kinks control current
Technology Research News
The steady increases in computer speed
we’ve gotten used to over the past four decades are largely due to the
shrinking of transistors. Smaller transistors mean shorter paths for electrical
current to signal the ones and zeros of digital computing, which in turn
speeds the process. Only a dozen years ago, the state-of-the-art 486 sported
1.2 million 1-micron transistors. Today’s Pentium 4 packs 42 million 100-nanometer
Researchers at Delft University of Technology in the Netherlands are trying
to lower the transistor size barrier much further with a single-electron
transistor (SET) made of a single-wall carbon nanotube that is 1.5 nanometers
wide and about 50 nanometers long. A nanometer is a millionth of a millimeter.
To create the nanotube transistor, the researchers used an atomic force
microscope (AFM) to put two kinks in a nanotube. “We have strongly bent,
or buckled, a nanotube twice with an atomic force microscope. In this
way, we created an ultra-tiny conducting island within the nanotube,”
said Cees Dekker, a professor in applied physics at Delft University of
Technology in the Netherlands.
In the nanotube, the area between the kinks is the island, or conducting
part, and measures about 25 nanometers in length. Using a single nanotube
as a transistor is the ultimate level of miniaturization, according to
Today’s current state of transistor miniaturization allows about 750 transistors
to fit in 3 square millimeters, which is about the size of the average
flea. More than 100,000 of the nanotube transistors could fit in the same
The nanotube transistor, which resembles a slightly mistreated drinking
straw, uses a single electron rather than the several million required
to turn on a typical computer transistor today. The key to the tiny transistor
is its ability to function at room temperature.
Single-electron transistors have historically required severely low temperatures
because the energy of warmer molecules drowns out a single electron’s
signal. The researchers got around this central problem by reducing the
size of the transistor. At the nano scale, heat fluctuations don’t matter.
“As a rule of thumb, the smaller the device, the larger the charging energy
for a single electron is,” said Henk Postma, a graduate student at Delft.
“If the charging energy associated with [passing] a single electron is
larger than the energy you have available from temperature and bias voltage,
the current cannot run and you have a functioning SET,” he said. “Our
device is so small, that the charging energy is large enough to operate
at room temperature.”
In a transistor, electrons flow from a source electrode to a drain electrode
through the island. At two ends of the island are junctions that connect
the island and the electrodes. On either side of the island are gates.
When there is no voltage moving through the gates, electrons are blocked
from moving through the island. Current flowing through the gates turns
the transistor on allowing electical current to flow from the source to
Electrons flow through the nanotube transistor by tunneling through one
by one. When electrons tunnel, they exhibit that peculiar quantum trick
of disappearing, then reappearing somewhere else without traversing the
space between. As one electron tunnels from the island to the drain electrode,
another electron takes its place by tunneling from the source electrode
to the island, a process known as coupling.
“In conventional SET's, people believe that electrons hop onto the island
and off the island in an uncoupled manner…the electron that hops on does
not know about the electron that hops off the island. We have shown in
our experiment that the electrons in our device only hops on when another
electron hops off,” said Postma.
This correlated, or coupled tunneling, is like entering a subway station
by dropping a coin into a turnstile machine. While the coin is traveling
to the belly of the machine, you can’t enter the station. When the coin
has settled in, the turnstile rotates just once to let a person through.
The research is novel because the whole device is one single molecule
and it operates at room temperature, said Jie Han, a research scientist
of Nanotechnology at NASA’s Ames Research Center. “This may have future
applications in nanoelectronics if metallic tubes can be made and buckles
can be created in a controlled and large-scale manner,” he said.
“[The] work definitely will get attention from the general body of research
on SET's [because it is] a molecular solution to SET technology and application.
However, it cannot be expected that many researchers will be able to improve
or even repeat this work. It is still very difficult to only make metallic
tubes and then to buckle them in right positions,” Han said.
The researchers are currently working on nanotube logic and the issue
of assembling nanoelectronics, Dekker said. The device will not be applied
in nanoelectronics for at least a decade, he said.
Dekker and Postma’s research colleagues were Tijs Teepan, Zhen Yao, and
Milena Grifoni of Delft University. They published the research in the
July 6, 2001 issue of the journal Science. The research was funded by
the Dutch Foundation for Fundamental Research on Matter (FOM) and the
European Community’s SATURN research network.
Timeline: >10 years
TRN Categories: Nanotechnology; Integrated Circuits; Quantum
Story Type: News
Related Elements: Technical paper, "Carbon Nanotube SET
at Room Temperature," Science, July 6, 2001.
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