Choosing a transistor for an active-circuit design, such as a low-noise or power amplifier, was once as simple as picking a proven silicon bipolar transistor. But several decades ago, gallium arsenide (GaAs) field-effect transistors (FETs) became the favored active device for many RF/microwave amplifier circuits. And today, pursuits of higher-frequency operation and higher solid-state power levels in both commercial and military applications has fueled the development of silicon-carbide (SiC) and gallium-nitride (GaN) transistorstransistors boasting considerably higher power densities than their silicon-bipolar and GaAs-FET predecessors. This abundance of different solid-state devices makes the task of selecting an optimum active device for a given high-frequency application ever more challenging.

In terms of solid-state power, wide-bandgap materials like SiC and GaN and their high breakdown voltage fields lend themselves to high output power levels compared to silicon and GaAs devices, although SiC suffers from poor electron transport properties which will limit its top operating frequency. A report from ABI Research published last year, "RF Power Semiconductors," noted that GaN devices provided the high output-power levels at relatively high frequencies that enabled them to bridge the gap between GaAs FET devices and higher-power silicon laterally diffused metal-oxide-semiconductor (LDMOS) transistors, especially for military applications. The study focused on applications of 5 W or more at frequencies to about 3.8 GHz, which it stated represented the bulk of current high-frequency, high-power applications for solid-state devices. Because cellular/wireless base stations typically operate at +28 VDC, where GaN works quite well, these devices are also very attractive for the growing number of wireless amplification applications.

In anticipation of the GaN market growth, Nitronex successfully completed qualification of its GaN-on-silicon NRF1 discrete process for volume production at Global Communication Semiconductors (GCS). As part of a long-term supply agreement, GCS will exclusively provide Nitronex with NRF1 foundry services for discrete and monolithic-microwave-integrated-circuit (MMIC) devices. Qualification includes extensive DC, RF, thermal, reliability, and other parametric testing to ensure that devices fabricated at GCS are equivalent in every way to those from Nitronex's facility in Durham, NC. "The combination of our proprietary 100-mm GaN-on-Si process, and the full suite of production and new process development capabilities at GCS, gives us the ability to be a leader in the rapidly emerging market of GaN RF power devices," says Charlie Shalvoy, the company's Chief Executive Officer.

Recently, Nitronix unveiled its model NPA1004 GaN-based power amplifier for the military communications market. It is based on the firm's mature +28-VDC NRF1 GaN process technology. With 27-dB small-signal gain from 1 to 2 GHz, the device demonstrates the high power density of the technology, delivering 50 W saturated output power from a package measuring just 0.28 in.2 With 55% typical drain efficiency, the high gain allows designers to eliminate a driver stage in some circuit and system designs. The company also recently announced its +48-VDC NRF2 GaN-on-silicon process with double the power density and higher gain than the NRF1 process. This new process is also claimed to represent a significant advance in GaN-on-silicon reliability, with a mean time to failure (MTTF) of more than one million hours (114 years) at an operating junction temperature of +230C.

A large number of commercial GaN devices are fabricated on SiC substrates to take advantage of the latter material's excellent thermal properties. Model MAGX-003135-180L00 from M/A-COM Technology Solutions, for example, is a gold-metallized GaN-on-SiC transistor optimized for pulsed civilian and military radars from 3100 to 3500 MHz. It delivers 180-W peak output power when operating with 300-s pulses at 10% duty cycle, and is designed for a typical supply of +50 VDC. The Class AB device is built into a thermally enhanced Cu/Mo/Cu flanged ceramic package which provides excellent thermal performance. The company also offers its model MAGX-001214-250L00 gold-metalized GaN-on-SiC transistor for pulsed L-band applications. It provides 250-W peak output power from 1200 to 1400 MHz under the same pulse conditions as its higher-frequency part. The device is projected to have a mean time to failure (MTTF) of 114 years based on a channel temperature of +200C.

Mitsubishi Electric Corp. set some new marks for GaN efficiency last year with a GaN HEMT device developed for C-band satellite communications. Unveiled at the 2011 IEEE International Microwave Symposium (IMS) in Baltimore, MD, the high efficiency GaN power amplifier was achieved by placing a harmonic tuning circuit in front of each GaN HEMT cell on the substrate. Capable of 107 W (+50.3 dBm) output power with 67% PAE at C-band in a package measuring just 17.4 x 24.0 x 4.3 mm and weighing just 7.1 g, the internally impedance-matched amplifier design was meant as a possible replacement for much larger traveling-wave-tube-amplifier (TWTA) components in C-band satcom systems.

Cree, Inc., which is well known as a supplier of light-emitting-diode (LED) chips and components for lighting applications, and also has extensive solutions for power electronics based on SiC devices, has developed several robust GaN HEMT packaged devices for S-band radar. These include the model CGH31240F packaged transistor, designed for operation at +28 VDC with almost 12-dB gain from 2.7 to 3.1 GHz. It delivers 240 W output power when working with pulses less than 300 s wide at duty cycles of 20% or less. The transistor, which is supplied in a ceramic/metal flange package, boasts power-added efficiency (PAE) of better than 59% to 3.0 GHz and 52% at 3.1 GHz.

Microsemi has been a strong proponent of SiC device technology. Its model 0405SC-1500M is a common-gate N-channel depletion-mode Class AB SiC static-induction transistor (SIT); the unit is capable of 1500 W pulsed RF power from 406 to 450 MHz for high-power amplifiers in UHF weather radar and long-range tracking radar. The output-power rating is based on the use of 300-s pulses at a 6% duty cycle. With 55% typical drain efficiency, this SiC transistor draws 125 mA average quiescent drain current from a +125-VDC drain voltage.

TriQuint Semiconductor, long a supplier of defense-electronics-driven solid-state devices, has begun work on Phase II of the Defense Advanced Research Projects Agency (DARPA) multiyear Nitride Electronic NeXt-Generation Technology (NEXT) program as a prime contractor. TriQuint has received $12.67 million in support of the NEXT contract to date, with Phase II designed to build upon the results of Phase I. TriQuint's Vice President and General Manager for Defense Products and Foundry Services, James L. Klein, explains: "The devices developed under NEXT open-up applications for lower voltage GaN-based products, which achieve power densities at least four times higher than GaAs devices." The new devices are expected to find applications in phased-array radar and communications systems. TriQuint has already reported cutoff frequencies (ft) in GaN exceeding 240 GHz, with Phase II work to include increasing yields while pursuing operating frequencies to 400 GHz. Phase III will seek to extend the operating frequency to 500 GHz with still higher yields and reduced circuit size.

Of course, TriQuint built its strong position as a device/foundry supplier in the RF/microwave arena by building its GaAs resourcesand doing so long before GaN became a popular option. One of the world's largest commercial GaAs foundries, TriQuint offers GaAs pseudomorphic high-electron-mobility-transistor (pHEMT) and metal-epitaxial-semiconductor-FET (MESFET) semiconductor technologies in addition to GaN devices and foundry services and indium-gallium-phosphide (InGaP) heterojunction bipolar transistors (HBTs). The foundries employ optical photolithography to form the features of larger devices and electron-beam lithography for its 0.25-m pHEMT devices. Its highest-power individual devices are based on GaN-on-SiC technology, while 0.35-m GaAs pHEMTs are not far behind.

Among TriQuint's highest-output GaAs amplifiers, model TGA2517 offers +42 dBm output power from 7.5 to 11.5 GHz with 28-dB gain and 35% power-added efficiency (PAE). Suitable for radar and military communications applications, it draws as much as 2 A quiescent current from voltage supplies of +9 to +12 VDC. In contrast, the firm's model TGA2572 GaN amplifier delivers 20 W output power (+43 dBm) from 14 to 16 GHz with 23-dB gain and 30% PAE. It draws 2 A quiescent current from a +35-VDC supply.

For lower-power, higher-frequency operation, InP-based HEMTs still show the highest cutoff frequencies and lowest noise of all three-terminal devices, and silicon-germanium (SiGe) transistors and diodes have routinely been fabricated for applications well into the millimeter-wave range, including for integrated-circuit (IC) transceivers at 160 and 165 GHz. But silicon CMOS has also shown a great deal of life at higher frequencies, with 90-nm silicon CMOS capable of fabricating ICs with +1-VDC transistors operating at 60 and 77 GHz, and more expensive 65-nm silicon CMOS processes typically delivering transistors operating beyond 100 GHz.