Slimmer chips handle fast nets

By Kimberly Patch, Technology Research News

IBM researchers have built a bipolar transistor that will potentially double the speed of the chips that connect optical networks to electric computers. The transistors could also be used to lower power usage in devices like cellular phones.

The transistors could boost chip speeds to as fast as 100 gigabits per second, according to Seshadri Subbanna, a senior engineering manager at IBM Fishkill and IBM's T. J. Watson research center. One hundred gigabits translates to enough speed to feed 1.25 billion characters -- or about 300 books -- into an optical fiber every second.

The prototype operates at 210 gigahertz, or cycles per second, which breaks a theoretical speed barrier of 200 gigahertz for silicon based transistors, Subbanna said. "200 gigahertz was thought to be some fundamental limit that silicon had, based on calculations. This proves that the barrier isn't really there at all," he said.

The transistor measures 50 nanometers across and is made from silicon germanium. The researchers increased its speed by making it thinner and adding more germanium to the mix, said Subbanna. Thinner transistors are faster simply close it takes electrons less time to pass through them. The transistor uses one milliamp of power to drive the electrons through at a speed of 210 gigahertz.

Alternatively, less power can be used to drive electrons through the transistor at slower speeds, said Subbanna. "There are two ways to use this technology. You can use the same power to do much faster processing, or use the same chip at much less power," he said. The transistor uses 20 percent of the power of its silicon germanium predecessor and 50 percent of the power of transistors made from indium phosphate, which are also common in high-speed networking devices, he said.

Unlike the etched silicon transistors commonly used in personal computers, the bipolar silicon germanium transistors are grown. The key to making faster transistors out of the material is growing thinner layers that are free of defects. The researchers were able to grow thinner silicon germanium wafers by lowering the temperature, which decreased the rate of growth, allowing the researchers to more finely control the process.

Bipolar transistors are used in devices whose transistors are either all on or all off, like the transistors that control the lasers that send pulses of light down optical networking equipment, the network interconnects that join networks, and the chips that control cellular phones.

This type of transistor is often made from hybrid materials like silicon germanium or indium phosphate and electrons travel through the mix more quickly than through the plain silicon used in standard computer processors. The new transistor is about five times faster than an equivalent transistor made of silicon and double that of the fastest indium phosphate transistors on the market, said Subbanna.

The speed boost is important in applications that interface with faster optical communications equipment. "They're used for driving the lasers and detectors the go into optical fiber networks. So as soon as the light gets converted to electricity -- that's when these chips come in," he said.

Because fiber optic lines can carry more information than wire, there has always been a fire hose problem in converting the light signals used by fiber optics to the electric signals used by electronics. Just as it's difficult to drink from a fire hose due to the abundance and speed of the water coming out, electronics has trouble keeping up with optics. Optical communications equipment regularly achieves speeds of 160 gigabits per second and has been pushed to about seven terabits per second in the lab. Faster electronics helps to close this gap.

Bipolar transistors have two electrodes: an emitter, or source and a collector, or drain. Current flows from the emitter to the collector through the base, in this case silicon germanium. The more complicated field effect transistor (FET) used in personal computers has a third electrode, which controls the flow of electrons through the transistor and allows it to be turned off independently from the other transistors around it. "Ninety-nine percent of the transistors in the microprocessor are always off so you save power when you do it that way. Network interconnects are very different, the transistors are always working so there's no advantage [to using more complicated] FETs," he said.

The results are impressive and surprising, said Mark Lundstrom, a professor of electrical and computer engineering at Purdue University. "It had been commonly thought by most of us that silicon -- and silicon germanium is basically silicon -- would top out at about 100 gigahertz, and beyond that you have to go to other materials such as indium phosphide. I had assumed that there would be a lot of work to try to push the performance up to 100 gigahertz and maybe a little bit beyond, but to go this far beyond is very interesting," he said.

Being able to double the speed of silicon germanium at the same power "means you can take well developed low-cost silicon technology and continue to use it in the regime where it was commonly thought you'd have to go to much more expensive technologies," Lundstrom added.

The researchers are currently working on integrating the new generation of bipolar transistors into circuits. "We have prototype transistors and we are working with customers to design circuits with them and we expect to demonstrate those circuits, which would be basically the building blocks of network [interconnects] by the end of the year," Subbanna said.

The new circuits could be put to practical use in optical networks within two years, and in cellular phones within two to three years, Subbanna said.

Subbanna's research colleagues were Bernard Myerson and Greg Freeman of IBM Research. The research was funded by IBM.

Timeline:   2 years
Funding:   Corporate
TRN Categories:  Semiconductors; Integrated Circuits
Story Type:   News
Related Elements:  


June 27, 2001

Page One

Chemists concoct tiny lasers

Slimmer chips handle fast nets

Prototype transistors promise speedy chips

Molecules make short-term memory

Micromachine parts relax into place


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