Combining Two Types Of Transistors Results In Improved Circuits

July 15, 1997

CHAMPAIGN, Ill. -- Researchers at the University of Illinois have developed a method of combining two types of transistors in high-performance devices with a variety of applications, including wireless products and optical communications.

"Compared to circuits employing only conventional depletion-mode technology, circuits that utilize both enhancement-mode and depletion-mode technology enjoy several advantages, including a reduction in both chip size and power consumption," said Ilesanmi Adesida, a U. of I. professor of electrical and computer engineering and a researcher in the university's Microelectronics Laboratory. "In addition, the integration of both types of transistors allows for single power-supply operation in circuits, which is difficult to achieve with depletion-mode-only technology."

The new devices were fabricated on an indium phosphide heterostructure grown by molecular beam epitaxy. "The excellent electrical properties of these devices ­ coupled with the small variation in threshold voltage across the wafer ­ make them ideal candidates for integration in large-scale integrated circuits," Adesida said.

Because devices built with depletion-mode technology are normally "on" at all times, they continuously consume power and generate considerable quantities of heat that must be dissipated. Devices based upon enhancement-mode technology, however, are normally "off" when not in use. Only when the device is actually being used does it consume power and generate heat.

An integrated circuit combining enhancement-mode and depletion-mode high-electron-mobility transistors requires less power, generates less heat and results in a smaller chip. These

high-performance devices are ideally suited for high-speed digital applications.

"We have demonstrated that we can put both types of transistors together in a simple integrated circuit," Adesida said. "Now we are pursuing more complicated circuits. Our ultimate goal is to combine a photodetector, amplifier and digital processor in a single chip."

Such a device would be extremely useful for future high-speed, low-power, optical communications networks.

Adesida's research team included graduate students Aaditya Mahajan and Mohamed Arafa, and visiting postdoctoral researcher Patrick Fay. In addition to their work in indium phosphide, the researchers also are developing high-performance field-effect transistors in other heterostructure materials, including silicon-germanium.

The team's latest findings appear in the June and August issues of Electronic Device Letters.

University of Illinois at Urbana-Champaign

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