Tiny
tubes make logic circuits
By
Kimberly Patch,
Technology Research NewsThe 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," he said.
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, he said.
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," he said.
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
Funding: Government
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 journal Science.
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October 10, 2001
Page One
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