Research by IBM and the Georgia Institute of Technology suggests that the upper bound for performance in silicon-germanium (SiGe) devices may be higher than originally expected—and they have evidence to support their claim. The joint research team has demonstrated what it bills as the first SiGe transistor able to operate at frequencies above 500 GHz.
The SiGe heterojunction bipolar transistors operated at frequencies above 500 GHZ at 4.5 K, a temperature attained using liquid helium cooling. Computer simulations suggest that the germanium chip could ultimately support even higher (near terahertz) operational frequencies, even at room temperature (see the figure).
Ultra-high-frequency SiGe circuits have potential applications in many communications systems, defense systems, space electronics platforms, and remote-sensing systems. Achieving such extreme speeds in silicon-based technology, which can be manufactured using conventional low-cost techniques, could provide a pathway to high-volume applications. Until now, only integrated circuits fabricated from more costly III-V compound semiconductor materials have achieved such extreme levels of transistor performance.
The devices used in the research are from a prototype fourth-generation SiGe technology fabricated at IBM on a 200-mm wafer using an older un-optimized mask set. The experiments are part of a project to explore the ultimate speed limits of SiGe devices, which operate faster at very cold temperatures.
The frequency experiments are part of a larger project to explore the ultimate speed limits of silicon germanium. The next step in the research will be to understand the physics behind the SiGe devices, which display unusual properties at such extremely low temperatures. John Cressler, a professor at Georgia Tech’s School of Electrical Engineering, said that the team observed effects in the devices at cryogenic temperatures that would potentially make them faster than simple theory would suggest.
SiGe technology has been of great interest to the electronics industry because it fuels substantial improvements in transistor performance while using fabrication techniques compatible with standard high-volume silicon-based manufacturing processes. By introducing germanium into silicon wafers at the atomic scale, engineers can boost performance while retaining the many advantages of silicon.
