(From left to right) Mazumdar, Cardamone and Stafford. Picture by Lori Stiles.

Silicon is the edifice of the electronics revolution. It’s a wonderful semiconductor. Transistors made of silicon switch electrical current on and off, just like a valve turns water on and off in a garden hose.

By putting hundreds of thousands of tiny transistors on a silicon single chip, engineers shrank the size of a computer from as big as a room to a pocket-sized PC in a matter of four decades.

And this reduction in dimension did not affect the performance of computers — in fact, their power grew exponentially. For instance, while a 8088 microprocessor chip released by semiconductor giant Intel Corporation had just 6000 transistors, today’s Intel Pentium IV chip carries as many 50 million transistors. This miniaturisation and concurrent improvement in computing power because of rapid advances in microprocessor development is governed by what is know as Moore’s Law, named after Gordon Moore, the co-founder of Intel. Moore’s Law says loosely that the computing power of a computer will double every 18 months.

Indeed, if Moore’s Law were applicable to the airline industry, a flight from New York to Paris in 1978 that took seven hours and cost $ 700 would now be carried out in less than a second at a cost of 1 US cent — so remarkable has been the development of the microprocessor in the last few decades. One giant snag: silicon-based transistors are not expected to go very far and will soon hit a technology roadblock. Scientists don’t give silicon more than another 20 years of life as far as computers are concerned.

The good news is that the Cassandras may be wrong, if a team of University of Arizona scientists has its way. Arizona physicists, including non-resident Indian Sumit Mazumdar, feel that they have made a breakthrough. They have shown — at least in principle — that single organic molecules can replace silicon-based transistors.

Their paper appears as the cover story in the November 2006 issue of Nano Letters journal. Apart from Mazumdar, another University of Arizona professor Charles Stafford and a post doctoral fellow Cardamone explored this theoretical possibility.

The paper argues that this novel design concept can lead to molecular transistors that can make the next generation of tiny, powerful computers. Such transistors will be as small as one nanometre. In comparison, the smallest transistor in use today is 65 nanometres in size. Theoretically speaking, current transistor technology can’t shrink smaller than 25 nanometres, or one-forty thousandth (1/40000) the width of a pinhead.

Overcoming barriers

The problem with all transistors in current technology, and almost all proposed transistors, is that they regulate current flow by raising and lowering an energy barrier. “Using electricity to raise and lower energy barriers has worked for a century of switches, but that approach is about to hit a wall,” says Stafford, one of the co-authors of the study. This is because scaling down transistors beyond 25 nanometres can create serious energy problems.

The Mechanism: When voltage is applied through electric lead III (C), it regulatesflow of current between the other two leads (A and B).

Even if it were possible to build an ultra-advanced laptop computer with molecule-sized transistors using current transistor technology, it would take so much electricity to run the laptop that the resultant heat generated would probably vapourise the computer itself.

The Arizona scientists started thinking about the next generation of transistor technology three years ago. They felt that there should some other way to work around the problem. If they resorted to quantum mechanics, they could get around the problem of power dissipation, which leads to the generation of excessive heat. Moreover, regulation of current flow can be thus made easier, making these single-molecule transistors work at room temperatures. Earlier, such minute transistors required very low temperatures to function.

“It is a very different and fascinating approach,” says Robert Wolkow, the nanotechnologist at the University of Alberta in Canada who has done pioneering work in this area. In 2005, his team showed for the first time that a single charged atom on a silicon surface can regulate the conductivity of a nearby molecule, thereby allowing current to pass through.

The Arizona scientists have already applied for a patent on their device, called Quantum Interference Effect Transistor, nicknamed “QuIET.”

The simplest molecule they propose for a transistor is benzene, a ring-like molecule. They propose attaching two electrical leads to the ring to create two alternate paths through which current can flow.

They also propose attaching a third lead opposite one of the electrical leads. Other researchers have succeeded in attaching two contacts to a molecule this small, but attaching the third is the trick — and the point. The third lead is what turns the device on and off, the “valve.”

“Our work is unique in that we propose a new mechanism to control the flow of current through a single molecule. In the “off” state of our proposed device, current is blocked by perfect destructive intereference stemming from molecular symmetry,” Mazumdar told KnowHow. Perturbing the molecule so as to break the symmetry allows current flow and this perturbation is done in a such a way that this does not affect all parts of the molecule in the same way, Mazumdar says. .

Back to roots

A regular visitor to India and an adjunct professor at the Indian Association for the Cultivation of Science in Calcutta. Mazumdar has been working closely with scientists at the Indian Institute of Science in Bangalore and at S N Bose Institute in Calcutta for several years.

In classical physics, the two currents through each arm of the ring would just add. But using quantum mechanically, the two electron waves interfere with each other destructively. So no current gets through. That’s the ‘off’ state of the transistor.

The transistor is turned on by changing the phase of the waves so they don't destructively interfere with each other, opening up additional paths through the third lead.

“It took a while to go from the idea of how this could work to developing realistic calculations of this kind of system,” Stafford said. “We were able to do the simplest kind of quantum chemical calculations which neglect interactions between different electrons within a few weeks. But it took some time to put in all the electron interactions that demonstrate that this really is a very robust device.”

According to Wolkow, though the ideas are not entirely new, the work offers a clever and rather specific recipe for finding such a quantum interference phenomenon on a molecule. Mazumdar is confident about the practical potential of QuIET. It promises to overcome two of the most fundamental obstacles to further miniaturising transistors to a one nanometre scale, he says. “First, since current is regulated by quantum interference, instead of raising and lowering an energy barrier, QuIET could be operated at a much lower power level. Second, the interference effect upon which QuIET is based is insensitive to other electrostatic changes that occur to molecules,” he says. .

QuIETly making waves?

Wolkow says the current silicon technology will not have a life beyond 15 years. “But new lithographic (transistors are etched on a silicon chip using lithography) tricks will probably keep it going somewhat further,” he says.

So, will QuIET will give a longer life to Moore’s Law, which is threatened by the industry’s eventual inability to shrink the transistor further? “Probably not. It may in fact leapfrog the pace of Moore’s law, if the device could be quickly commercialised,” Mazumdar says. Hence it would go to the endpoint of Moore’s law faster. Still, other experts are not completely convinced about the potential benefits. Supriyo Bandyopadhyay, professor of electrical engineering at Virginia Commonwealth University in Richmond, says that all quantum interference devices are irreproducible, delicate and unstable. Even after nearly 20 years of research not a single potential device exists today. “The present work is no exception,” he argues.

The silicon transistors now used are extremely resilient. “Several years of research and several billions of dollars of investment underpin this behemoth. So far, they have withstood every challenge thrown at them,” says Bandyopadhyay, who also specialises in nanoelectronics. Meanwhile, Semiconductor Research Corporation, an ensemble of semiconductor chip making firms, says that it typically takes a dozen years for a new idea to go from initial scientific publication to commercial technological application. So will QuIET take years to come to market?