NRI's make world's smallest transistor
SAN FRANCISCO, SEPTEMBER 05, 2005
PTI
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.
