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Semiconductor Safari: Exotic Materials Beyond Silicon

At this moment, the semiconductor industry is justifiably focused on the impending inflection in Moore’s Law, the famous technology projection that underlies the astonishing progress of the microelectronics era, and its potential impact on the continuing advance and dominance of silicon technology. That means it’s worthwhile to consider another visionary perspective in the history of microsystems. As the nascent Advanced Research Projects Agency (ARPA) passed its one-year anniversary in 1959, Prof. Richard Feynman at Caltech delivered one of his most famous and consequential talks, titled “There’s plenty of room at the bottom.”

In particular, gallium arsenide (GaAs) and its alloys emerged as new wonder materials that allowed transistors to operate well beyond the performance limits of silicon.

Similar to Gordon Moore, Feynman anticipated many of the opportunities for technological advance that lie within the realm of microscale systems. However, Feynman took a much broader view that highlighted the exotic possibilities that would emerge with the ability to manipulate structures at the atomic scale. DARPA has played a central role in bringing many of these “exotic” structures, including semiconductors, to life with capabilities beyond the binary-processing feats that silicon electronics have been pulling off for half a century.

Feynman’s talk inspired a resurgent interest in the 1980s as his speculative notions of nanotechnology and the ability to tailor materials at the atomic scale were becoming tantalizingly close to realization. At that time, emerging crystal growth techniques were enabling the creation of a class of materials known as compound semiconductors, where the exact chemical composition or alloy could be varied at the atomic level on a layer-by-layer basis. In particular, gallium arsenide (GaAs) and its alloys emerged as new wonder materials that allowed transistors to operate well beyond the performance limits of silicon. DARPA identified the potential for the new GaAs transistors to move electrons faster and therefore operate at higher frequencies in the electromagnetic spectrum. While this new technology would not displace silicon technology for highly integrated digital logic, DARPA anticipated its value to enable the next generation of radar and communications systems. To that end, DARPA in 1988 took over the baton from the Office of the Secretary of Defense (OSD) to run the Microwave and Millimeter Wave Integrated Circuit (MIMIC) program, which OSD had stood up two years earlier.

Precision Fires-Dismounted system DARPA-web

A soldier with 3-6 FA 1BCT 10th Mountain Division, 1st Brigade Combat Team, 10th Mountain Division, navigates the new Precision Fires Dismounted system that allows soldiers to view live-streaming full-motion video from unmanned aerial vehicles through an app on approved smartphones. The ubiquitous cellphone technology used worldwide owes its existence in part to the development of the GaAs semiconductor industry made possible by DARPA-funded research.

The MIMIC program, which ran until 1995, had a profound impact on industry, as it sought to develop the ways and means of integrating higher-frequency materials and components into military-relevant technologies, such as radio and radar, and to establish a reliable industrial base to do those things. In fact, the MIMIC program was able to realize GaAs transistor technology that led to a new class of RF (radio frequency) “front end” components. The front end of an RF system is the amplifier technology that sends and receives signals in the electromagnetic spectrum. DARPA’s MIMIC technology, particularly the techniques of integration that came out of it, enabled the Department of Defense (DOD) to make radios and radar systems that engage the spectrum at higher frequencies and bandwidths than ever before. The use of GaAs technology in DOD systems continues to this day.

Ultimately, the GaN technology delivered on its promise and is now being used in the next generation of radar technology, such as the Navy’s Air and Missile Defense Radar (AMDR).

Beyond defense applications, the high-frequency GaAs amplifiers provided a key piece of the puzzle to the commercial sector as it sought to establish newly developed cellular phone technology in the 1990s. GaAs transistors enabled handheld phones with small batteries to establish the critical communications link to the towers. To this day, every smartphone contains a small piece of GaAs to perform this critical function, and the United States enjoys a dominant share of the suppliers of this multi-billion-dollar semiconductor industry as a result of DARPA’s investment in the MIMIC program.

Gallium-arsenide wafer

A pristine 2-inch single crystal gallium arsenide wafer. Purple features are a reflection of a nitrile glove.

The success of GaAs technology proved the defense relevance and the commercial viability of semiconductor technology beyond silicon and made a once-exotic research material into a commodity technology. However, even as GaAs was maturing into an industry, researchers sponsored by the Office of Naval Research (ONR) and elsewhere had already begun to identify the next leap in semiconductor materials. Wide band gap semiconductor (WBGS) materials were identified as promising due to their ability to move electrons rapidly like GaAs but also handle large electric fields as well. This combination of high current capability and high voltage drives the ability to deliver more RF power. While several candidate materials were being developed around the world, DARPA considered gallium nitride (GaN) and its alloys the most promising and established the Wide Band Gap Semiconductor-RF (WBGS-RF) program to rapidly advance the technology.

The high-profile successes of these GaAs, GaN, and SiGe transistor technologies exemplify the ongoing innovation that is possible through manipulation of the crystal structure at the atomic scale.

The WBGS-RF program sought to mature an unproven material with obvious potential into an industrially relevant technology that could further the cause of national defense. Launched in the early 2000s, the program started with GaN material that was delivered on small semiconductor wafers (2-inch diameter) that had large numbers of micropipes or holes, similar to a slice of Swiss cheese. From this inauspicious state, the WBGS-RF program systematically addressed the materials challenges before progressively and successfully taking on the device and circuit-design challenges. Ultimately, the GaN technology delivered on its promise and is now being used in the next generation of radar technology, such as the Navy’s Air and Missile Defense Radar (AMDR). And there is way more to come: GaN is now part of the technology portfolios of all major RF semiconductor players. Once again the United States has a dominant role in this emerging market.

GaN on diamond HEMT

DARPA’s GaN-on diamond high electron mobility transistor (HEMT) demonstrated improved thermal properties that may lead to better performance for RF systems.

DARPA’s efforts have enabled compound semiconductors to move from the research fringes to a mainstream semiconductor industry. They have also driven mainstream silicon technology to embrace variants that include alloys of silicon. In particular, the mixing of silicon with germanium is a technology that DARPA championed during the 2000s with the Technology for Efficient, Agile Microsystems (TEAM) program. Germanium (Ge) was the material basis for the original Bell Labs transistor created in 1947; however, this material was soon abandoned in favor of silicon due to germanium’s reliability problems and the processing advantages of silicon. The insight that brought Ge back was that while it was not useful on its own, a materials stack that included a mix of Ge with Si, or SiGe, allowed for the atomic-level engineering of devices with enhanced RF performance to be built right alongside conventional silicon logic devices in high density. This technology did not possess the complete performance advantages of the other compound semiconductors, such as GaAs and GaN, but it had the ability to produce chips that mixed analog and digital functions. This trait proved highly useful, and SiGe technology is now dominant for delivering low-power commercial solutions for applications like local WiFi amplifiers and now potentially for phased-array systems for 5G communications radios.

TIMU

DARPA-supported researchers at the University of Michigan made significant progress with a timing & inertial measurement unit (TIMU) that contains everything needed to aid navigation when GPS is temporarily unavailable. The single-chip TIMU prototype contains a six-axis IMU (three gyroscopes and three accelerometers) and integrates a highly accurate master clock into a single miniature system, smaller than the size of a penny. This IMU chip integrates breakthrough devices (clocks, gyroscopes and accelerometers), materials, and designs from DARPA’s Micro Technology for Positioning, Navigation and Timing (Micro PNT) program. University of Michigan photo

The high-profile successes of these GaAs, GaN, and SiGe transistor technologies exemplify the ongoing innovation that is possible through manipulation of the crystal structure at the atomic scale. Yet, even these efforts took place within the relatively well-understood paradigm of transistor physics that was established by the silicon semiconductor community. The wider frontier of microsystems goes beyond the electronic properties of materials, as illustrated by some of the more exotic technologies that have emerged along the way. For instance, DARPA championed microelectromechanical systems, or MEMS, in the 2000s through a series of programs that leveraged semiconductor processing to create tiny structures that move and flex rather than just conduct electrons. MEMS technology blossomed with DARPA support into a multi-billion-dollar industry today. MEMS motion sensors and actuators are the heart of protective air-bag systems, navigational and gaming products, and even chips with millions of micro-mirrors that project movies onto theater screens.

In more recent years, DARPA has pioneered work to leverage so-called phase-change materials to create RF switches that operate by a toggle in a material’s crystal structure rather than through conventional transistor action. This wholesale shift to another physical basis and set of materials for digital switching has enabled the demonstration of RF switches with cut-off frequencies in the terahertz (THz) region of the spectrum, which is about 1,000 times the frequency of cell phone operation.

The menagerie of semiconductor technologies that has emerged, in part through sustained DARPA investments, reinforces Feynman’s notion of the broad opportunity that exists in the realm of the microsystem. While these micro- and now nano-landscapes are not the terra nova they were at DARPA’s inception, there’s still plenty of room at the bottom!


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