A Posteriori

Attempts to grapple with and elucidate empirical knowledge

Graphene ICs around the corner? January 3, 2010

Filed under: Electronics,Semiconductors — Rāhul @ 13:51

2009 marked the 50th anniversary of the Physicist Richard Feynman’s speech to the American Physical Society in 1959 where he foresaw the coming age of nanotechnology. Much of his vision of atomic level fabrication and nanoscale surgical robots in the bloodstream are yet to be realised. However, the Electronics industry has surged relentlessly down this path, following the now famous Moore’s law which says that the number of electronic building blocks called transistors on a chip roughly doubles every two years.

Ever since 1965 when Gordon Moore, the co-founder of Intel, foresaw this exponential decrease in the sizes of individual transistors on a computer chip, this unbridled advance of electronics has allowed progressively more powerful and compact computers. But as applications and user aspirations demand ever more powerful computers, Moore’s law is finally coming against the brick wall of Quantum Physics. Transistors on today’s chips are only of the order of a millionth of an inch, nearing the subatomic level where large scale physical laws break down and the uncertainties of Quantum Mechanics take over.

Scientists are now looking at ways to redesign the transistor in such a way as to take advantage of the quantum laws. All such approaches mean a shift from Silicon, which is used near-universally today. One promising new material is Graphene which is a one atom thick sheet of Graphite, one of the pure forms of elemental Carbon.

Graphene was only isolated in 2004 by peeling off a layer from Graphite using scotch tape. Since then, ways have been found to grow Graphene on Silicon Carbide and to deposit it from a solution. The thermal and electronic properties of Graphene have been found ideal for transistor operation. Current carriers in Graphene can travel very fast while picking up very low noise. Very high ability to conduct heat also makes it attractive in densely packed integrated circuits which need to dissipate heat efficiently.

Integrated circuits found on computer chips are essentially a clever combination of millions of transistors and other electronic elements in a logical circuit that achieves a certain output depending on the inputs. Individual transistors, connected to form electronic switches lie at the heart of the circuit. Mechanical switches can be flipped on and off to control the flow of current through a circuit. Similarly, transistors have a channel through which the flow of current is regulated by the voltage at a terminal called gate. When the gate voltage is flipped between high and low, the transistor channel can be flipped on and off. By cascading many thousands of such switches or gates, complicated logical operations can be performed electronically.

Although single transistors of Graphene had already been demonstrated, it was only recently that a Graphene Gate was demonstrated. Floriano Traversi and Roman Sordan from Politecnco di Milano and Valeria Russo from the Department of Energy, Micro and Nano structured materials laboratory, both in Italy reported in Applied Physics Letters, their demonstration of a Graphene Inverter. An Inverter is the most basic logic gate, which gives a low output if the input is high and vice versa, essentially inverting the input at the output.

The researchers deposited two adjacent Graphene layers on a Silicon substrate cove red with oxide to form the channels of two different transistors. Metal contacts were then formed behind the Silicon substrate to act as the controlling gate and in between the Graphene layers so that they were connected end to end. In an ingenious step, they then electrically annealed just one of the graphite channels so that the gate voltage at which it flips from off to on changed. If a constant voltage is now applied across both the transistors together, the output voltage at the terminal they share will depend on which transistor is off and which is on. By controlling the gate voltage, the scientists then controlled the channel conductivities and hence the output voltage.

Tying together many such switches in a circuit similar to the Silicon Integrated Circuits, much faster computer chips with lesser noise can be obtained. But this technology is yet only in a nascent stage. Output voltage from the demonstrated gate does not switch between values that can be directly fed to another gate input. Also, unlike today’s Silicon switches, this Graphene gate cannot be turned fully off shutting off current completely. So, they dissipate power even while not switching.

Silicon electronics has matured today after continuous improvements over half a century. The intense research interest and rapid progress in Graphene based devices and this demonstration of the feasibility of Graphene integrated circuits indicates that post Silicon Electronics might be just around the corner. Moore’s law will probably continue its resilience into the near future.