Rather than find a single semiconductor technology that is ideal for all applications, the RF/microwave industry relies on numerous device processes, each with its own set of strengths and weaknesses.
Semiconductor technologies are now more diverse than at any time in the history of the RF/microwave industry. At one time, they were entirely based on silicon. Then, with more than a little push from military funding, gallium arsenide (GaAs) became the “darling” semiconductor technology of the high-frequency industry. GaAs was projected in various 1980s studies to be the ultimate RF/microwave semiconductor material, and GaAs monolithic microwave integrated circuits (MMICs) would eventually replace silicon and provide all transistors, diodes, and integrated circuits (ICs) needed for any electronic market. Today, producers and providers of gallium nitride (GaN) say that it is the semiconductor material to beat, and they are predicting the kind of dominance for GaN that was once projected for GaAs.
Admittedly, GaN is an impressive semiconductor substrate material, especially for high-frequency applications. It can support operating frequencies well into the millimeter-wave range, at very high output-power levels, and with high gain. GaN may lack the excellent noise characteristics of GaAs that enable high-performance low-noise amplifiers (LNAs) at microwave and millimeter-wave frequencies, but it has clearly become the device material of choice for high-power microwave semiconductors.
GaN devices work well with very high supply voltages, such as +48 and +50 VDC. For example, NXP Semiconductors has developed a number of +50-VDC discrete power transistors based on its GaN high-electron-mobility-transistor (HEMT) process technology, with provides generous output-power levels with high efficiency. The firm’s model CLF1G0035-50 is usable from DC to 3.5 GHz with better than 50 W output power and 43% power-added efficiency at test points of 1 and 2 GHz. It is suitable for a number of different applications, including in jammers, radar transmitters, commercial cellular systems, and public mobile radios.
1. GaN devices such as these are capable of high output levels, and packaging is critical to provide effective thermal management. (Photo courtesy of NXP Semiconductor.)
For even higher power levels, the company’s model CLF1G0035-100 is a high-power 100-W transistor that is usable from DC to 3.5 GHz. It provides 11.2-dB gain at 1 GHz and 11.7-dB gain at 2 GHz, with continuous-wave (CW) PAE of 47.9% at 1 GHz and better than 53% at 2 GHz. Both GaN HEMT transistors are supplied in flange-type packages with or without mounting holes (Fig. 1). But while GaN performance is impressive, the technology must overcome some hurdles for broader customer acceptance. “GaN’s current biggest challenge is that it is expensive,” admits Mark Murphy, NXP’s Marketing Director for RF Power. “The challenge is to find a better cost structure that will allow it to be used in more commercial applications.”
In an attempt to address the cost issues, M/A-COM Technology Solutions recently introduced a line of GaN wideband power transistors in plastic packages. Although data is not yet available for these devices, they are supplied in 3 x 6 mm plastic packages and designed for low-cost L- and S-band radar and satellite-communications (satcom) applications. As Damian McCann, M/A-COM’s Director of New Technology Initiatives, explains: “We have created a range of rugged and reliable high voltage, unmatched transistors with exceptional electrical and thermal performance. Building on M/A-COM’s deep history and understanding of the pulsed radar market, these products will enable our customers to create the next generation of small size and low-cost phased-array radars for L-Band and S-Band applications.”
High-power GaN transistors are often mounted on silicon-carbide (SiC) substrates because of the latter material’s excellent thermal characteristics. As an example, the model T1G4005528-FS transistor from TriQuint Semiconductor is a GaN-on-SiC device usable from DC to 3.5 GHz and capable of 55 W output power and 15-dB linear gain at 3.5 GHz. It is supplied in a metal-ceramic flange package and operates from a +28-VDC supply. The company offers GaN-on-SiC devices at power levels as high as 100 W from DC to 18 GHz (model TGF2023-20).
SiC is also a starting material for many current high-power transistor products. Like GaN, SiC received a big push from military funding, and some of the results showed the promise of SiC devices at high power levels. Unfortunately, the technology has so far been limited to RF and lower microwave frequencies, and is actually gaining many followers for power supplies and power-switching applications.
Nevertheless, Cree has developed some higher-frequency devices based on SiC substrates—including the model CRF-22010 transistor, a SiC MESFET with 10 W output power and 12-dB gain at 2.2 GHz. The firm, which is also active in GaN technology, has also developed lower-frequency SiC devices operating at much higher voltages: model CMF10120D is a high-power transistor that draws 24 A at +1200 VDC and is suitable for both motor drives and switched-mode power supplies.
Microsemi has likewise applied SiC transistor technology to motor-drive and power-supply activities. But this is also a company that has developed a number of different technologies to cover a broad range of markets, including automotive, industrial, medical, commercial, and military applications. The firm recently announced availability of a complete medical network radio link for implantable medical devices such as pacemakers and cardiac defibrillators. The new radio link is comprised of the company's ZL70321 implantable radio module and its ZL70120 base station radio module, both of which are based on the firm’s low-power model ZL70102 medical implantable communications service (MICS) band radio transceiver chip.Martin McHugh, Microsemi’s Product Line Manager, notes: “With Microsemi’s two-module radio link, companies can now focus research dollars and development efforts on new therapies that enable a better quality of life.”
Also at lower frequencies, silicon laterally diffused metal oxide semiconductor (LDMOS) technology has been a dominant device material for cellular base stations and RF/microwave radios for some time (although proponents of GaN feel that the newer technology may one day replace LDMOS as the technology of choice in high-power wireless base stations). Like GaN, silicon LDMOS transistors can work at high voltages, such as +50 VDC, making the two device technologies almost interchangeable for broadcast and commercial communications applications.
Although LDMOS devices are more typically designed for pulsed and CW applications at frequencies below about 1500 MHz, they are capable of impressive output levels. Drawing from the legacy of Motorola Semiconductor’s LDMOS technology, the model MRF6VP41KH is a +50-VDC LDMOS transistor in an air-cavity ceramic package with 1000 W output power from 10 to 500 MHz. Suitable for commercial aerospace and industrial-scientific-medical (ISM) band applications, it has excellent thermal performance and can achieve high drain efficiency.
While GaN is now being touted as the successor to GaAs, the latter is hardly finished as a viable high-frequency semiconductor technology. The technology and its suppliers have received generous support over the years from government agencies such as the Defense Advanced Research Projects Agency (DARPA). As a result, GaAs in its many forms—in low-noise and higher-power circuits—is well entrenched throughout the RF/microwave industry. Major GaN suppliers such as TriQuint Semiconductor, continue to support GaAs foundry services. In fact, TriQuint’s GaAs foundry, which was established in 1985, is one of the world’s large GaAs foundry facilities and works with both 100- and 150-mm wafers. TriQuint recently announced a low-cost PA module for cellular GSM and EDGE applications in smartphones, computer tablets, and other wireless consumer products. The PA measures just 5.0 x 3.5 mm and is based on the firm’s GaAs heterojunction-bipolar-transistor (HBT)/CuFlip™ technology. In addition to GaAs and GaN, the foundry also provides indium-gallium-phosphide (InGaP) HBT processing services.
TriQuint recently added to its GaAs product portfolio with a number of packaged GaAs pseudomorphic-high-electron-mobility-transistor (pHEMT) PA modules for applications from 6 to 38 GHz. Suitable for commercial satcom systems, point-to-point radios, and military radar and electronic-warfare (EW) systems, they are supplied in packaging that supports multilayer printed-circuit-board (PCB) layouts.
The PAs use the company’s Die-on-Tab packaging approach to simplify handing. The amplifiers benefit from vacuum reflow process which creates nearly void-free bonds between dies and bases with excellent thermal stability. One of the packaging approaches (Fig. 2) is a ground-signal-ground (GSG) package configuration that allows users to mount the amplifiers “right-side up” or “upside down” on a PCB. Examples of the new PA line are models TGA2502-GSG with 3.6 W output power from 13 to 16 GHz and model TGA2575-TS with 3 W output power from 32 to 38 GHz.
2. These GaAs PA modules are mounted in novel ground-signal-ground (GSG) packages that allow flexible mounting on a PCB. (Photo courtesy of TriQuint Semiconductor.)
GaAs technology is also well supported by commercial foundry and device supplier United Monolithic Semiconductors. UMS touts GaAs-based solutions through 100 GHz, and recently took the “plastic-package” approach to create a low-cost GaAs MMIC circuit for W-band applications from 76 to 77 GHz, including in automotive commercial radar systems and industrial sensors. Model CMH1270a98F is a dual-channel transmitter/receiver that works with intermediate-frequency (IF) signals from DC to 100 MHz. It is also available in chip form, measuring just 2.95 x 2.00 x 0.10 mm.
Hittite Microwave Corp. is another company well known for its lower-cost GaAs-based MMIC solutions, with GaAs products available through 100 GHz. A recent example is the model HMC3653LP3BE, a GaAs HBT amplifier supplied in a 3 x 3 mm plastic QFN package. It delivers 15 dB gain from 7 to 15 GHz and as much as +15 dBm output power at 1-dB compression with +28 dBm output power at the third-order intercept point. The plastic-packaged MMIC amplifier, which boasts a noise figure of only 4 dB, is ideal for point-to-point and point-to-multipoint radios, VSAT, broadcast relay links, and X-band communications systems. It is designed for +5-VDC supplies and draws about 44 mA current over a wide operating temperature range (-40 to +85°C).
Using a slightly different process, ANADIGICS recently introduced its model AWB7129 power amplifier (PA) for small cellular-communications infrastructure stations, such as picocells and customer premises equipment (CPE). It is based on the company’s patented InGaP-Plus™ HBT MMIC technology to deliver a combination of high efficiency, linearity, and effective thermal management.
At lower power levels, GaAs is facing some competition from semiconductor processes that have moved higher in frequency in recent years. Silicon complementary-metal-oxide-semiconductor (CMOS) processes have gained ground on GaAs. Some firms, such as TowerJazz, now offer devices and processing based on RF CMOS and other lower-power technologies such as silicon-germanium (SiGe) BiCMOS well into the microwave frequency range.
Marco Racanelli, TowerJazz Senior Vice President, sees the firm’s SiGe products as legitimate threats to the market share long help by GaAs in lower-noise applications: “We also see the highest performance SiGe technologies now reaching noise-figures that can compete head to head with GaAs for applications such as Global-Positioning-System (GPS) low-noise amplifiers (LNAs) and so believe some of these applications will move away from GaAs in the future.” Racanelli adds that “GaAs will retain some market share of high-end Wi-Fi PAs. In cellular communications, GaAs PAs will continue to dominate, but in-roads on the low end of the market are being made by CMOS…we see a bigger threat looming for the rest with SiGe, which can reach performance and size/cost to be a stronger threat to GaAs than CMOS has been thus far.”
Dr. Keith Strickland, Chief Technical Officer at Plessey Semiconductors Ltd., says it may be a bit premature to cast off GaAs and start awarding prizes to GaN devices. He notes that GaN is still limited in terms of suitable substrates, and each substrate poses problems.“Sapphire, for example, has a relatively poor thermal conductivity which spoils the advantage of GaN, and the GaN may need to be transferred to another substrate, such as SiC.”
Strickland adds that the excellent high-frequency characteristics of GaAs devices still make them valuable: “GaAs as a current state of the art through about 100 GHz. It has an advantage of being relatively well understood, with substrates that are native to the technology and will keep it cost-effective for very high frequency front end and microwave applications…but because GaAs does not have the thermal performance of GaN, Si, or SiC, it may lose out to GaN or SiC in the future for high-power applications.”