COMPUTERS that store bits of information as single electrons could be working by 2010, according to Yutaka Kuwahara, head of R&D at Hitachi Europe. Such machines would pack the power of a thousand PCs onto a chip the size of a postage stamp. Kuwahara鈥檚 prediction follows the news that researchers backed by Hitachi have made logic circuits in which one bit is represented by fewer than 10 electrons. In today鈥檚 silicon chips, one bit is equivalent to 500 000 electrons. 鈥淭his is a major development,鈥� says Kuwahara.
The circuits are the fruit of research by Haroon Ahmed of the Microelectronics Research Centre at the University of Cambridge and Kazuo Nakazato of Hitachi鈥檚 Cambridge laboratory. Their team has already created the first 鈥渟ingle-electron memory鈥� in a thin wire of silicon sandwiched between layers of gallium arsenide.
To make their new devices, the researchers chop the wire into silicon islands, 10 nanometres across. Electrons can tunnel from one island to another, but if a small number collect on a single island they repel other electrons that attempt to get on. The presence or absence of a chosen number of electrons can then be used to represent either a 鈥�1鈥� or a 鈥�0鈥� (Technology, 6 November 1993).
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Ahmed and Nakazato have now succeeded in using a small number of electrons in one circuit to control the motion of electrons in another 鈥� a critical step towards processing data stored in single-electron memories. They apply a voltage that makes electrons hop along a chain of silicon islands, and use these to control the flow of electrons along an adjacent chain.
In the (Diagram), A and B are islands in two chains connected to separate parts of a circuit. When A contains no electrons, electrons flow freely in the other chain from a 鈥渟ource鈥� through B to a 鈥渄rain鈥�. But when electrons occupy A, they inhibit the flow of electrons beyond B.
The electrons on A impede the progress of electrons beyond B by repelling them. Normally, the effect of this force (X) would be limited because other electrons on B would exert an even stronger repulsive force (Y). But Ahmed and Nakazato create a 鈥渟pace charge鈥� by permanently removing electrons from a region of B. The resulting weak positive charge is enough to screen the repulsive forces between electrons on B. A single electron on A can hold as many as three electrons in place on B.
The influence that A holds over B means that Ahmed and Nakazato can transfer information from one circuit to the other. By applying this basic technique, the Cambridge scientists have made a NOR gate, one of the building blocks of computer circuits. They have also made silicon islands only 1 to 2 nanometres across and created a functioning single-electron transistor 鈥� a chain of islands controlled by a gate. So far, the devices work only at 78 K, the temperature of liquid nitrogen, but the researchers鈥� aim is to make their devices work at room temperature.
