17-19 20 Years v2 10/9/09 13:25 Page 17
LEDs technology
rhombus4
20 years and counting
for the GaN LED
A commercially viable GaN LED was an incredibly hard nut to crack
that required the development of a buffer layer and a novel approach
to p-type doping. But 20 years ago it all came together.
Richard Stevenson looks back at the device’s birth.
I
f the readership of Compound Semiconductor were followed a few years later, along with the previously
asked to name the inventor of the GaN LED, the most mentioned success at the RCA labs.
common reply would probably be Shuji Nakamura, the
former Nichia researcher who is now an academic at the But progress was short-lived and by the mid-1970s many
University of California, Santa Barbara. But this is not the researchers were turning their backs on GaN and looking
correct answer. The first GaN LED was actually made in to other materials that might produce more fruitful results.
1971 by a team at the laboratories of the Radio The main problems with GaN were its low crystal quality,
Corporation of America (RCA), which included Herb and very high residual donor concentrations that made it
Maruska, Jacques Pankove and Ed Miller. impossible to realize p-type conduction. Controlling the
levels of n-type conduction was not easy, either.
This device employed a metal-insulator-semiconductor
structure, and its incredibly low efficiencies prevented it These issues did not deter Akasaki, and throughout the
from ever being a commercial success. The far more early 1970s he focused on the research and development
important breakthrough is, without doubt, the first LED of a GaN LED while working at Matsushita. And some
with a p-n junction, because this is the architecture success came in 1975, when he produced single-crystal
employed in all of today’s blue, green and white-emitting GaN by MBE. Material quality was not great, but the
devices. This LED emanated from the Isamu Akasaki’s advance was still large enough to merit a three-year
group at Nagoka University, Japan. Nakamura contribution research grant by Japan’s Ministry of International Trade
was to quickly build on this work by optimizing the design,
which led to a hike in device efficiencies and enabled
Nichia to launch its first commercial LEDs in 1993.
Blue, green and white
One of the striking things about Akasaki’s achievement is LEDs are widely used in
that it came when he had just turned 60, a time in most the keypads and
researchers’ lives when they have lost their drive for backlights of mobile
striving for success. But his breakthrough was the phones. Falling
culmination of many years of nitride development that average selling prices,
stretched back to the early 1970s, when he worked as a coupled to modest
research scientist at Matsushita Research Institute Tokyo. rises in the number
of handset sales
Nitrides were a relatively hot topic back then, thanks to each year, have
several recent breakthroughs. These included the caused the value
fabrication of the first single crystals of GaN on sapphire of this market to
substrates in the late 1960s, which were produced by plateau. However, it is still
HVPE. Stimulated emission from optically-pumped GaN the biggest market for GaN LEDs
Credit: Nokia
September 2009
www.compoundsemiconductor.net 17
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