Breaking the digital speed limit

2 min read

A new world record in silicon-germanium chip speed brings the prospect of a step-change in digital devices closer. This heralds a class of low-energy chips for applications that will deliver DVD-quality video to mobile phones, boost vehicle intelligence and dramatically increase the capability of networks.

IBM and Georgia Institute of Technology have demonstrated the first silicon-germanium (SiGe) based chip capable of operating at 500GHz, 250 times faster than those used in today’s mobiles. The laboratory experiment cryogenically froze the chip to 4.5K, a fraction above absolute zero, which allowed electrons to flow rapidly through the doped material.

Even at room temperature the chip operated at a staggering 350GHz, and the researchers believe the technology could
ultimately support near-terahertz frequencies (1,000GHz).

Achievable

‘For the first time Georgia Tech and IBM have demonstrated that speeds of half a trillion cycles/second can be achieved in a commercial silicon-based technology, using large wafers and silicon-compatible low-cost manufacturing techniques,’ said Georgia’s Prof John Cressler.

‘There have been theoretical simulations made by us, IBM and others showing the chip could operate at 500GHz or perhaps even higher, but nobody had demonstrated it before,’ he said. ‘This work redefines the upper bounds of what is possible using silicon-germanium nanotechnology.’

With this record under his belt, Cressler is bullish about the future. ‘It shows that there’s a lot of steam left in silicon technologies. We have to understand better what is limiting operating speed, and already at low temperatures we see some very interesting physical phenomena — because at these temperatures they become exaggerated,’ he said. ‘Then we have to take what we learn and use it to improve the semiconductors. It is probably several years before half-terahertz chips become widely available, but this is an encouraging result.’

It will also fulfil one of Gordon Moore’s lesser-known predictions. The co-founder of Intel is famous for having said that the number of transistors would double every 18 months (Moore’s Law) but he also observed that the number of instructions per second performed by a chip will also double every 18–24 months.

The chips used in the research are from a prototype fourth-generation SiGe technology fabricated by IBM on a 200mm wafer. By doping silicon with germanium the lattice structure of the molecules allows electrons to move much more freely than through silicon
alone.

The trick is to get the right quantities of the two materials and the structure. While it is currently more expensive to fabricate than pure silicon, it is said to be more cost-effective than III-V compound semi-conductor materials that have already been demonstrated at such high operating speeds.

Innovation

IBM has shipped hundreds of millions of SiGe chips since it began producing them in high volume in 1998. ‘This groundbreaking collaborative research by Georgia Tech and IBM redefines the performance limits of silicon-based semiconductors,’ said Bernie Meyerson, chief technologist of IBM Systems and Technology Group.

‘IBM is committed to working closely with its academic and industry partners to deliver the insight and innovation that will enable a new generation of high-performance, energy-efficient microprocessors.’

The Georgia Tech group has acquired a vast amount of expertise in SiGe chips through working both with IBM and NASA. Cressler’s team has worked with Boeing to develop SiGe chips that will operate in the coldest lunar environments (The Engineer 19 June), and Cressler said that the NASA work and this research with IBM were ‘very synergistic activities’.