GaN adoption solves power, performance issues in military applicationsStory
October 04, 2018
Radar, electronic warfare (EW), and communication systems are increasingly leveraging gallium nitride (GaN) technology to meet stringent high-performance, high-power, and long life cycle demands of these systems Meanwhile, increased commercial volumes is resulting in price reductions for GaN components across multiple markets.
GaN is a semiconductor material that in recent years has become a key component in enabling higher performance systems in military applications such as active electronically scanned array (AESA) radars and electronic warfare systems that require more power, reduced footprints, and more efficient thermal management.
In a nutshell: “There are a lot of different aspects of GaN, and we’ve just started to tap the possibilities,” says Dean White, Director of Defense & Aerospace Marketing Strategy, at Qorvo (Greensboro, North Carolina).
GaN delivers because “at a high level, GaN is really being considered for military applications because of its high power density, high efficiency, wide bandwidth, and exceptionally long life,” says Jim Milligan, Senior Director Foundry, Aerospace and Defense, at Wolfspeed, a Cree company (Research Triangle Park, North Carolina).
GaN applications abound from electronic warfare to radar to communications: Popular applications such as “wideband communications [are] especially benefitting from GaN technology, because it can enable a radio that traditionally would use two transistors operating multioctave to move to just a single device,” says Gavin Smith, Product Marketer for the NXP RF Multi-Market Team (Chandler, Arizona). “This is important, because it helps save space and – in some cases – complexity of a design.”
Radar systems – to take another example – are requiring wider bandwidths and more power for operation. “GaN is proving to be an ideal technology for AESA radar systems,” says Deepak Alagh, Senior Director & General Manager, Mercury Systems RFM Group (Andover, Massachusetts). “The high power density allows the solid-state power amplifier (SSPA) to be much closer to, or even integrated with, the radar tile. By reducing the line lengths, losses are kept to a minimum. Additionally, compared to a one large traveling wave tube (TWT) amplifier and multiple phase shifters and power splitters, GaN SSPAs near the tile enable digital beam steering. Through the use of these multiple, compact GaN amplifiers paired with FPGA modules, the AESA achieves a much higher level of flexibility. Also, since GaN-based SSPAs are much smaller than the alternative technologies, it helps control the total size of the radar system.” (Figure 1.)
Figure 1: Mercury Systems’ amplifiers cover most radar and communications bands as well as other popular frequencies, including millimeter wave. Photo courtesy of Mercury Systems.
Putting GaN in radar systems helps reduce footprints while still increasing system efficiency. “Designers need higher-power solutions that fit into size-constrained spaces, that also don’t require too much heatsinking,” NXP’s Smith says. “Like LDMOS [laterally diffused metal oxide semiconductors], customers using GaN continue to ask for higher-power solutions. To meet such size requirements, some applications require higher-power transistors, cutting out the need to combine several devices to achieve desired power levels. We also see applications that aren’t as concerned with SWaP requirements but are determined to achieve certain levels of efficiency for their systems.”
GaN enables smaller footprints in EW systems
GaN technology is helping designers meet reduced size, weight, and power requirements for EW systems. Like radar systems, EW applications are benefitting from the reduced footprints GaN enables. “You go from having large systems that are vehicular-mounted in the old days, to systems now that a soldier could actually carry in a backpack,” says Dean White, Director of Defense & Aerospace Marketing Strategy, at Qorvo (Greensboro, North Carolina). “The broader bandwidth allows them to get voice and video, very similar to what you’d have in a handheld smartphone. There’s also less concern about cooling because of GaN’s ability to operate at higher channel temperatures.
“Most EW systems are very broadband, requiring high amounts of RF output power, but these are key areas where GaN really performs well,” White continues. “Because of the impedance on the input of the transistor of GaN, it’s easier for designers to match to it. Many EW systems used to have to use switches to switch from band to band; it’s better now that they have a continuous bandwidth to operate over. What used to be broadband amplifiers in a fairly large brick module, are now condensed into a single mimic or maybe two mimics in a single package. Things are becoming smaller.”
GaN technology is good in general for EW systems, explains Alagh: “Since a GaN device is smaller than its equivalent GaAs device, the parasitic gate capacitance is reduced. This yields a smaller input impedance that makes broadband matching much easier. Also, the high power density of GaN-based devices results in smaller EW systems, which enables their deployment on a wider range of platforms.”
GaN power benefits
“As a general trend, we’re starting to see higher-powered transistors,” Milligan says. “When we first introduced GaN transistors for high-power applications such as radar applications, they were at the 100- to 200-watt level for a single package transistor. Now we’re starting to see the trend going to larger and larger power levels.”
Radar systems are now seeing “a kilowatt or more per package transistor for some L-band radar applications,” he adds. “We’re starting to see that trend in general as you go through L, S, C-band, X-band radar. This is really to service a lot of centralized transmitter type of applications.”
One significant GaN application is in power amplifiers, White says. “They are the number-one choice because of the power density of GaN and its high level of power efficiency over a wideband bandwidth.” In addition, he continues, there is also the move toward using GaN for low-noise amplifiers (LNAs): “GaN surprisingly has very good noise performance. A typical GaN device may be able to withstand anywhere from 50 milliwatts to 100 milliwatts, whereas a GaN LNA could be anywhere from two to four watts of incident power on the input, thereby reducing or eliminating the need for a limiter on the input of an LNA.”
Phased-array radars are also benefiting from GaN power devices, Milligan says. “Phased-array radar has really sort of come of age over the last decade or so. For those applications you are using a GaN power transistor in every element of a phased array. In other words, it’s distributed elemental power. As a result of that, for those applications, you deal with lower power levels anywhere from 10 watts to 50 watts peak power, depending on the application.”
For example, Milligan states, Wolfspeed developed two C-band products for use in phased-array antennas. “One will be a 25 watt part, the other is a 50 watt part, that will cover 5.2 to 5.9 gigahertz, which is within the C-band radar operating band.” (Figure 2.)
Figure 2: CMPA5259025F - 25 W, 5.2-5.9 GHz, GaN MMIC power amplifiers for C-band radar applications. Photo courtesy of Wolfspeed.
Thermal management benefits
Increased power can also mean increased thermal management challenges, but GaN has upsides when it comes to keeping the system cool. “When you start to migrate to the use of GaN transistors, you can operate those at much higher channel temperature,” Milligan explains, citing this example: “Our standard operating channel temperature, junction temperature is 225 °C, which is merely 100 degrees higher than the equivalent LDMOS part. As a result of that, you’re able to operate GaN at higher power levels and do it in a way that sort of liberates the system designer from a thermal design prospective.”
In addition, he adds, “applications where you might have to use liquid cooling, if you’re using a silicon-based transistor, you can now migrate to air cooling if you’re going to use the GaN part.”
Specifically, “GaN has a clear advantage in short-pulse/low-duty-cycle radar,” explains Paul Scsavnicki, applications engineer, at NXP (Eindhoven, Netherlands). “From the higher power densities, drain efficiencies, and lower am/pm distortion at P-3dB, GaN demonstrates better performance than LDMOS.”
GaN enables better thermal management and offers new options, White says. “In particular, Qorvo uses GaN on silicon carbide (SiC) because it is an excellent thermal conductor. It’s better than many of the metals that are used to attach to it and much better than silicon. With GaN and SiC, you can operate at 200, 225 °C channel temperatures, as opposed to other technologies, in particular like GaAs, which you can operate at only about 150 °C, without giving up the reliability of the device.”
LDMOS versus GaN: When is GaN the better choice?
Many see GaN as a replacement for LDMOS technology, but is that the case for every application? “I wouldn’t say GaN is always the better choice than LDMOS for all radar systems,” NXP’s Smith says. “When choosing GaN or LDMOS, it depends on the frequency, power level, efficiency, and price.”
“LDMOS has been around for many years, especially for lower frequencies; S-band, L-band, and even down in the low L-band, the UHF bands. GaN originally started out replacing LBMOS S-band,” Qorvo’s White explains.
What GaN accomplishes and LDMOS does not is delivering the power that radar systems demand in certain frequencies. Alagh agrees: “GaN is the better choice since it balances bandwidth, power, and size. LDMOS can provide high output power in a small package, but only for low frequencies. GaAs can provide high frequencies, but not high power.”