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NRIs make world's smallest transistor

September 05, 2005 13:14 IST
Two non-resident Indian scientists have created history by making the world's tiniest transistor entirely from carbon nanotubes.

Nanotubes are rolled up sheets of carbon atoms and are more than a thousand times thinner than human hair.

The discovery heralds a new era of ultra miniature electronics where standard silicon transistors are replaced with much smaller versions fashioned from carbon nanotubes.

The new transistor is a Y-shaped nanotube with two branches that meet a central stem at a junction.

The current flowing from one branch to another can be switched on and off by applying a voltage to the third. Such binary logic called "gating" is the basis of nearly all transistors.

"The small size and dramatic switching behavior of these Y-shaped nanotubes makes them candidates for a new class of all-carbon transistor," says Prabhakar Bandaru, a materials scientist at the University of California, San Diego who led the team that included his colleagues Sungho Jin, graduate student Chiara Daraio and physicist Apparao M.Rao at Clemson University in South Carolina.

Their work published in the September issue of 'Nature Materials' has won instant acclaim from international science community.

The demonstration of switches and logic devices made purely from Y-junctions puts nanotubes "at the forefront of next-generation electronics," Professor Hongqi Xu of the Nanometer Structure Consortium at Lund University in Sweden commented in the journal Nature.

Conventional transistors like those in Pentium chips are built from layers of semi conducting silicon but the chip size has already shrunk to a size that cannot get any smaller.

The quest for ever smaller chips has driven scientists worldwide to explore nanotubes. They have already made logic circuits using nanotubes but these required metal 'gates' to control the flow of current.

Thus a true nanocircuit built entirely from nanotubes seemed like a dream until the pioneering work by Bandaru and colleagues.  "The Y-junction based carbon nanotube transistor incorporates a gate as part of the structure and is fully self-contained," Bandaru told PTI.

"It is very novel as it dispenses with the need for an external gate." However, the US based Indians were not the first to make the Y-junction, however.

Four years ago renowned chemist C.N.R. Rao at the Jawaharlal Nehru Centre in Bangalore produced a Y-junction nanotube and even showed that it behaved like a diode allowing current flow in one direction but not the other.

"We have considerably extended the measurements (made by C.N.R. Rao's group) and made a practical transistor like device," Bandaru said.

The scientists made their Y-shaped nanotubes by adding a titanium-iron catalyst to a pot of straight nanotubes while they are growing. When a catalyst particle is trapped in a nanotube, the tube branches forming Y-junction.

"Y-junction nanotubes have been previously synthesized but not much work has gone into assembling them for practical devices," Bandaru said.

"Our group was one of the first to actually assemble them and experimentally demonstrate their switching and logic functionalities without the need for an external gate," he said.

Commercial applications are however still years away. Bandaru agrees that to make a working chip, engineers would need to assemble millions of evenly spaced, predictably shaped nanotube transistors.

"Many smart people are working on this and the problem will likely be solved soon," he said.

One must remember that even for the Pentium chips that are used in our computers (which now have over 300 million transistors), the progenitor was a simple integrated circuit with two transistors in 1958, Bandaru pointed out.

"We are probably in the same stage with Y-junctions and the future looks good with so many possibilities."

The researchers plan to experiment with various other catalyst particles in order to tailor the three-way gating properties of the Y-junctions.  They are also trying to make T and X-shaped nanotubes that could allow different functions.

According to Bandaru one other novel feature of the transistor is that the catalyst particle at the Y-junction can be "nano-engineered" either during synthesis or by focused ion-beams.

"This gives rise to a whole series of possibilities, including giving each Y-junction device its own character and switching properties."

Indian scientists may have missed the semiconductor revolution of the 1960's that heralded the era of computers based on silicon. But when the era of nanoelectronics dawns on the world scene they are surely to be in the driver's seat.

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