of an organic gas. It deposits the mixture as an additional layer of atoms on the surface of a hot semiconductor wafer under ultrahigh vacuum conditions, growing layer upon layer of crystals. Building up precise, controlled layers can create specifically optical and electrical properties. The process was scalable to industrial needs.
Akasaki upgraded the MOCVD technology by developing it to produce blue, group III high brightness and efficient nitride LEDs. These were made by adding layers of GaN, AlGaN (aluminium gallium nitride) and a form of GaN that was doped with p-type magnesium on sapphire. The development of an aluminium nitride buffer layer was a major breakthrough but the use of aluminium also resulted in poor reproducibility.
A crucial aspect of developing LEDs that emit white light was the requirement for a blue LED. It remained very difficult to create efficient blue LEDs throughout the 1980s. In 1993 Nakamura overcame these issues by inventing a two-flow MOCVD reactor - introducing a subflow of nitrogen and hydrogen gas which gently pushed the carrier organic gases down to the substrate, improving the thermal boundary layer. It removed the need for aluminium and produced uniform and extremely high-quality growth of gallium nitride (GaN). This made the LED emission of white light possible by converting part of the blue light to yellow using a phosphor. The human eye perceives the combination of blue and yellow light as white. Nakamura’s achievements allowed white and bright LEDs to be produced commercially, creating the foundation of modern solid-state lighting.