Advantages
It is no secret that silicon devices dominate the microelectronics industry. Indeed, they account for more than 98% of sales in the global semiconductor market, largely because of their low manufacturing cost. However, the driving force behind today’s growth in high-speed optical networking and inexpensive, lightweight personal-communications devices is not silicon but silicon–germanium (SiGe). This technology increases operating speed, reduces electronic noise, lowers power consumption, supports higher levels of integration, and, thus, enables the design of more functional components on a chip.
Thanks to SiGe’s substantial performance benefits, it is quickly becoming the material of choice for both wireless integrated circuits (ICs) and low-power radio-frequency (RF) chips. And the number of SiGe applications is expected to explode over the next few years, with a concomitant increase in chip production. Strategies Unlimited (Mountain View, CA), a market research firm, predicts that sales of SiGe wireless and digital semiconductor devices will increase from $450 million in 2002—up from $15 million in 1999, an annual growth rate of 200%—to $1.8 billion by 2005. By then, SiGe ICs are expected to have captured nearly 10% of the total $19 billion market for high-speed devices in competition with silicon and gallium arsenide chips. Another firm, IC Insights (Scottsdale, AZ), takes an even more bullish view, estimating that the market will grow to about $2.7 billion by 2006.
Applications such as wireless and satellite-based voice and data services are expected to drive 79% of the demand, with highspeed computer networking making up another 16% of the market, according to Strategies Unlimited. SiGe is already widely used in a range of high-speed and lowcost wireless gear, including RF components in cellular handsets, wireless local-area network chipsets, highspeed test and measurement equipment, and chipsets for optical data-transmission systems.
“Wireless is such a huge volume of the market,” says Cliff King, R&D technical manager at Agere Systems, which spun off from Lucent Technologies last year to focus on communications components. “That’s the sweet spot for SiGe technology in terms of turning a profit.” That volume will allow the technology to move into other market sectors in which its benefits outweigh the extra cost incurred in manufacturing. Not surprisingly, telecommunications companies worldwide are taking note. More than 30 companies are developing SiGe ICs. In North America, IBM leads the industry, followed by other giants such as Lucent Technologies, Texas Instruments, National Semiconductor, Nortel, and SiGe Microsystems. A host of smaller component companies are interested in licensing the chips for their products. Overseas, the major players actively investigating SiGe applications are Alcatel, Daimler-Benz, Philips, and STMicroelectronics in Europe, as well as several Japanese corporations such as Hitachi, Toshiba, and NEC.
SiGe (dubbed “Siggie” by industry insiders) involves a revolutionary
process technology in which the electrical properties of silicon are augmented
with germanium to make the chips operate more efficiently. Introducing germanium
into the base layer of an otherwise all-silicon bipolar transistor improves
operating frequency, current, noise, and power capabilities. As such, SiGe
offers a bridge between low-cost, low-power, low-frequency silicon chips and
high-cost, high-power, high-frequency chips made from class III-V semiconductor
materials such as gallium arsenide and indium phosphide.
Scientists first became interested in the compound in the early 1980s, when
researchers at Bell Laboratories discovered that SiGe has a smaller bandgap
than conventional silicon, making it useful for building transistors.
“That is what really got people excited about using this material
for heterostructures, similar to what was being done at the time with III-V
heterostructures,” says King. SiGe technology also maintains the key
advantages of state-of-the-art silicon processing. Unlike the manufacture
of other high-speed semiconductors made of two or more materials, SiGe processing
is relatively simple because silicon and germanium have similar chemical and
physical properties. Initially, SiGe was built using molecular-beam epitaxy,
a labor-intensive process that has never been successfully transferred into
silicon technology, according to King. But by the end of the 1980s, researchers
had learned how to grow SiGe using chemical-vapor deposition. That feat proved
a critical turning point for the material’s commercial potential because
it combines the two elements into high-quality thin films at relatively low
temperatures and cost. And unlike chips created using more-expensive and less-reliable
gallium arsenide, chip-makers can run SiGe on existing production lines with
minimal changes and retooling.
Ordinary silicon does not operate at frequencies above a few gigahertz, which
has hampered the development of higher-speed wireless telecommunications devices,
and manufacturers are beginning to reach the physical limits of what they
can do to improve the performance of ordinary silicon-based materials. In
contrast to silicon-based chips,
SiGe semiconductors have speeds of up to 120 GHz, increasing traditional current
speeds by a factor between 2 and 4. “As the circuitry on chips was continually
scaled down to achieve greater speeds, it became clear that the performance
characteristics of bipolar silicon chips would eventually break down at very
tiny dimensions,” says Jack
Hurt, director of semiconductor foundry relations at Tektronix (Beaverton,
OR), a manufacturer of test, measurement, and monitoring equipment. “If
bipolar silicon were to continue to advance in speed, an alternative technology
was needed.” SiGe gives IC manufacturers the ability to reengineer the
bandgap of the silicon for higher performance, resulting in a two-material,
or heterojunction system that is compatible with silicon technology. Although
silicon and germanium have the same crystal structure, there is a greater
than 4% difference in their lattice constants, according to David Leet, director
of transparent modeling and technical strategic planning for Rudolph Technologies
(Sunnyvale, CA), a metrology instruments company. Introducing the larger germanium
atoms into a silicon lattice causes its structure to expand and become strained
to create the alloy material. It is this stress that changes the bandgap.
