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03/18/94

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Silicon-germanium can push back chip barrier

STANFORD -- Replacing pure silicon with a mixture of germanium and silicon can push back the fundamental and practical limits to making faster and more powerful integrated circuits, without giving up the microelectronic industry's massive investment in silicon technology.

That is the message Stanford electrical engineering research associate Judy Hoyt delivered on Monday, March 21, to scientists attending the spring meeting of the American Physical Society in Pittsburgh.

"The silicon-germanium field has been around for a while. In the late 1980s, several breakthroughs occurred, and today integrated circuits incorporating silicon-germanium are on the verge of commercialization," Hoyt said.

Researchers have shown that the addition of germanium to silicon can increase the speed at which electrons travel through the transistors that form the basic building blocks in integrated circuits. This provides a means by which computer chips and other microelectronic devices can be made faster and more powerful, Hoyt said.

In the past, improvements in the speed of integrated circuits have come largely from miniaturization. But the size of individual transistors in silicon chips is approaching a point that will be very difficult to go below, according to many experts.

Not only is it becoming more and more difficult to etch ever- smaller features onto the chips, but as they are squeezed closer and closer, the interference between individual transistors increases.

Another semiconductor material, gallium arsenide, is even faster than silicon-germanium, but it is also more expensive to fabricate into chips. Compatibility with existing silicon technology is another significant motivation for silicon-germanium's use.

Although silicon-germanium alloys are not as difficult to process as gallium arsenide, they are harder to use than pure silicon, Hoyt said. Because the germanium atoms are larger than silicon atoms, adding them causes strain within the material's crystalline structure. This strain produces beneficial electrical properties, but it also limits the thickness of the layers that can be grown without introducing undesirable defects.

Silicon-germanium is closest to commercialization in the bipolar transistor market, she reported. Bipolar transistors are used in telecommunications equipment, amplifiers and other applications where high speed and high power output are required.

In the last four years, researchers have demonstrated that silicon-germanium transistors can operate about 60 percent faster than those made from pure silicon. In addition to speed, several aspects of bipolar transistor performance have been improved by the addition of germanium to silicon.

Development of metal oxide chips (the type used predominantly in computers), that are based on germanium-silicon alloys is not as far along, but is making significant progress, Hoyt said.

At Stanford, she and her colleagues have produced metal oxide transistors with electron mobilities 180 percent above that of pure silicon. These initial results are very encouraging and indicate a strong potential for future integrated circuit applications, Hoyt said.

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