ELECTRONICS took another step into the nanoworld this week, as three research groups announced new ways to make molecule-sized transistors-the beating hearts of microchips.
The smaller the components on a chip, the closer they can be packed together-and the faster they can communicate. That means denser memory chips and faster microprocessors. Transistors in today鈥檚 commercial silicon microchips measure about 100 nanometres wide and are still shrinking, but the day will dawn sometime in the next two decades when existing etching technologies will be unable to make transistors any smaller. So engineers want to know if single molecules-at just a few nanometres thick-can function as transistors.
Transistors act as switches that turn currents on and off-representing digital 1s and 0s. The most common type consists of a channel through which current can flow between two contacts, called the source and the drain. A third contact, known as the gate, controls the flow by opening and closing the source-to-drain current route depending on the applied voltage. It鈥檚 like squeezing a hosepipe in the middle to stop water flowing.
Advertisement
The three teams used very different approaches to make logic circuits out of molecule-sized transistors. The simplest (published at www.scienceexpress.org) came from Jan Hendrik Sch枚n and a team at Lucent Technologies in New Jersey. Sch枚n鈥檚 team fashioned a transistor using a single organic molecule called 4,4鈥�-biphenyldithiol as the current channel. Gold electrodes at either end of the molecule serve as the source and drain, and a silicon block covered by a thin layer of insulating silicon dioxide serves as the gate. By linking two such transistors together, Sch枚n鈥檚 group made an inverter-or NOT gate-which changes a 1 to a 0 and vice versa.
Meanwhile, an alternative approach has been hatched by Cees Dekker and his colleagues at Delft University of Technology in the Netherlands (Science, vol 294, p 1317). They made transistors from carbon nanotubes-sheets of carbon arranged like chicken wire rolled up into a 1-nanometre-wide tube (see Graphic). In Dekker鈥檚 transistor, the nanotube itself acts as the current channel. Two gold electrodes placed at either end serve as the source and drain, and an aluminium wire covered in an insulating layer of aluminium oxide serves as the gate. Making the transistor is rather hit and miss at the moment-Dekker scatters nanotubes on top of the chip, choosing just those that lie along the aluminium wire. But his idea is a first because each transistor in an array has a separate gate and thus is individually controlled. By wiring a few of these transistors together, Dekker has made a number of different types of logic circuits.FIG-mg23173701.JPG
Another promising idea published this week (Science, vol 294, p 1313) is the use of semiconducting nanowires, which range from 2 to 20 nanometres in diameter. Charles Lieber and his group at Harvard University have formed a grid of nanowires in such a way that some act as current channels, while a wire that bisects it becomes the all-controlling gate electrode. By choosing a more complex arrangement of wires, Lieber created a number of circuits including an AND gate, a logic circuit that can be used to add two binary numbers.
Lieber says his approach, means that circuits can be created cheaply without expensive lithographic etching techniques used to make today鈥檚 silicon chips and also needed for both Sch枚n and Dekker鈥檚 molecular and nanotube transistors.
Not everyone in the nanotechnology world is convinced any of these gadgets will replace today鈥檚 silicon. Phaedon Avouris, a nanotube transistor researcher for IBM in Yorktown Heights, New York, is cautious. He says molecular electronics may be more suited to niche applications like sensors and flexible electronics, than to a broad-brush replacement for today鈥檚 microchips.
