University of Southampton's School of Electronics and Computer Science (ECS) is combining nanomachining and single-electron transistors, aiming to make sensitive sensors.

"This is the first time that anyone has combined these two nanotechnologies to develop a smart sensor," said project leader Professor Hiroshi Mizuta. "The traditional CMOS approach has many limitations so we needed to find a new approach."

Mizuta and his team are part of the three year European FP7-funded NEMSIC (nanoelectromechanical system integrated circuits) project which will make these devices possible.

"As well as being the smallest sensor on the market to date, it will have extreme sensitivity and very low power consumption," said the University

Details of the exact structure of the proposed devices will remain secret until the intellectual property is protected.

However, Mizuta's team has already published details of a single electron transistor whose channel is an under-etched bridge of silicon with air-spaced gates either side. Etched bulges, or embedded silicon nanocrystals, on the bridge act as quantum dots to confine electrons.

Speaking to Electronics Weekly about the sensor, Mizuta said: "We will develop two kinds of nanosensor devices on silicon-on-insulator substrates. A common and key structure used for both devices is a very thin suspended silicon bridge."

One sensor will work by detecting charge from molecules of interest.

"In the first nanosensor device, the suspended nanobridge, with or without an integrated silicon quantum dot cavity, is used as a channel of a transistor with side gates," Mizuta told Electronics Weekly. "Small charge transfer caused by the molecules captured on the surface of the nanobridge channel is detected electrically as a change in the conductance."

The other is mechanically closer to the classic vibrating cantilever micromachine sensor.

"In the second nanosensor device, the suspended nanobridge is used as a movable gate of a nanoscale transistor," said Mizuta. "A small mass change due to the molecules captured on the nanobridge gate surface is detected electrically as a change in the resonant frequency of the gate."

Why a single-electron transistor and not a conventional mosfet?

"Our nanobridge transistor with a suspended quantum dot works as an extremely sensitive charge detector which in principle senses charge transfer of even a single electron occurring on the surface of the quantum dot. This cannot be achieved using a conventional mosfet structure," said Mizuta.

"The single-electron transistor combined with nanomachine device technology reduces power consumption at both 'on' and 'off' states of the sensor. Stand-by power is reduced to zero by having a complete sleep with a mechanical switch when it is off."

Nanomachining gets a boost at Southampton as its new electron beam lithography machine comes on stream when the £55m rebuilt Mountbatten building, originally burned down in 2005, opens this month.

The European project NEMSIC is headed by Professor Adrian Ionescu of Ecole Polytechnique Fédérale de Lausanne.

Other partners are: Delft University of Technology, Stitching IMEC Nederland, Commissariat à l'Energie Atomique - Laboratoire d'Electronique de la Technologie de l'Information, SCIPROM Sarl, Interuniversity Micro-electronics Center, Honeywell Romania SRL - Sensors Laboratory Bucharest, and Université de Genève.