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very conductive on its own, but a p-n junction of the
two types in a single crystal can become depleted
of charge carriers. The junction element will then
have either forward bias or reverse bias depending
on the direction of an imposed electrical current.
As electrons move from the n-side to the p-side
across the junction, holes ‘move’ in the other di-
rection. As electrons recombine with holes there
is spontaneous energy emission. Depending on
the type of dopant, this energy can be released as
visible light.The pursuit of different frequencies, or
colours, of light-emitting diodes becomes, then, the
pursuit of the right type of dopant.
As soon as the physics of the interactions were
understood back in 1947 with the invention of the
transistor, work began on light emission. By the
1960s, companies across the world were making
red and green LEDs using Gallium Phosphate (GaP)
as the dopant.
The wave-length of light emitted depends on
the band gap between the n- and p-type elements.
Researchers at Philips tried GalliumNitride (GaN)
to grow crystals in the 1950s. They believed this
would offer an appropriate band gap.
GaN is a very hard material with a Wurtzite (a
hexagonal crystal system based on binary com-
pounds) structure. Its wide band gap permits
high-frequency light emission as well as high-speed
field-effect transistors. But first crystals of the stuff
had to be produced.
Unfortunately, researchers managed only a
powder and were unable to create p-n junctions.
Research continued with GaN using different tech-
niques to grow the crystals – HydrideVapour Phase
Epitaxy, for one – but without success.
JI Pankove, a leading scientist working on blue
LEDs, in 1973 said, “In spite of much progress in
the study of GaN over the last two years, much re-
mains to be done.The major goals in the technology
of GaN should be: 1) The synthesis of strain-free
single crystals, 2) the incorporation of a shallow
acceptor in high concentrations.”
Isamu Akasaki began studying GaN in 1974. In
1981, he became a professor at Nagoya University
andwas joined by Hiroshi Amano. Only in 1986were
they able to grow useable quantities of GaN using
the Metalorganic Vapour Phase Epitaxy technique.
As explained in the Nobel Prize review, “The
breakthrough was the result of a long series of
experiments and observations. A thin layer (30 nm)
of polycrystalline AIN was first nucleated on a
substrate of sapphire at low temperature (500 °C)
and then heated up to the growth temperature of
GaN (1000 °C). During the heating process, the
layer develops a texture of small crystallites with a
preferred orientation on which GaN can be grown.
The density of dislocations of the growing GaN
crystal is first high, but decreases rapidly after a
few µm growth. A high quality surface could be
obtained, which was very important to grow thin
multilayer structures in the following steps of the
LED development.”
Separately, Shuji Nakamura at Nichia Chemical
Corporation developed a similar method where AIN
was replayed with a thin layer of GaN which was
grown at low temperature.
This was only the beginning as efficiency needed
to be improved if it was to be at all commercially
viable. Previous LED developments demonstrated
that heterojunctions and quantum wells were nec-
essary to deliver high efficiency. New types of GaN
compounds were needed.
Over the next few years, Akasaki focused on
AlGaN/GaN and Nakamura worked on InGaN/GaN
and InGaN/AlGaN heterojunctions.
By 1996 the fundamental work had been com-
pleted and commercialisation began.
Nichia Chemical Corporation, which had forced
Nakamura to stop work on his research though he
had continued with his own time and resources,
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