Portland, Ore. - The Department of Defense is attempting to leverage silicon-germanium ICs to create a low-cost, high-performance technology to handle radar and communications on earth and radiation-hardened electronics in space.
The program represents a new research strategy for the DOD, which traditionally has funded expensive new technologies that had a low probability of seeing the light of day because of their exorbitant development costs. In a reversal of that trend, the Pentagon now is looking to develop less-expensive solutions with the potential for commercial as well as military applications.
In particular, the program has the potential to lead to the development of cheap silicon-germanium chips for less-expensive weather radar for aircraft or as collision-avoidance radar for automobiles.
"We've teamed up to work on a new approach that could revolutionize the way radar systems are built," said professor John Cressler, an EE in the Georgia Institute of Technology's School of Electrical and Computer Engineering. Cressler heads a team at Georgia Tech Research Institute (GTRI) and the Georgia Electronic Design Center (GEDC). Cressler's GTRI-GEDC group claims to be the largest university-based silicon-germanium research group in the world, with 20 scientists and graduate students involved in research on the emerging technology.
"Silicon germanium is an enabler for rethinking the way business-as-usual is done across a wide array of electronics applications-that makes it really exciting to work on," said Cressler.
The program, called the Silicon-Germanium Transmit-Receive Module Project by the DOD, should dramatically lower the cost of modern phased-array radar systems. And since a specially equipped vehicle would no longer be needed just to transport the antenna, it should also enable the systems to be portable.
If the silicon-germanium chips work as well as the researchers hope, then the naturally radiation-hardened chips could also downsize and lower the cost of critical space applications.
"By putting all the functionality of those complex modules onto a single chip, we are essentially reaching for the same level of integration in radar systems as in consumer electronics," said co-principal investigator Mark Mitchell, a GTRI senior research engineer.
Researchers have been studying silicon germanium as an alternative to gallium arsenide since the early 1980s. The problem has always been that silicon has an indirect bandgap, as opposed to a direct bandgap, like gallium arsenide.
Direct-bandgap materials enable the extra energy to be injected electrically to boost gallium-arsenide electrons into larger, higher-energy orbits. Then when the energized electrons fall back to their normal, smaller-sized orbits, they directly translate the extra energy into emitted photons. Indirect-bandgap material like silicon, on the other hand, tends to shed extra injected energy stored in higher-electron orbits into phonons-lattice vibrations-instead of photons. The Georgia Tech researchers are taking a different tack by doping silicon with germanium so it can imitate a direct-bandgap material.
"Silicon-germanium devices enable conventional silicon integrated circuits to use nanotechnology techniques to introduce germanium inside the silicon on an atomic scale," explained Cressler. "Silicon-germanium layers can double or even triple chip performance with no cost penalty."
Silicon-germanium devices are not likely to replace gallium-arsenide devices for most optical applications, however, because their power-handling capabilities are about 10-fold lower than for high-power gallium-arsenide discrete devices. But for low-power applications like radar, the ability to integrate the silicon-germanium optics on the same chip with pure-silicon electronics could enable developers to dramatically lower the cost.
Potentially, silicon germanium could be 100- to 1,000-fold less expensive, even though silicon-germanium circuits need more devices per function than pure-silicon circuit elements. They also require very large antennas, but since they can be very thin, the U.S. Missile Defense Agency is also funding an effort to make foldable antennas for portable phased-array radar systems.
The natural radiation hardness of silicon germanium, plus its ability to work in the -230 Kelvin of space, has spawned other projects at Georgia Tech. One program for NASA aims to simplify interplanetary space probes as well as landers designed for lunar and Martian exploration.