Transistors get a single atom switch

Dutch researchers have found that a single electron makes the difference between ‘on’ and ‘off’ for a new transistor made from a single carbon nanotube. The single electron transistor is the first to operate efficiently at room temperature and could be an ideal device for molecular computers.

‘We’ve added yet another important piece to the toolbox for molecular electronics,’ said Cees Dekker of Delft University of Technology.

Transistors may be best known as the workhorses of the computer industry. Working together, million of transistors on a single silicon chip help perform logic functions or store information.

In their ‘off’ state, transistors block the flow of current, but current flows when a small voltage is applied.

As computer chips shrink in size the idea of using a ‘single electron transistor,’ (SET) has become increasingly appealing.

The particular advantage of SETs is that they only require one electron to switch between on and off states. In contrast, transistors in conventional microelectronics use millions.

Researchers currently foresee a limit to how densely they’ll be able to pack such conventional transistors together, because the abundance of electrons moving around would ultimately produce too much heat for the chip to function. SETs might provide a means to avoid this problem.

The researchers believe an SET can be compared to a one-way bridge with tolls at each end that control whether cars can cross, one by one.

Specifically, it consists of a metallic ‘island,’ separated from ‘source’ and ‘drain’ electrodes by two barriers, through which electrons can tunnel.

A gate attached to the island tunes the voltage of the whole system. Controlling the voltage on the gate regulates the number of electrons hopping on or off the island, one at a time.

SETs have, up until now, operated at extremely low temperatures, because heat can also provide the energy necessary to add electrons to the island.

Now, Dekker’s group has made a device so small that heat fluctuations are irrelevant, even at room temperature.

That’s because the smaller the space in which electrons are confined on the island, the more energy it takes to add them.

Dekker and his colleagues started with a single carbon nanotube, and used the tip of an atomic force microscope to create sharp bends, or buckles, in the tube.

These buckles worked as the barriers, only allowing single electrons through under the right voltages. The whole device was only one nanometre wide and 20 nanometres long.

Researchers may someday assemble these transistors into the molecular versions of silicon chips, but there are still formidable hurdles to cross.

‘Only four years ago did we measure for the first time any electronic transport through a nanotube,’ said Dekker. ‘Now, we are exploring what can be done and what cannot in terms of single-molecule devices. The next step will be to think about how to combine these elements into complex circuits.’