The quest to develop a single electron memory chip has moved one step closer with the announcement of a technique to manipulate specific numbers of electrons to perform present-day semiconductor logic functions. The development comes from researchers at Hitachi Euorpe Ltd’s Cambridge Laboratory, who have been working with Cambridge University’s Microelectronics Research Centre to develop a single electron circuit since 1991. The first major breakthrough came in 1993 when the research partnership produced a memory cell with a single electron which could store one bit of information (CI No 2,110). By adding logic to the memory cells the group developed the first single electron processor but the device could only be controlled at minus 269.8¦C – which was of little commercial use as such devices need to operate at room temperature. Present-day semiconductors use an operating principle based on the average behaviour of a large number of electrons. If the number of electrons is reduced the semiconductor loses performance. But the number of transistors within one chip is rapidly increasing and the group estimates by the year 2015 there will be 10,000m transistors per chip. To achieve this, the number of active electrons in one transistor will need to be reduced to 10 to avoid excessive heating. The control of individual electrons is therefore needed if 10 is to become the magic number to avoid overheating. The group has now developed new fabrication techniques to increase the temperature at which the device would work to minus 196¦C. The new technique enables the fabrication of nanostructures – minute metallic structures two nanometres in size, which enable the behaviour of single electrons to be closely observed. From such observations researches o bserved the Coulomb Blockade effect – the principle by which an isolated area of a conductor is made small enough so that the change of stored energy from the loss or addition of an electron prevents further electrons from leaving or entering the area. To increase the temperature at which electrons work effectively, the group found they need to decrease the conductor space by developing new materials capable of containing electrons within a smaller area. Such material has now been found and the research group is now carrying out the first tests using silicon structures. The researchers expect to produce a device that can fully function at room temperature by 2010. The joint development project has received ú1.5m from Hitachi Ltd this year, ú350,000 of which has gone towards an extension of the Cavendish Laboratory, which will open on July 6. Japan’s Ministry of International Trade & Industry has also invested ú1m in the project which is scheduled to run to the year 2000.
