Bijan Davari has worked with semiconductors for most of his adult life. Born in Tehran, Iran, he received his Ph.D. from Rensselaer Polytechnic Institute, where he completed his thesis on the interface behavior of semiconductor devices. On joining IBM’s Thomas J. Watson Research Center in 1984, he began working on ways to improve Complementary Metal-Oxide-Semiconductor (CMOS) technology, which provides the basis for much of today’s semiconductor processing.In 1985, Davari began the task of defining IBM’s next generation of CMOS integrated circuits, which came to be called CMOS-5X. One approach to achieving the goal of integrating more transistors onto a single piece of silicon is to make the devices smaller. However, simply scaling down the dimensions of a device won’t work if the voltage powering the circuits remains the same. The heat generated by the densely packed transistors at higher voltages would become excessive, and the higher voltage in itself would cause reliability problems, resulting in the malfunctioning of the devices. So as devices grow smaller, their working voltages have to be reduced.
At the time Davari started, the emerging industry standard voltage was 3.3 volts. Meanwhile, several technological advances made by Davari and his colleagues were pointing the way to a new voltage standard. Dual-polysilicon gates provided better control of current through small devices, while shallow-trench isolation offered a way to build electrical separators between transistors to allow for greater circuit densities. “These innovations removed the barriers to reducing voltages and enhanced the packing densities,” explains Davari.
Beyond those barriers, the trick was to determine a new standard voltage to provide low power and adequate reliability without sacrificing performance. The answer emerged quickly. “The sweet spot for the 0.25-micron devices was 2.5 volts,” Davari recalls. That voltage not only generated less heat, it also produced a device that was more reliable. Indeed, the device actually gained in performance. “We changed the perception in the industry,” says Davari. “We showed that, if the geometries were scaled down properly, you could scale down the voltage and gain significant performance.”
IBM introduced CMOS-5X in 1994. It serves as the basis for the PowerPC® 601+ and at least nine other microprocessors, including the microprocessors used in the latest announcement of the IBM System/390 servers.
Today, along with the advanced CMOS development, Davari spends his time on system scale integration, figuring out how to use all the transistors that can now be integrated onto a chip. “We are moving toward putting a whole system on a single chip,” says Davari. “With this kind of integration, the gains in performance can be substantial. You could create tiny devices that connect you to huge networks. Also, high-end servers will be significantly affected because most of the hardware just goes onto a very few silicon chips.”