Silicon is a type of element known as a metalloid from column IV in the periodic table. It's a poor electrical conductor, but can be made more conductive by doping – adding small amounts of elements from adjacent columns in the periodic table.
Doping with a group V element (often phosphorous or arsenic) produces an n-type semiconductor, while doping with a group III element (typically boron) produces a p-type semiconductor. Combining n-type and p-type semiconductor materials allows the basic building block of digital electronics, the MOSFET transistor, to be created.
Silicon isn't the only metalloid with this property; in fact, some of the first transistors were made of germanium. Recently though, attention has turned to using compound semiconductors – that's semiconductors made from more than one metalloid, usually one from group III and the other from group V, although they can also be made from more than two metalloids.
As we look at the performance of new types of transistors (including ones made of materials other than compound semiconductors that we investigate elsewhere) we'll refer to the maximum switching speed, but don't confuse this with processor speeds. Although microprocessors have plateaued at just less than 4GHz, this is by no means a fundamental speed limit for silicon.
Individual silicon transistors have been demonstrated at over 100GHz – the fact that processors are slower than this is mainly due to how difficult it would be for millions of them to work on the same chip without overheating.
Using a single chemical element as a semiconductor doesn't give designers a lot of choice, especially when they have so few to play with. Essentially they're stuck with what's available. But lots of alloys have semiconductor properties and since the constituent elements and their ratios can all be controlled, this allow the various properties (band gap, carrier mobility and lattice constant) to be fine-tuned in the search for the perfect transistor.
As Rob Willoner, Technology Analyst at Intel's manufacturing group explained, "The main advantage of compound semiconductors is better mobility. Electrons flow more smoothly than through silicon, much like a particularly well oiled machine. This higher mobility means faster switching speeds and lower power (think less friction) at the same voltage, at the same speed at a lower voltage, or a combination".
Compound semiconductors like these are usually referred to by the chemical symbols of their constituents, and include GaAs, InP, AlGaAs and GaInNAs.
Intel has a well-established compound semiconductor programme, and in 2005 it announced an InSb (indium and antimony) transistor that was a remarkable five times faster than its silicon equivalent, with a 10th of the power consumption.
More recently, Intel has been working with InGaAs (indium, gallium and arsenic) and, in some variants of the technology, combining it with germanium, but except for referring to "very high performing devices", it has been tight-lipped about what sort of performance it's seen.
With the world's largest manufacturer of x86 processors apparently committed to compound semiconductors, it might seem reasonable to assume that they'll soon be the new silicon. However, Intel's Rob Willoner indicates a rather more cautious technical approach.
"Basically, compound semiconductors are being looked at as a replacement for the silicon channel of transistors – the region where current flows when transistors are on," he says. "They aren't being looked at as a replacement for silicon substrates. Wafers will continue to be made of pure silicon, because that's what the industry has about half a century of experience with."
So what does all this mean to the likelihood of finding a compound semiconductor processor in a mainstream PC in the near future?
Again Rob Willoner was cautious in his outlook. "We have been working on this for many years, and there are many years of work still ahead before this technology gets into production, if ever", he told us.
"A huge challenge is integrating these new materials with the silicon substrate. They have different lattice spacings – that's the spacing between the atoms – and this leads to dislocations at the junction." The research is promising, but domestic applications are a long way off.
Tiny terahertz tubes
For years, two forms of carbon were known: diamond and graphite. This all changed in 1985 with the discovery of Buckminsterfullerene – a form of carbon with 60 atoms arranged as a spherical cage.
Lots of other forms of carbon have been produced since then, and two of them – graphene and carbon nanotubes – are showing potential as the basic material of high-speed transistors.
Graphene is a one-dimensional form of carbon, in which each atom bonds to its four closest neighbours in a hexagonal arrangement to form huge flat sheets. Dr Xiangfeng Duan of the University of California in Los Angeles says it's a wonder material for creating electronic circuits.
"Graphene is of significant interest for highspeed transistor applications because of its combination of several important characteristics, including the highest carrier mobilities of any known material" he told us.
Carrier mobility is the speed at which charge carriers (an electron or a 'hole') propagate through a metal or a semiconductor when an electric field is applied. Partly as a result of this super-high mobility, switching speeds of 300GHz have already been demonstrated, and over 1THz (1,000GHz) is thought to be entirely possible.
But although graphene transistors are fast, they don't switch on and off positively enough, so they're currently being targeted at analogue applications like generating the radio signals for Wi-Fi and other data communication equipment.
Mind the band gap
That doesn't mean scientists have given up on them for logic applications though, and Dr Duan described several initiates that might produce the necessary 'band gap', to use the technical jargon.
"There is still a substantial challenge for application in processors", he said – and it doesn't end with the band gap. "In general, the conventional dielectric integration and device fabrication processes cannot be readily applied to graphene transistors because they can often introduce defects into the monolayer of carbon lattices and degrade device performance."
However, if these obstacles are overcome, the benefits could be huge, as Dr Duan explained. "If implemented for logic applications, it could create a paradigm-shift by moving electronics from a single crystal substrate onto glass or a flexible substrate."
Dr Duan also enthused about faster, lower cost processors and devices with a variety of form factors including flexible, wearable and disposable computing devices. As for whether graphene will supplant silicon, we got a familiar response. "It's going to impact various areas of electronics, but it's difficult to say whether it would eventually replace silicon."
Carbon nanotubes can be thought of as long, hollow cylinders made by rolling up sheets of graphene. In fact there are lots of different types of nanotube, which differ in their diameter and chirality – the angle at which they're rolled up with respect to the molecular lattice. Some types of nanotube are inherently semiconducting, even without doping (the addition of small amounts of other elements needed to turn silicon into a semiconductor).
Fast and efficient
Professor Subhasish Mitra of Stanford University is one of the leading researchers in this field and gave us a feel for what could be just round the corner.
"If you do the correct optimisations, then potentially you can obtain a nanotube processor that can run five times faster than a silicon-CMOS processor at 11nm technology at the same power."
Indeed, figures as high as a terahertz have been suggested as the potential for carbon nanotube transistors, though the Stanford researchers are more concerned with the density at which nanotubes can be grown to form a chip, making the design immune to imperfections, making good connections to the nanotubes and engineering the doping process.
Professor Mitra was quick to point out that good progress is being made. Indeed, moving on from the individual transistors demonstrated several years ago, the team at Stanford have managed to create working arithmetic circuits and memory, and have developed techniques for building 3D chips.
"I think we have come a long way, and this is an exciting time", Professor Mitra said. "Carbon nanotube technology should have the ingredients to be a great complement to silicon CMOS."