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Beyond silicon: the processors of the future

What might replace silicon chips when the technology reaches its limits? We investigate the options.

The world’s first microprocessor, the Intel 4004, was launched in 1971. It was a 4-bit design with a clock speed of 740kHz, and contained a single core. Today we have 64-bit chips, clock speeds of 4.4GHz, and up to a dozen cores. This phenomenal rate of change would be awe-inspiring had we not come to expect constant improvements as the norm in the world of computing.

See also: What's so great about Haswell?

Some analysts and scientists are suggesting that such complacency might be misguided as the laws of physics could soon step in and bring a halt to further improvements.

Fortunately, silicon transistors aren’t the only way to make processors, and even the familiar concept of executing instructions sequentially, on digital data, has its alternatives. Here we look at some of these different and, in some cases, bizarre technologies to get a view of what might be driving our computers in a decade or two's time.

You might also like: Haswell laptops: All day, and all of the night

Future processors: the non-silicon alternative

Nanotube

Transistors made from carbon nanotubes have the potential to operate at 1THz. (Photo: Standford University)

A transistor is an electronic component that either amplifies a signal or allows one signal to control another. They form the basis of nearly all electronic equipment, indeed today’s most complicated processors contain no fewer than 2.5 billion transistors. Although the term “silicon chip” is a familiar one, the element silicon isn’t the only substance that can be used to make transistors. Indeed in the early days, germanium was also used.

No-one is suggesting a return to germanium of course, but other semi-metallic elements do look promising when used in mixtures. Intel has been experimenting with several of these compound semiconductors. Mixing elements allows the various electrical properties to be fine-tuned, whereas using single elements provides no such flexibility, and this has provided improved performance compared to silicon.

Back in 2005 the company announced an InSb (indium and antimony) transistor that was five times faster than its silicon counterpart but consumed a tenth of the power. More recently, Intel has used a combination of indium, gallium and arsenic and has referred, tantalisingly, to “very high performing devices”.

While these substances might provide a stop-gap measure, an alternative with the potential for even higher performance – albeit a potential that is by no means guaranteed and much further away – is carbon.

For many years, carbon was known to exist into two forms, namely graphite and diamond. Then, in 1985, Buckminsterfullerene was discovered. This form of carbon has molecules with 60 carbon atoms arranged as a sphere, and many more new forms of carbon have been discovered since. Two which are attracting a great deal of interest are graphene, which comprises sheets of carbon atoms, and a group known as carbon nano-tubes in which the atoms are arranged in cylinders of various sizes. Among their many other uses, both these forms of carbon are able to be used as transistors and can operate much more quickly than silicon.

Silicon and other semi-metallic elements have small amounts of impurities added to them – a process known as doping – to give them the semi-conductor properties needed for them to act as transistors. Some of these esoteric forms of carbon, on the other hand, are inherently semiconducting so don’t need doping.

More significantly, though, an electrical current travels more quickly through graphene than any other known substance. As a result, IBM has demonstrated a 300GHz graphene transistor and experts believe that both these forms of carbon have the potential to operate at 1THz. As yet the transistors are more suitable for analogue electronic circuits, such as those used in mobile phones, than digital circuits, but you can bet that researchers will do their upmost to change all that.

Next page: Optical computers

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