tubes make logic circuits
Technology Research News
The transistors that make up today's computer
chips are fairly simple devices, and they are microscopic, but they are
not as small as they could be.
A group of researchers from the Netherlands has demonstrated several types
of simple logic
circuits using transistors made of single-molecule nanotubes, which are
rolled-up sheets of carbon atoms. The nanotube transistors are 1.4 nanometers
in diameter, or about 100 times smaller than a standard silicon transistor
and nearly 1,000 times thinner than an E. coli bacterium.
A transistor is a switch that either blocks or lets electrical current
pass through. Computer
chips are made of transistors wired into circuits constructed so that
electrical current can flow in different patterns to represent the basic
logic operations like "and" and "or" that underpin a computer's calculations.
The chips that power today's personal computers contain millions of transistors;
our society is also full of simpler chips that control things like fuel
injection in cars and heating systems in houses. Smaller circuits could
speed up these chips and would also come in handy in the burgeoning field
of nanotechnology, which promises to provide microscopic machines.
To make a logic circuit from nanotubes, the researchers first had to make
more practical nanotube transistors. "We made some improvements like using
a thinner gate oxide," said Peter Hadley, a researcher at Delft University.
A transistor's gate oxide controls how much current runs through the transistor.
The new design showed an increase in transconductance, or the ability
to transfer electric charge from one device to another. "When we measured
the transistors, we realized that we could use them to make logic circuits.
We connected a few transistors together and demonstrated... circuits,"
The researchers fashioned several types of logic circuits using one, two
or three nanotube transistors. Logic circuits operate between two voltage
levels that represent the ones and zeros of digital logic. The researchers
made an inverter, or NOT gate, which reverses an input, converting a 1
to a 0, and a NOR gate, which has two inputs and one output, and returns
a 1 only if both the inputs are 0. They also made a static random access
(SRAM) memory cell, which retains the results of these logical machinations.
In a related development, researchers at IBM recently modified a single
nanotube to act as a NOT gate.
To make their circuits, the Delft researchers placed aluminum gates that
were one micron wide on a silicon substrate, or base, then poured the
solvents containing the much smaller carbon nanotubes over the substrate.
"As the solvent dried, the nanotubes were deposited willy-nilly on the
substrate," said Hadley. The researchers then looked at the sample with
an atomic force microscope, searching for tubes that were lying on the
aluminum gates. When they found tubes that were aligned correctly, they
deposited gold electrodes on top of them using electron beam lithography,
said Hadley. The method uses tightly focused beams of electrons followed
by chemical solvents to carve microscopic molds in plastic.
Along the way, the researchers had to solve a couple of technical challenges.
"The aluminum gates were originally too rough... we solved the problem
by depositing the aluminum at low temperature," said Hadley. The researchers
also had to develop the technique that allowed them to deposit gold directly
on the tubes, he said.
The researchers' results prove that such small-scale circuits can be made,
but the tedious technique cannot be used to make them in bulk.
One way around this scalability problem is finding a chemical process
that would allow the transistors and their connections to assemble automatically,
said Hadley. "As with any molecular components there's the hope that self-assembly
could [eventually] be used to fabricate the circuits. A solution containing
these molecular transistors would be poured over a circuit and the transistors
would stick in the right places because of chemical interaction between
the transistor molecules," said Hadley.
A process like this could provide a cheap way to make billions of devices,
There are also size improvements to be made. Although the nanotubes are
very small, the gate oxide is about twice as large as those in today's
silicon transistors. "We have not tried to to make a particularly small
transistor. The important point is that we use a single-molecule as a
component of a transistor," said Hadley.
The researchers have made a "major advance" in nanotube logic
circuits, said Steven Kornguth, assistant director of the Institute for
Advanced Technology at the University of Texas. Key to the advance is
the nanotube transistors better transconductance. "The advance realized
is a gain in voltage, or signal output by a factor of 10," he said.
The Delft and IBM advances show that, in principal, it is possible to
make practical logic devices from carbon nanotubes, said Michael Fuhrer,
an assistant professor of physics at the University of Maryland.
The Delft improvements to nanotube transconductance were "clever", Fuhrer
said. Although using oxide on aluminum as a way to control electricity
in electronic devices is not new, "applying this technique to produce
the gate dielectric in nanotube devices is new, and noteworthy," he said.
More research is needed to further explore the electrical properties of
the tiny nanotube transistors, which are substantially different from
silicon transistors, Fuhrer said. The single molecule nanotubes transport
electrical charge differently, which results in a different distribution
of charge inside the transistor, he said. This leads to a fundamental
difference: "nanotube transistor properties do not scale with length in
the same way that silicon transistor properties do," he said.
Nanotube circuits could be used in sensing applications within three to
five years, while simple circuits that perform calculations will probably
take more than five years, said Hadley. "Most of the short-term potential
for molecular electronics lies in small circuits such as sensors. Once
molecules are incorporated into electrical circuits, the chemical and
optical properties of the [molecules] can be utilized in the circuit,"
Using nanotube circuits in place of today's established silicon to power
the much more complicated chips that run computers is much further off,
he said. "It will likely take more than 20 years before any technology
can displace silicon from its dominant position in computation," he said.
Hadley's research colleagues were Adrian Bachtold, T. Nakanishi and Cees
Dekker at Delft University in the Netherlands. The research is slated
for publication in an upcoming issue of the journal Science. The research
was funded by the European Community (EC) and by the Dutch foundation
for Fundamental Research on Material (FOM).
Timeline: 3-5 years, 20 years
TRN Categories: Integrated Circuits; Semiconductors; Nanotechnology;
Story Type: News
Related Elements: Technical paper, "Logic Circuits with
Carbon Nanotube Transistors select," slated for an upcoming issue of the
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