Germanium atoms are creeping back into a silicon-dominated world, a half century after the happy marriage of silicon dioxide to silicon-based transistors left germanium out in the cold.

Early transistors were made in both germanium and silicon, until silicon dioxide proved such an excellent insulator that it moved the industry toward silicon.

Silicon got the edge when, in early 1955, Carl Frosch, a veteran diffusion chemist at Bell Labs, accidentally ignited hydrogen gas, introducing water vapor into the diffusion chamber. The result, say authors Michael Riordin and Lillian Hoddeson, was a green silicon dioxide film that formed a "smooth, hard, protective layer on the silicon surface that kept it from eroding, pitting or evaporating away." They describe the scene in Crystal Fire, a wonderful history of those early pioneers and the early efforts at Bell Labs to develop transistors.

Germanium's return is part of a wider transformation in materials engineering that includes SOI wafers, SiGe BiCMOS, strained-silicon channels and new dielectrics at the interconnect and the gate.

The strained-silicon research offers a way to significantly enhance carrier mobility. MIT's Gene Fitzgerald, Judy Hoyt and Dmitri Antoniadis, IBM Microelectronics' Ken Rim and Jeff Welser, and others are working to build alloys of silicon and germanium that strain the upper silicon lattice.

Fitzgerald, a founder of strained-silicon intellectual-property provider AmberWave Systems, said exploratory work at MIT shows the way toward 400 percent improvement in CMOS stage delays. By adding compressed germanium between the active silicon and the relaxed SiGe layer, the MIT group says it has created "dual-channel" PMOS devices with encouraging mobility enhancement. "The compressed germanium results in less scattering of the holes," he said.

The goodness of adding germanium to a silicon platform is that it may be less expensive to create these more-complex materials stacks, compared with the advanced equipment needed to shrink device dimensions.

Indeed, we're less than a year away from buying Intel's 90-nanometer processors with strained-silicon channels, part of an important revolution in materials. Carl Frosch would be pleased.

David Lammers covers SoC process equipment. contact him at dlammers@cmp.com.