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February - 2003 - issue > Cover Feature
SiGe More Zip Per Chip
Pradeep Shankar
Thursday, January 30, 2003
EVEN AS THE PC CHIP MAKERS INTEL AND AMD are engaged in the so-called gigahertz wars, focus is now beginning to move away from the performance of chips inside individual PCs toward the performance of the chips inside communications devices.

And paving the way for a new class of communication chips is IBM's silicon germanium (SiGe) chip-making technology. Last year, IBM created the world's fastest silicon-based transistor, which operates at a speed of 350Ghz. The transistor, built using the SiGe technology, is said to be nearly 300% faster than today's devices, and 65% faster than previously reported ICs. The key to these fantastic speeds isn't the usual smaller transistors and thinner circuit lines, which have long been the main drivers of ever-increasing chip speed. In fact, this record-setting chip has relatively fat 0.13-micron-wide transistors and lines—sizes that date back to the late 1990s. Instead of smaller lines, the chip’s blistering speed stems from a new silicon-and-germanium recipe.

IBM began developing SiGe materials two decades ago, when industry pundits were warning that pure-silicon chips would top out in the 1990s. IBM is now making the technology, available to top-tier communications equipment makers to help increase the speed of today’s networks.

Communications equipment makers have adopted the SiGe technology for a variety of applications, including RF components in cellular handsets, Wireless Local Area Network (WLAN) chipsets, high-speed test and measurement equipment, and chipsets for optical data transmission systems. Chipmakers such as Agere, Atmel, Conexant, Infineon, Maxim, Motorola, Intel and Texas Instruments have entered the SiGe market, thus increasing the competition with IBM.

However, IBM claims it is still far ahead of the competition-both in current and future processes, and its new 350 GHz transistor leapfrogs the competition. While many companies are introducing their first versions, IBM is in the fifth generation of SiGe technology. However, SiGe technology is suitable only for small chips (communication chips) that have several hundred or a few thousand transistors as compared to several million transistors in PC chips.

For years now, the semiconductor industry has been relying on the gallium arsenide-based (GaAs) and indium phosphide (InP) technology to manufacture microchips. New products based on SiGe technology are now rolling into the market, replacing GaAs and InP products. Transistor speeds are largely determined by how quickly electricity passes through them. This is dependent on the material of the transistor and the distance electricity must travel through it. In standard transistors, electricity travels horizontally, so shortening the path requires that the transistor be made thinner—an increasingly difficult task with diminishing returns using today’s chip manufacturing techniques. Standard transistors are made of ordinary silicon. Traditionally, silicon integrated circuits such as those found in computers, appliances, and many other applications have used Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and Bipolar Junction Transistors (BJTs), but neither of these transistors operate above a few gigahertz because of the material properties of silicon.

SiGe is a process technology in which the electrical properties of silicon, the material underlying virtually all modern microchips, is augmented with germanium to make the chips operate more efficiently. In addition, SiGe provides increased integration capabilities, enabling designers to pack more function onto a single chip, resulting in speed, power, cost and weight savings. SiGe technology can also be used to extend the function and battery life in cellular phones and other radio frequency, or wireless, communications products.

Over the years, IBM researchers have discovered that adding the element germanium to the silicon—typically used in chips—speeds up the flow of electrons on the chip and, consequently, its ability to process information. SiGe technology permits the fabrication of Heterostructure Bipolar Transistors (HBTs) in which the electrical flow is vertical. In IBM's SiGe manufacturing process, the height of the transistor is more easily reduced by thinning the SiGe layer, thereby shortening the path of electrical flow and improving performance. Silicon implanted with germanium can also achieve high frequency. The high-speed switching capability of SiGe has obvious benefits in telecommunications switching and routers. The downside is a slight increase in manufacturing complexity.

But that has not deterred the chip manufacturers. Everyone is racing to add SiGe technology to their arsenal and launch new kind of chips. Arizona-based research firm, IC Insights, in their “2002 McClean Report,” estimates that IBM SiGe revenues grew 86 percent 2001 over 2000, representing more than 80% of total 2001 SiGe business. The report also estimates that SiGe sales totaled $320 million in 2001and are projected to grow to about $2.7 billion by 2006.

In the next few years, as the SiGe technology matures, we will find that wireless networking devices, mobile phones and optical networking gear will have chips based on the new technology. There will be many new players entering the SiGe market. Competition will heat up.

“When we started out a couple of years ago, people said SiGe couldn't go much further, so we had some optimism that we could improve performance then. But in our wildest dreams, we didn't think we could do what we have done so far. IBM’s breakthrough disproves a widely held belief that 100GHz was a fundamental limit for transistor performance beyond which silicon technology would not be able to go,” says Senior Engineering Manager Seshadri Subbanna, who is credited for leading the fastest chip’s development team.

At IBM Microelectronics sprawling campus in East Fishkill, New York, Subbanna overlooks a team of 50 engineers who are now working towards a even faster transistor. “Still faster chips could emerge when IBM gets around to shrinking line widths to 0.1 microns or below, now the cutting edge of semiconductor engineering. Refinements in the chipmaking process have improved Big Blue's ability to control the composition of SiGe layers and reduce their thickness— both keys to the new speed champ. We're very optimistic about future performance improvements,” says Subbanna.

Whether IBM will maintain its leadership position in SiGe or not, the SiGe technology shows promise to bring in a paradigm shift in the semiconductor space.

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