Having moved the silicon-germanium (SiGe) process out of the labs and onto the manufacturing floor a few years ago, researchers at IBM Microelectronics have reformed the structure of the SiGe heterojunction-bipolar transistor (HBT) to catapult its cutoff frequency (fT) to 210 GHz. At this speed, the new HBT is almost twice as fast as the previous generation. This dramatic improvement comes from thinning the transistor's vertical transport layer and collector depletion region.
According to the company, the thickness of the transport layer has been reduced by half, to about 0.025 µm (see the figure). "We can realize this thin vertical layer using conventional lithography equipment," says Greg Freeman, senior engineering manager for advance technology definition and evaluation at IBM's Communications R&D Center in Fishkill, N.Y.
This ultra-thin film has been deposited using low-temperature epitaxial methods employing ultra-high vacuum chemical vapor deposition. The real achievement, Freeman notes, is in the fact that the ultra-thin film has been maintained throughout the HBT's fabrication process.
At this switching speed, the SiGe HBT draws about a milliampere of current, representing an 80% performance improvement and a 50% reduction in power consumption over current designs. Implemented in 0.18-µm design rules, the device offers a breakdown voltage of 1.8 V, with an open base. However, Freeman ex-plains, the device's usable voltage is 3.5 V with a realistic impedance in the base region.
The company believes this transistor will drive communications and networking chips to 100 GHz within two years. This HBT also will deliver this high speed while consuming low power. Top-tier communications equipment makers are already working with the new technology to build faster, more efficient products to increase the speed of today's networks, the developer says.
Researchers at IBM hope to achieve device-level qualification for the latest SiGe biCMOS process by the end of next year. Production is planned for sometime in 2003 at the company's facility in Burlington, Vt.