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Scientists tune in to molecule-sized transistors

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Boffins continue to try to shrink the size of the micro-circuitry from which processors are built, to allow chips to be designed that offer ever more features yet don't rapidly become too hot to operate - and before their transistors reach one clear physical limit: the size of a single molecule.

Working molecular transistors, then, are the new goal in microchip development. Here are the three latest attempts, details of which were published recently in learned journal Science.

Wires

Charles Lieber's team at Harvard have produced a range of semiconductor 'nanowires' measuring between 2nm and 20nm, out of which they have woven a grid. The current along one wire can be controlled by a voltage applied to a wire running across it. Build a sufficiently complex grid, reckons Lieber, and you can form logic gates - the 'brain cells' of microprocessors.

And indeed that's just what Lieber and co. have done - using, more to the point, techniques that aren't based on lithography, the process used today to etch chip designs onto silicon. To build smaller chips, lithography requires ever smaller optical wavelengths to work with, making them expensive to design and construct.

Tubes

A second investigation, conducted at the Delft University of Technology in the Netherlands by a team headed by Cees Dekker, is based on lithography. Dekker and co. have created 1nm diameter 'nanotubes' constructed from carbon. Current runs down the tube, controlled by the voltage applied to an aluminium strip on which the tube sits.

The snag here is that it's easy to build the basic structure of the transistor - the aluminium base and two gold contacts that conduct a signal into and out of the carbon tube. The difficult bit is getting the tube into place - effectively, Drekker and co. currently sprinkle tubes over the transistor and hope one lands in the right place. However, they've made sufficient transistors to string them together into logic gates.

Organics

What do you get if you fit a molecule of organic compound 4,4'-biphenyldithiol between gold electrodes and sit the lot on top of a block of silicon, a thin slice of silicon dioxide between them? According to Lucent researcher Jan Hendrik Schön, you get a transistor, and he and his team made some to prove it. They put two of them together and made a NOT gate, which flips a 1 into a 0 and vice versa - ie. flips a positive voltage into a negative one.

Dekker and Lieber and their teams created a number of different logic gates, such as AND, which generates a 1 only if two binary inputs are 1, and OR, which generates a 1 if either of two inputs are 1. ®

Related Links

Lieber and co.: Logic Gates and Computation from Assembled Nanowire Building Blocks
Drekker and team: Logic Circuits with Carbon Nanotube Transistors
Schön and co: Field-Effect Modulation of the Conductance of Single Molecules
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