People have been waiting for the end of silicon technology for decades. The smaller things get and the faster things get—its physical limits are a strike against it. But combined with germanium, it has been breaking all the speed records for years.
Silicon isn’t exactly the fastest transistor material. Besides a transistor’s material properties, the thing that influences its switching speed the most is its size. So the tachometer needle is pretty much maxed out. After all, further miniaturization would run up against its physical limitations.
Still, companies have invested billions in silicon technology. And who wants to part with manageable costs, high yields, small dimensions, high integration and proven manufacturing processes? What we need is a technique that can speed up the second most common element after oxygen without further miniaturization.
A good twenty years ago, researchers at IBM discovered that combining silicon with its related element germanium resulted in amazingly fast transistors—and they could be made on existing production systems. Conventional silicon wafers are enhanced by the addition of a silicon-germanium (SiGe) layer.
Incidentally, now proponents of miniaturization are putting their money on germanium. As a result, the “German” semiconductor is improving electron speed in IBMs first 7-nanometer chip (market launch 2017). Intel has announced a similar development.
SiGe setting world records
But let’s reflect on the past. Together with Analog Devices, IBM produced the first commercial products using SiGe (silicon-germanium) bipolar transistors in 1994. At the time—in the infancy of silicon-germanium technology—the maximum switching speed that could be reached was “just” 100 GHz (100 billion cycles per second). The era of world records started after that. And IBM, the Georgia Institute of Technology or the Leibniz Institute for Innovative Microelectronics (IHP) all had a finger in the pie.
2001 Conexant Systems Inc. got the ball rolling in 2001 with SiGe processing technology for transistors with switching frequencies up to 180 GHz.
2002 Just one year later, IBM took back the laurel wreath with cycle times of 350 GHz for SiGe transistors that were manufactured in a 9HP BiCMOS process.
2006 In mid-2006, IBM and the Georgia Institute of Technology took things to the next level and operated SiGe transistors on 200-millimeter wafers at 500 GHz at near absolute zero. They reached nearly 350 GHz at room temperature.
2014 After that, IHP and the Georgia Institute of Technology made the record books with a maximum frequency of 798 GHz—also under “ice cold” conditions near minus 273 degrees Celsius. With speeds of 417 GHz at room temperature, the SiGe transistor is still significantly faster than 98 percent of all currently available transistors. The heterojunction bipolar transistor (HBT) designed by IHP and manufactured using a 130-nm BiCMOS process was made out of a silicon-germanium nanoalloy and embedded in a silicon transistor.
2016 And now in early December, once again IHP announced another heterojunction bipolar transistor that operates at a maximum frequency of 720 GHz at room temperature. The new world record was the result of the DOTSEVEN project funded by the EU. Participants included IHP, Infineon and twelve other partners from six countries. The next SiGe target: the terahertz barrier, what else?