A carbon nanotube computer processor is comparable to a chip from the early 1970s, and may be the first step beyond silicon electronics.
For the first time, researchers have built a computer whose central
processor is based entirely on carbon nanotubes, a form of carbon with
remarkable material and electronic properties. The computer is slow and simple,
but its creators, a group of Stanford University engineers, say it shows that
carbon nanotube electronics are a viable potential replacement for silicon when
it reaches its limits in ever-smaller electronic circuits.
The carbon nanotube processor is comparable in capabilities
to the Intel 4004, that company’s first microprocessor, which was released in
1971, says Subhasish Mitra, an electrical engineer at Stanford and one of the
project’s co-leaders. The computer, described today in the journal Nature, runs
a simple software instruction set called MIPS. It can switch between multiple
tasks (counting and sorting numbers) and keep track of them, and it can fetch
data from and send it back to an external memory.
The nanotube processor is made up of 142 transistors, each
of which contains carbon nanotubes that are about 10 to 200 nanometer long. The
Stanford group says it has made six versions of carbon nanotube computers,
including one that can be connected to external hardware—a numerical keypad
that can be used to input numbers for addition.
Aaron Franklin, a researcher at the IBM Watson Research
Center in Yorktown Heights, New York, says the comparison with the 4004 and
other early silicon processors is apt. “This is a terrific demonstration for
people in the electronics community who have doubted carbon nanotubes,” he
says.
Franklin’s group has demonstrated that individual carbon
nanotube transistors—smaller than 10 nanometers—are faster and more energy
efficient than those made of any other material, including silicon. Theoretical
work has also suggested that a carbon nanotube computer would be an order of
magnitude more energy efficient than the best silicon computers. And the
nanomaterial’s ability to dissipate heat suggests that carbon nanotube
computers might run blisteringly fast without heating up—a problem that sets
speed limits on the silicon processors in today’s computers.
Still, some people doubt that carbon nanotubes will replace
silicon. Working with carbon nanotubes is a big challenge. They are typically
grown in a way that leaves them in a tangled mess, and about a third of the
tubes are metallic, rather than semiconducting, which causes short-circuits.
Over the past several years, Mitra has collaborated with
Stanford electrical engineer Philip Wong, who has developed ways to sidestep
some of the materials challenges that have prevented the creation of complex
circuits from carbon nanotubes. Wong developed a method for growing mostly very
straight nanotubes on quartz, then transferring them over to a silicon
substrate to make the transistors. The Stanford group also covers up the active
areas of the transistors with a protective coating, then etches away any
exposed nanotubes that have gone astray.
Wong and Mitra also apply a voltage to turn all of the
semiconducting nanotubes on a chip to “off.” Then they pulse a large current
through the chip; the metallic ones heat up, oxidize, and disintegrate. All of these
nanotube-specific fixes—and the rest of the manufacturing process—can be done
on the standard equipment that’s used to make today’s silicon chips. In that
sense, the process is scalable.
Late last month at Hot Chips, an engineering design
conference hosted, coincidentally, at Stanford, the director of the
Microsystems Technology Office at DARPA made a stir by discussing the end of
silicon electronics. In a keynote, Robert Colwell, former chief architect at
Intel, predicted that by as early as 2020, the computing industry will no
longer be able to keep making performance and cost improvements by doubling the
density of silicon transistors on chips every 18 to 24 months—a feat dubbed
Moore’s Law after the Intel cofounder Gordon Moore, who first observed the
trend.
Mitra and Wong hope their computer shows that carbon
nanotubes may be a serious answer to the question of what comes next. So far no
emerging technologies come close to touching silicon. Of all the emerging
materials and new ideas held up as possible saviors—nanowires, spintronics,
graphene, biological computers—no one has made a central processing unit based
on any of them, says Mitra. In that context, catching up to silicon’s
performance circa 1970, though it leaves a lot of work to be done, is exciting.
Victor Zhirnov, a specialist in nanoelectronics at the
Semiconductor Research Corporation in Durham, North Carolina, is much more
cautiously optimistic. The nanotube processor has 10 million times fewer
transistors on it than today’s typical microprocessors, runs much more slowly,
and operates at five times the voltage, meaning it uses about 25 times as much
power, he notes.
Some of the nanotube computer’s sluggishness is due to the
conditions under which it was built—in an academic lab using what the Stanford
group had access to, not an industry-standard factory. The processor is
connected to an external hard drive, which serves as the memory, through a
large bundle of electrical wires, each of which connects to a large metal pin
on top of the nanotube processor. Each of the pins in turn connects to a device
on the chip. This messy packaging means the data has to travel longer
distances, which cuts into the efficiency of the computer.
With the tools at hand, the Stanford group also can’t make
transistors smaller than about one micrometer—compare that with Intel’s
announcement earlier this month that its next line of products will be built on
14-nanometer technology. If, however, the group were to go into a
state-of-the-art fab, its manufacturing yields would improve enough to be able
to make computers with thousands of smaller transistors, and the computer could
run faster.
To reach the superb level of performance theoretically
offered by nanotubes, researchers will have to learn how to build complex
integrated circuits made up of pristine single nanotube transitors. Franklin
says device and materials experts like his group at IBM need to start working
in closer collaboration with circuit designers like those at Stanford to make
real progress.
“We are well aware that silicon is running out of steam, and
within 10 years it’s coming to its end,” says Zhirnov. “If carbon nanotubes are
going to become practical, it has to happen quickly.”
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