ANDOVER, Mass.—At the front door of Raytheon's Integrated Air Defense Center, there's a reminder of how big microwave electronics used to be—the original microwave oven. The now ever-present kitchen device was invented after a Raytheon engineer discovered his candy bar melted while he was standing near a magnetron used in a radar system the company was developing. Nearly the size of a refrigerator, the original microwave looks like it would cook a whole lot more than whatever was put within its metal grate, which was meant to contain the microwaves from its magnetron.

A few hundred yards away from the relic, Raytheon manufactures a much different microwave technology today. In a semiconductor fabrication facility, built to resemble a giant integrated circuit from above, the company produces many of the chips that go into its modern radar systems, including monolithic microwave integrated circuits (MMICs). These tiny radio frequency amplifiers are similar to ones found in cellular phones, Wi-Fi adapters, and other wireless communications devices.

This technology is currently in the process of getting a major upgrade as the result of more than 16 years of research by Raytheon. And any MMIC evolution will be driven by the same substance that has made power-sipping LED light bulbs, Blu-ray players, and game consoles possible: gallium nitride (or, in chemical shorthand, GaN).

The reason Raytheon and other semiconductor manufacturers have put so much effort into gallium nitride is the semiconductor's unique attributes. GaN is what is known as a "wide band gap" semiconductor. It can be used in devices that operate at much higher voltages, temperatures, and frequencies than other semiconductors. As a result, GaN chips are more power-efficient, and they have a longer life than other semiconductor chips because of their heat tolerance. Those characteristics are a trifecta for military applications.

As it turns out, Raytheon and some commercial manufacturers of GaN chips have found a way to produce them more cheaply than other semiconductors—including silicon chips made for the same applications. The ability to handle high power and have fast switching speeds makes GaN interesting, even essential, for a whole host of applications from cellular communications to renewable and portable energy. And as the cost of making GaN chips falls, GaN could start to challenge silicon for more tasks. Particularly because of its ability to operate at much higher frequencies, GaN might even become the next thing to help extend the life of Moore's law—not in terms of actual transistor density, but in terms of increases in speed of processors.

Mind the gap

To truly understand GaN, first you need to look to its predecessor. For the past 30 years, wireless devices have used chips made with gallium arsenide (GaAs) as transmitter and receiver signal amplifiers. Once considered the future of computer semiconductors because of its high switching speeds, gallium arsenide was used for the chips in the Cray 3 supercomputer. GaAs chips have been used for the LEDs used in DVD and CD player lasers, but the chips never went mainstream. Their adoption has been limited for a variety of reasons, but it's largely because of how expensive GaAs is to manufacture.

Still, GaAs was a natural choice for RF transmitters and receivers for a number of reasons. GaAs, like GaN, is what is referred to as a Category III-V semiconductor—that is, it's a compound of an element with three valence electrons (gallium) and an element with five valence electrons (arsenic). A quick high school chemistry refresher: valence electrons are the electrons that get shared with other atoms in covalent bonds, with each atom contributing one electron to a pair.

Silicon (a Category IV semiconductor) has four valence electrons, so each silicon atom bonds with four other neighboring atoms. GaAs molecules, however, create covalent bonds with other molecules by sharing eight pairs of electrons. Because of that, electrons move more quickly through it than through silicon—GaAs have about six times the "electron mobility" of silicon. That faster movement means that GaAs transistors can switch at much higher frequencies than silicon transistors, which is important for tasks like amplifying radio frequency (RF) signals.

Another upside of GaAs is its "band gap"—the amount of energy that is required to push a substance to the point of becoming conductive and allowing electrons to flow when subjected to an electrical field. Conductive metals have no band gap. Insulators, which allow no electron flow, have band gap ranges that are impossibly wide to overcome. Semiconductors, however, have band gaps in between—they're small enough that when enough energy is applied, their molecules will begin to free up electrons and allow electricity to flow.

At normal temperature ranges, gallium arsenide has a bigger gap than silicon (1.43 electronvolts versus silicon's 1.1). In applications like microwave circuits, that gap is important because it means the chip can handle higher power levels and still act as an insulator. And because GaAs can do that more efficiently than silicon, they continued to attract some demand.

"Gallium arsenide amplifies signal without adding a lot of noise to it," Colin Whelan, technical director of Advanced Technology Programs at Raytheon Integrated Defense Systems, told Ars. "That's important if you want to pick up signals from far away without introducing noise."



The heat is on

GaAs has a number of downsides, of course. GaAs semiconductors are still expensive to produce. And while it can handle higher power inputs than silicon, GaAs also handles heat poorly in comparison to silicon. The chips also leak a significant amount of the power pushed into circuits as heat. GaAs chips have about a third of the thermal conductivity of silicon, and heat tends to build up in some areas rather than distribute evenly across the surface of the chip. That can seriously reduce the expected life of GaAs RF components, since they typically operate at temperatures above 120 degrees Celsius (up to as much as 150 degrees Celsius in military radar systems).

That may not be a huge concern for consumer electronics manufacturers. In fact, given the planned obsolescence of many wireless devices, manufacturers probably like it that way. "In the commercial world, they're often going for lowest cost," Whelan said. "If your cell phone or Wi-Fi stops working—my Wi-Fi range gets shorter and shorter every year—that could be things like the power amplifier degrading over time. But for our systems, they usually have 30 year lives, running continuously, which requires our systems to be ultra-reliable."

You can't just wait around for 30 years to see if components last that long before putting them in the field. So to accelerate testing of failure times for semiconductors, engineers use high heats as a sort of time machine.

You can't just wait around for 30 years to see if components last that long before putting them in the field. So to accelerate testing of failure times for semiconductors, engineers use high heats as a sort of time machine—using the "Arrhenius-type behavior" of the semiconductor to extrapolate how long it would last during normal operation. Using a formula developed by Swedish scientist Svante Arrhenius (the man who first tried to calculate the greenhouse effect on global climate), it's possible to get a fairly accurate prediction of how long a semiconductor circuit will last. "If you heat them up to 300 to 400 degrees Celsius, you can force them to degrade more quickly," Whelan explained. This degradation happens in a predictable enough way to get a statistical picture of how long the chips will last.

It turns out that it's very hard to get GaAs chips through the Arrhenius test with the kind of performance the military is looking for. Depending on the design, researchers at the Naval Research Laboratory found that GaAs MMICs under continuous use will experience gaps between failures from as little as a year up to 22 years when operated at 120 degrees Celsius (failure being defined as a 20 percent drop in performance).

Running under high power to generate strong enough radio signals for radar generates a lot of heat, so this can be a problem. Phased-array radars operate constantly, at temperature ranges up to 150 degrees Celsius, and the military expects them to have a lifetime of more than 30 years. To achieve that, Whelan said, "you need this incredible reliability performance to make sure none of them degrade over a 30 year lifetime." After all, there are somewhere in the range of 50,000 to 100,000 transistors packed into the radar.

To make sure that the radar isn't being taken offline frequently for service, statistically speaking, Raytheon is looking for an average service life on each semiconductor component of a million hours of operation—more than 100 years. This ensures the semiconductor wasn't the weakest link in radar systems' reliability. Raytheon was able to hit that mark with GaAs transistors largely because of the three decades of development work the company has done with gallium arsenide. "We're driven by performance and reliability, so we tend to develop our [chips] differently, which is why we do things in-house," Whelan said.

That doesn't do a lot for the overall affordability of the systems, however, and that's where GaN research comes in. As such, early gallium nitride material research in the late 1990s got the attention of Raytheon, the military, and a lot of other people.

Listing image by Raytheon