US military plays the integration game

A critical driver in the development of compound semiconductors in the US has been the US Department of Defense s (DoD) need for next-generation communications, radar and power systems. The Army, Navy and Air Force, along with the Defense Advanced Research Projects Agency (DARPA), often require the DoD to sponsor the research and development work needed to build these systems.

Once those technologies have been developed for military needs they inevitably make their way into the commercial sector, opening up whole new industries. A prime example of this phenomenon was the development of GaAs through the DoD s Microwave and Millimeter Wave Monolithic Integrated Circuit Program in the 1980s and 1990s, leading to the creation of a wide spectrum of commercial GaAs companies that are now key players in the ever expanding wireless marketplace.

More recent DoD programs have used InP, SiC and GaN, and devices based on these material systems are starting to make a commercial impact. Therefore, if you want to get a peek of the new technologies and capabilities that will be developed in the compound semiconductor arena, history has shown what a good indicator the DoD s future programs and applications can be.

Laura Rea is the technology development lead of the Electronic and Optical Materials (EOM) branch within the Air Force Research Laboratory s (AFRL) Materials and Manufacturing Directorate, located at Wright-Patterson Air Force Base in Dayton, Ohio. Not only is Rea involved in DARPA s current wide-bandgap semiconductor technology initiative (see web link at the end of this article), but her branch is one of several military research and development offices that will help shape the type of compound semiconductor solutions sought to fulfill the Air Force s future mission. But just what is the mission of the Air Force s EOM branch?

Rea responds in no uncertain terms: "Complete and total domination of the electromagnetic spectrum for Air Force requirements." Her branch s role in that mission addresses the needs of three primary DoD customer communities: "First the intelligence, surveillance and reconnaissance community – where electrons and photons play a critical role," she says, "second, the communications and computations community where we must move decision-quality information quickly and effectively and get it in the hands of operators in real time. And third, the emerging area of directed energy."

With similar directness, Rea explains the approach used to meet the Air Force s need for next-generation semiconductor-based devices and systems: "Bulk and epi," is how she characterizes the DoD s historical examination of a new material system, looking at features such as breakdown voltages, carrier mobilities and thermal conductivities. If those characteristics led to electronic and photonic devices with the sort of enhanced performance that the DoD needed for its next-generation systems, it would initiate a development program focused on developing those solutions.

The GaAs materials research program outlined above is a good example: what was first considered as an exploratory research program evolved into a Defense Production Act Title III program, aimed at improving the quality and increasing the diameter of GaAs substrates. Sandwiched between these programs was DARPA s MMIC effort, where epitaxy developments resulted in the fabrication of advanced MESFET, HEMT and HBT devices. This is the traditional "bulk and epi" technique that Rea refers to and this model has since been repeated with other material systems – the most recent being GaN.

So, after GaN, which new material system will enable the Air Force to "dominate the electromagnetic spectrum," as Rea puts it? The antimonides, ZnO or perhaps diamond could be considered as candidates. The reality shows a sea-change in the DoD s thinking, however. While Rea says that advanced material systems such as these will be investigated, she believes that there will not be a single next-generation material system per se.

In fact, she sees a paradigm shift occurring not only within the Air Force but also within the other services and DARPA. The approach will no longer be the tried-and-tested bulk and epi, explains Rea. "We are very clearly headed into the realm of multifunction, integrated, miniaturized components." Rather than one single material, this means different materials and different devices folded into a single component. What might this component look like? "We need products that can carry out some level of silicon-like processing, and are able to communicate and execute some level of sensing," says Rea. "In other words, we are talking about a component that that can see, talk and think."

While she feels that this goal requires some form of materials integration, Rea says that it is not yet clear which path will best achieve this. "Will it be wafer bonding, direct-write epitaxy or will it be..." Rea hesitates because she is about to mention a much-discussed and forever promised buzzword, "system-on-a-chip?" In all likelihood, she admits, different approaches will be adopted for different materials, devices and applications.

These approaches may ring of "been there, done that", and resurrect images of the multiple failed attempts at GaAs-on-silicon, which often showed promise but were unable to make the jump to either military- or commercial-grade devices because of defect-related reliability issues. However, as Rea points out, things are different today. "Materials technology has advanced and we need to bear in mind that new approaches may solve old problems."

Reassuringly, Rea is also quick to point out that the Air Force is not moving away from the wide-bandgap approach, rather that its development program will evolve toward multifunctional materials research. Future programs will look to integrate the best devices, using the optimum materials to maximize performance, as well as new materials and processes – not all of them traditional semiconductor approaches – into "all-seeing", next-generation products.

"There are other classes of materials that we are also exploring, including biological materials and processes. For example, we are starting to explore the integration of DNA-based photonic devices with inorganic semiconductor technology," says Rea.

Much like the term itself, some might characterize the DoD s paradigm shift as "everything that is old is new again". But this would be unfair; Rea is a firm believer in the wisdom of the poet-philosopher George Santayana – that those who forget the past are condemned to repeat it. In her world, that would mean wasting too many precious research dollars reinventing the wheel. "Besides, it s one of the perks of being an old timer – a form of job security!" she laughs.

DARPA s wide-bandgap programs (SiC and GaN development aimed at both RF/microwave/millimeter-wave technology and high-power electronics): www.darpa.mil/mto/wbg/.