Researchers Build a Working Carbon Nanotube Computer
In a paper in the journal Nature
on Wednesday, the researchers reported that they had successfully built
a working computer — albeit an extremely simple one — entirely from
transistors fashioned from carbon nanotubes. The nanotubes, which are
cylinder-shaped molecules, have long held the promise of allowing
smaller, faster and lower-powered computing, though they have proved
difficult to work with.
The Stanford Robust Systems Group, however, has made significant
progress in the last 18 months, advancing from building individual
carbon nanotube transistors to simple electronic circuits made by
interconnecting the transistors, and this week to a complete computer
made from an ensemble of just 142 low-power transistors.
While Stanford’s prototype computer is assembled from transistors that
are gargantuan by industry standards — one micron vs. 22 nanometers — it
is what computer scientists refer to as a “Turing complete” machine,
meaning that it is capable of performing any computation, given enough
time.
“It can run two programs concurrently, a counting program and a sorting
program,” said H. S. Philip Wong, a Stanford University electrical
engineer, and one of the leaders of the group. “We’ve spent a tremendous
amount of time on this; in fact we’ve spent two generations of students
on this.”
The computer is based on a subset of 20 of the instructions used by the
commercial MIPS microprocessor, which itself was designed by a group of
Stanford researchers led by Stanford’s current president, John Hennessy,
during the 1980s.
“I think this is a really nice piece of work,” said Supratik Guha,
director of physical sciences at I.B.M.’s Thomas J. Watson Research
Center. “It’s a rudimentary demonstration that carbon nanotubes can be
used to build a universal computer, or a Turing-complete machine. This
is not the most efficient computer, but that wasn’t the point. It’s one
of the first steps.”
Because the factory processes that underlie the modern semiconductor
industry require such painstaking precision, any new technology that the
industry might use must be perfected more than three years before it
can be considered for use in commercial production.
Carbon nanotubes have continued to excite the material science field
because of their proliferating array of allotropes — different forms of
the material — all with potential. Dr. Guha complimented the Stanford
group for maintaining its focus on a single engineering advance.
Currently, semiconductor industry leaders can make integrated silicon
circuits with a feature size of 22 nanometers, roughly 4,000 of which
could be spread across the width of a human hair. With the arrival of a
new generation of smaller transistors roughly every two years, the
industry generally believes that silicon will be scaled down to a limit
of 5-nanometer transistors sometime after 2020.
The constant shrinking of transistor size over the last half-century has
been important because it has significantly lowered the cost of
computing, making it possible to build ever more powerful computers that
are faster and cheaper, and consume less power with each generation.
While Intel has been generally circumspect about what material
technology it plans to turn to when silicon ceases to “scale” down to
smaller transistor sizes, I.B.M. has been more vocal and optimistic
about the potential for carbon nanotubes.
The company has recently succeeded in creating an inverter, a basic
logic element used in electronic circuits, using two different types of
carbon nanotube transistors, and plans to demonstrate the device at a
technical meeting at the end of the year.
The researchers said that their advance was not a scientific
breakthrough, but it was a significant demonstration of the ability to
work with a material other than silicon with great precision.
They also stressed that their research project was entirely compatible
with industry-standard manufacturing processes based on silicon. This
suggests that in the future it will be possible to build hybrid chips
using carbon nanotubes at particular locations, and thus extend the life
of silicon in computing.
The researchers said they were proud of their tiny prototype.
“This is a general computer and we can do anything with it,” said Max
Shulaker, a Stanford graduate student who is a leading member of the
research group. “We could in principle run 64-bit Windows, but it would
take millions of years.”
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