Microwave and RF device designers working on commercial and defense designs rely on different semiconductor processes that are based on a range of materials, including silicon carbide (SiC), gallium nitride (GaN), silicon (Si), silicon on insulator (SOI), silicon germanium (SiGe), and gallium arsenide (GaAs). As process technologies mature, each is finding its niche. Generally speaking, the silicon technologies are working well for applications below 4 GHz, while GaAs and GaN look to be on target to satisfy high-power, high-frequency needs.
In the high-power market, SiC is being used for devices in VHF and UHF pulsed radar and applications at the kilowatt level and beyond. Microsemi, for instance, offers a line of UHF static induction transistors. There has also been some development work recently on SiC varactors for use in the dynamic load modulation of power amplifiers.1 Lance Wilson, Research Director for RF Components and Systems at ABI projects that SiC currently has less than 1% of the high-power market. Generally speaking, silicon carbide's advantages are its ability to operate at extremely high voltages and its thermal performance, but it doesn't offer quite the same performance levels as GaN. According to Jim Mielke, ABI's Vice-President of Engineering, neither SiC or GaN are having a significant impact on lower-power handset applications.
Silicon carbide's greatest success in RF/microwave applications might be to serve as the substrate for the majority of GaN devices being manufactured, according to Wilson. For microwave applications, GaN can either be used with a SiC or silicon substrate. RFMD, for example, has developed a GaN-on-SiC technology. Joe Johnson, the company's Vice-President of Advanced Engineering Technology & Product Deployment, explains that RFMD made this decision because of the higher thermal conductivity of the SiC substrate that supports the epitaxial GaN active layers. Johnson notes that GaN-on-SiC technology can support higher power levels and power densities as well as higher impedance (higher power density drives smaller device sizes), with higher breakdown voltages and efficiency than can be obtained with GaN-on-silicon technology.
Some of the first high-power products in GaN operated below 4 GHz, and Wilson says that these devices now have about 10 to 15% of the market. "Where GaN is really going to shine is above 4 GHz. I think it is the technology of the future for between 4 and 20 GHz, and it will displace GaAs for high-power RF (15 to 20 W and above)," notes Wilson. Recognizing that GaN is expensive, Wilson points out that GaAs is also relatively pricey for high-power applications. GaN's price point limits its applications, however. "GaN has advantages of good gain, efficiency, and high-voltage operation. This makes it well matched for applications below 4 GHz, but it cannot compete well on cost with the silicon technologies," he adds.
At Microsemi Corp., Mark Faulkner, Director of Engineering, believes that the major benefit of GaN is that it supports 10X the power density of GaAs. It also withstands a 50 to 100C higher junction temperature than GaAs given the same mean time before failure (MTBF). "In addition to its obvious application as a PA or switch, we use GaN for broadband LNAs," he says. "GaN LNAs carry a reasonable noise figure and support high input power handling, thus we can eliminate limiters and their associated loss in our receivers."
Mark W. Andrews of TriQuint Semiconductor adds that, in addition to power density, GaN offers greater bandwidth capabilities and efficiency. He notes that an RF transistor or MMIC device based on GaN technology can routinely offer 4X the power density of GaAs, with power added efficiency at 60% or more, compared to GaAs efficiency that is typically 35 to 50%.
RFMD's GaN technology is targeted at high-power applications (above 5 W output power) below 6 GHz, such as wireless infrastructure and radar, according to Johnson. RFMD is working to develop GaN technology processes for lower voltage and higher-frequency applications as well. Dr. Thomas Joseph, RFMD Foundry Service Technical Manager, explains that RFMD's GaN1 (high-power) process technology offers high breakdown voltage, high power density, and high peak efficiency, and it is optimized for constant envelope broadband applications, such as military communication systems, public mobile radio, military jamming, electronic warfare, and general purpose amplifiers, as well as pulsed applications such as air traffic control and military radar.
Joseph claims that RFMD's GaN1 leads the industry with qualification to 65-V operation, and it is optimized for maximum performance below 4 GHz. The company's GaN2 process is optimized for high linearity and targets applications such as CATV, power doubling amplifiers, and broadband amplifiers that are designed for the high peak-to-average-ratio (PAR) waveforms used in military communication systems and wireless base stations. How do the two processes differ? RFMD reduces power density in the GaN2 process as compared to GaN1, resulting in improved third-order-intermodulation-distortion (IMD3) performance.
Silicon CMOS has applications in low-power RF, and silicon LDMOS does work for high-power RF. In fact, according to Wilson, it has the bulk (90%) of the market for high-power RF below 4 GHz. "It's going to remain the dominant technology for high-power RF below 4 GHz," says Wilson. "It has a long track record, is easy to make, and inexpensive." Silicon bipolar, the legacy technology for high-power RF, still has part of the market, but market share decreases with each year, he observes.
For low-power applications, Mielke notes that "CMOS is making the biggest move right now with silicon-on-insulator (SOI) and silicon-on-sapphire (SOS) devices showing up in a lot of RF switchesA few companies are pursuing CMOS PAs, but no real mass production yet except for a rare Renesas PA found in a Nokia device last year."
One of the earliest proponents of SOI technologies, Peregrine Semiconductor has made dramatic inroads into the wireless market with its proprietary SOS technology. The company just shipped its one billionth product in December 2011 and this month introduced (and announced volume shipment of) its SteP5 UltraCMOS SOS process, which, like previous generations, uses standard CMOS manufacturing processes. "The best applications for UltraCMOS are RFIC devices that require exceptional RF performanceincluding high linearity across a broad bandwidth, excellent harmonic performance, high isolation, low insertion loss, and great ESD tolerance," says Rodd Novak, Peregrine's Chief Marketing Officer. The company just announced new SP8T and SP10T RF switches based on its SteP5 process. It also offers digitally tunable capacitors (DTCs) for 4G and LTE smartphones. Peregrine's SOS devices can be found in many leading smartphones, including those from Apple, Nokia, HTC, Samsung, LG, Motorola, and Nexus.
IBM's silicon-germanium (SiGe) process is renowned for its high-frequency performance, and it has proven to be a fundamental enabler for test gear. This especially holds true with oscilloscopes from leading manufacturers, including LeCroy, National Instruments, and Tektronix. Other target applications of SiGe include radar and millimeter wave applications, as well as high-speed switches. Advantages include low noise, high gain, and linearity. Mielke notes that SiGe is also used in some small-signal applications like low noise amplifiers (LNAs). Last fall, NXP Semiconductors announced a family of SiGe:C downconverters for satellite receiver applications. Analog Devices, Inc. is known for its work in SiGe, developing proprietary SiGeBipolar and SiGe BiCMOS processes that are used for a range of devices, including differential amplifiers, programmable variable gain amplifiers, and low-power opamps. Last year, Skyworks acquired SiGe Semiconductor, and now offers a range of SiGe products, including front-end modules, power amplifiers, GPS receivers, downconverters, and digital attenuators.
GaAs continues to enjoy the bulk of the high-power RF market below above 4 GHz, but Wilson expects that for high-power devices (15 W and above) we will soon see GaN eating into this market for applications between 4 and 20 GHz. GaAs is also still the major market player for mobile-phone power amplifiers.
Microsemi's Faulkner notes that GaAs PHEMTs with short gate lengths can yield high gain and low noise through millimeter wave frequencies at a reasonable cost. Some products that can be developed using this process include low noise amplifiers, converter LO chains, drivers to PA chains, and FET-based mixers. Microsemi uses a GaAs VPIN (vertical P-I-N) process for diode-based ICs such as switches, limiters, and attenuators. "Using clever circuit topologies, we apply this technology to high-power millimeter switches capable of surviving 15 W of CW input power at Ka-band," Faulkner says, adding that this makes the GaAs VPIN process well suited for transmit/receive switches in transceivers and monopulse radars.
According to Triquint's Andrews, the benefits of GaAs include a wide selection of mature processes targeting cellular frequencies (2G/3G/4G), network base station, point-to-point radio, VSAT, CATV, WLAN, WiFi, and other microwave applications, as well as the typically high frequency needs of defense/aerospace. He notes that TriQuint likes GaAs because it makes it possible to integrate on-chip and provide linear, low-noise/high-efficiency solutions that are superior to silicon in terms of power handling (high power density). "Plus," he adds, "it offers the ability to put up to three layers of metal into semiconductor designs and has two to three times the power density capability of silicon, so power amplifier modules, switch duplexer modules, and related devices targeting mobile device applications can be orders of magnitude smaller than devices constructed of other technologies."
Weighing Alternatives
As they work their new designs, engineers have plenty of process technologies from which to choose. Recognizing this, many RF and microwave device providers work in multiple processes. For example, Skyworks "uses a variety of III-V and silicon based processes for a number of product categories, including power amplifiers and switches," says Aniruddha Joshi, the company's Technical Director of Wafer Foundry Engineering. Specifically, the company offers III-V HBT, PHEMT, and E/D PHEMT; silicon BiCMOS; SiGe BiCMOS; silicon/SOI-based RFCMOS; and bipolar CMOS DMOS. Some vendors have their own fabs, such as TriQuint and RFMD, who each have their own GaAs and GaN foundry services.
According to Mike Mullins, Director of Business Development, RF Group, Analog Devices, Inc., his company uses specialized GaAs HBT and pHEMPT processes for RF power applications, complementary SiGe BiCMOS for very high dynamic range RF signal processing, and nanometer CMOS processes for highly integrated RF products. "ADI believes these mainstream process technologies are applicable to the majority of high-volume RF applications where cost and reliability are both important considerations. Many new process technologies have been and are being considered, and ADI will use them when they can meet our reliability and cost expectations," says Mullins.
As for alternatives, Faulkner sums them up this way: "Considering which technology is better, the key trade-offs are output power, input survivability, and cost. Our assessment is InGaP is the best semiconductor technology for low additive phase noise. When matched correctly, GaN beats GaAs for power-added efficiency, which is why we use it in short-lived military applications that have severely limited thermal paths."
"The main competition for our GaN is LDMOS and, in some cases, power GaAs," notes Johnson, adding, "LDMOS is the incumbent technology that GaN is attempting to unseat in many of our key markets of interest. Although the higher cost associated with GaN is currently an issue, the technology exceeds LDMOS in specifications such as efficiency, bandwidth, gain, and linearity. If cost is the major issue, then we can expect that to come down as the technology matures and begins to ship in volume."
The overwhelming impression given by all of the vendors mentioned here (only a small sampling of those supplying RF and microwave products) is that they are optimizing their processes, so as to better anticipate and satisfy the rapidly changing needs of RF and microwave designers.
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Reference
- Andersson, Christer M., Ejebjork, Niclas, et al. "A SiC with large effective tuning range for microwave power applications." IEEE Electron Device Letters, June 2011.