Micro miniaturisation of computer circuitry will drive the silicon chip out of reckoning in just four years, Indian-American scientist Suman Datta has warned.

The silicon chip, which has propelled decades' of remarkable increases in computing power and speed, seems incapable of sustaining this pace for long, Datta said Thursday at a conference on 'Condensed Matter and Materials Physics' at the Royal Holloway College, London.

According to Datta, an alumnus of IIT, Kanpur, who is currently with the Pennsylvania State University, the bid to miniaturise silicon circuitry has its limits.

For instance, they become leaky, rendering them incapable of holding onto digital information.

So if steady increases in computing capability that is taken for granted are to continue, some new technology will have to take over from silicon.

And according to other researchers who presented papers at the conference, carbon nanotubes are promising replacements.

Discovered in 1991, they are just the width of a protein molecule, tens of thousands of which can be crammed in the space of a single human hair.

Some nanotubes behave as semiconductors, like silicon; others carry currents like metal wires. Already, fundamental elements of computer circuits such as transistors have been made from individual carbon nanotubes.

But the problem is arranging nanotubes into circuit patterns. The difficulty arises out of their being made as mixtures of metallic and semi-conducting tubes, whereas just one kind or the other is needed for a specific component.

These electrical properties depend on the precise arrangement of carbon atoms in the nanotube, but that's hard to determine for single tubes.

Bryan Hickey and his co-workers of Leeds University in Britain have now developed a technique that will reveal an individual nanotube's structure and its electrical properties, and then allow it to be placed in a position on a surface with an accuracy of about 100 nanometres, a fraction of the width of a human blood cell.

Leeds University researcher Chris Allen said: "With this technique we can make carbon nanotube devices of a complexity that is not achievable by most other means."