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New Transistor First to Work at Speeds Above 500 GHz

Photonics.com
Jun 2006
ATLANTA, June 20, 2006 -- The development of the first silicon-germanium transistor able to operate at frequencies above 500 GHz -- a speed 250 times faster than the chips powering today's cell phones -- could lead to the development of communications and other systems that operate at very high speeds and are relatively inexpensive to manufacture, a researcher involved in the project said.GATechchip3.jpg
Georgia Institute of Technology Professor John Cressler and Phd student Ram Krithivasan examine a silicon-germanium chip inside a cryogenic test station at Georgia Tech's Georgia Electronic Design Center (GEDC) in Atlanta.

The record performance was attained by a research team from IBM and the Georgia Institute of Technology. Although their work was done at extremely cold temperatures, the researchers said, the results suggest that the performance limits in silicon-germanium (SiGe) devices may be higher than originally expected.

"For the first time, Georgia Tech and IBM have demonstrated that speeds of half a trillion cycles per second can be achieved in a commercial silicon-based technology, using large wafers and silicon-compatible low-cost manufacturing techniques," said John D. Cressler, Byers Professor in Georgia Tech's school of electrical and computer engineering and a researcher in its Georgia Electronic Design Center (GEDC). "This work redefines the upper bounds of what is possible using silicon-germanium nanotechnology techniques."

Silicon-germanium technology, first introduced by IBM in 1989, is a process technology in which the electrical properties of silicon, a material used in virtually all modern microchips, is augmented with germanium at the atomic scale to make chips operate more efficiently. SiGe boosts performance and reduces power consumption in chips used in cellular phones and other advanced communication devices and can be made using standard high-volume, low-cost silicon-based manufacturing processes, making it of great interest to the electronics industry, the researchers said.

Ultrahigh-frequency SiGe circuits have potential applications in communications systems, defense systems, space electronics platforms and remote sensing systems. Achieving such extreme speeds in silicon-based technology could provide a pathway to high-volume applications, the researchers said. Until now, only integrated circuits (ICs) fabricated from more costly III-V compound semiconductor materials have achieved such extreme levels of transistor performance.
 
"This new speed record provides encouragement to keep pushing forward on silicon-germanium devices," Cressler said. "There is a lot more fruit available from silicon-germanium technology if we invest the effort to get there."

The SiGe heterojunction bipolar transistors built by the team operated at frequencies above 500 GHz at 4.5 kelvins (-451 °F), a temperature attained using liquid helium cooling. At room temperature, they operated at approximately 350 GHz. Performance measurements were made using a specialized high-frequency test system at GEDC.

GAtechChip.jpg
A close-up view of silicon-germanium chips (black squares) inside a cryogenic test station in Georgia Tech's GEDC. The facility can cool electronic devices to temperatures near absolute zero.
The devices used are from a prototype fourth-generation SiGe technology fabricated at IBM on a 200-mm wafer using an older unoptimized mask set. Simulations suggest that the technology could ultimately support much higher (near-terahertz) operational frequencies at room temperature, Cressler said.

"Having a silicon-based technology that is compatible with low-cost IC manufacturing -- while still providing these extreme levels of performance -- allows us to envision integrating these devices into systems that would be affordable for emerging commercial markets as well as defense applications,” Cressler said.

The next step in this research, he said, will be to understand the physics behind the SiGe devices, which display some unusual properties at these extremely low temperatures.

"We observe effects in these devices at cryogenic temperatures which potentially make them faster than simple theory would suggest, and may allow us to ultimately make the devices even faster,” said Cressler. “Understanding the basic physics of these advanced transistors arms us with knowledge that could make the next generation of silicon-based integrated circuits even better.”

The research team also included Georgia Tech PhD students Ramkumar Krithivasan and Yuan Lu; Jae-Sun Rieh of Korea University in Seoul, South Korea (formerly with IBM); and Marwan Khater, David Ahlgren and Greg Freeman of IBM Microelectronics in East Fishkill, N.Y.

The project, which also received some support from NASA, will be reported in the July issue of the journal IEEE Electron Device Letters. For more information, visit: www.gatech.edu


GLOSSARY
transistor
An electronic device consisting of a semiconductor material, generally germanium or silicon, and used for rectification, amplification and switching. Its mode of operation utilizes transmission across the junction of the donor electrons and holes.
wafer
A cross-sectional slice cut from an ingot of either single-crystal, fused, polycrystalline or amorphous material that has refined surfaces either lapped or polished. Wafers are used either as substrates for electronic device manufacturing or as optics. Typically, they are made of silicon, quartz, gallium arsenide or indium phosphide.
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