Some years ago, I wrote about a prototype microprocessor from IBM that had a clock speed of 1 GHz (see “Tech Page: No GUTS, No Glory” in the May 1998 issue of EM). When processor speeds were in the 300 MHz range, 1 GHz seemed like science fiction, but now, more than eight years later, it is positively poky. It just goes to show how quickly technology marches onward, surpassing even the most optimistic prognostications.
FIG. 1: The black squares at the center of this photo are the SiGe chips that were pushed to 500 GHz clock speeds. They are mounted in a cryogenic test station that cools them to within a few degrees of absolute zero.
With microprocessor clock speeds now in excess of 3 GHz, what can we expect in the future? Once again, IBM is leading the way. In collaboration with the Georgia Institute of Technology, the company has demonstrated a prototype silicon-germanium (SiGe) heterojunction bipolar transistor able to operate at a record-setting speed of 500 GHz (see Fig. 1). That speed is made possible by cooling the device to 4.5° Kelvin (-451° Fahrenheit). At room temperature, the transistor achieves a speed of 350 GHz, which is still two orders of magnitude above common clock speeds today.
“Georgia Tech and IBM have demonstrated that speeds of half a trillion cycles per second can be achieved in a commercial, silicon-based technology, using large wafers and low-cost, silicon-compatible manufacturing techniques,” says John D. Cressler, Byers Professor at Georgia Tech's School of Electrical and Computer Engineering and a researcher at the Georgia Electronic Design Center (GEDC) at Georgia Tech. “This work redefines the upper bounds of what is possible using silicon-germanium nanotechnology techniques.”
IBM first announced its SiGe technology in 1989, introducing high-volume SiGe-based chips in 1998. The idea behind the technology is to enhance the electrical properties of silicon with the addition of germanium, boosting performance while reducing power consumption. Another key factor is the ability to use well-established fabrication techniques to manufacture SiGe chips.
“Having a silicon-based technology that is compatible with low-cost IC manufacturing while still providing these extreme levels of performance allows us to envision integrating these devices into systems that would be affordable for emerging commercial markets as well as defense applications,” says Cressler.
Now that they've achieved this milestone, Cressler's team wants to understand the physics of SiGe devices, which exhibit some unusual properties at extremely low temperatures. “We observe effects in these devices at cryogenic temperatures that potentially make them faster than simple theory suggests and that might allow us to ultimately make the devices even faster,” says Cressler. “Understanding the basic physics of these advanced transistors gives us knowledge that could make the next generation of silicon-based integrated circuits even better.”
The devices used in the current research are based on fourth-generation SiGe technology fabricated by IBM on a 200 mm wafer, using an older, nonoptimized mask set (a series of data that defines the geometry of each step in the manufacture of semiconductor chips). According to Cressler, computer simulations indicate that SiGe could support near-terahertz speeds at room temperature.
It will likely be quite some time before we see terahertz processors in personal computers and other electronic-music devices, but perhaps not as long as some might think. After all, it took eight years to go from 1 to 500 GHz (a factor of 500) in the lab, while commercially available processors went from 300 MHz to 3 GHz (a factor of 10) in the same time. If the increase in clock speed over time is exponential, processors could reach 300 GHz (a factor of 100) in the next eight years.
Cressler believes that SiGe has a lot of potential. “This new speed record provides encouragement to keep pushing forward on silicon-germanium devices,” he says. “There is a lot more fruit available from silicon-germanium technology if we invest the effort to get there.”