Semiconductor processes and materials continue to improve, leading the way for steady advancements in discrete-device and integrated-circuit (IC) performance and value. Several decades ago, high-frequency devices were almost exclusively based on silicon materials. Today, gallium arsenide (GaAs) has matured into a semiconductor material for commercial, military, and even commercial products, and a host of other semiconductor materials, such as gallium nitride (GaN) and indium phosphide (InP), are finding application in strong niche markets. Even silicon has matured to the point where basic CMOS processing can now yield RF and microwave devices at frequencies once considered the exclusive realm of GaAs. And the "offspring" of basic silicon—silicon germanium (SiGe) and silicon carbide (SiC)—are beginning to fulfill the promise of earlier research claims.

The list of high-frequency semiconductor suppliers has never been longer (see table). Driven in large part by expanding markets for Bluetooth, cellular, wireless-local-area-network (WLAN), radio-frequency-identification (RFID), and other large-volume applications, RF semiconductor suppliers offer everything from small-signal low-noise transistors and large-signal power transistors to complete radio receivers, transmitters, and transceivers on a chip. Although companies with their own semiconductor foundries still flourish, the number of "fabless" semiconductor companies (relying on outside foundries) has risen dramatically. Companies such as Hittite Microwave (Chelmsford, MA), for example, are not bound to any one semiconductor technology. Although founded by former Raytheon Company engineers with expertise in GaAs device design, Hittite has taking advantage of outside foundry services to develop a line of broadband 7-GHz direct modulator products and gain blocks based on SiGe (see October 2003, p. 78). Similarly, startup company Centellax has employed outside SiGe foundry services to fabricate its advanced fractional-N synthesizer design (see December 2003, p. 88).

How high can silicon go? That question certainly has come to haunt those with significant investments in GaAs foundries and semiconductor product lines. Although GaAs high-power transistors are well entrenched in a variety of terrestrial and satellite-communications bands from such suppliers as California Eastern Laboratories (NEC), Excelics Semiconductor, Fujitsu Compound Semiconductor, and Mitsubishi, GaAs is being strongly challenged by SiGe and even traditional Si CMOS for lower-power (1 to 2 W) applications through about 5 GHz. For example, SiGe Semiconductors, one of the earlier adopters of SiGe technology, offers ICs and RangeCharger RF modules for IEEE 802.11a/b/g WLAN systems operating at 2.4 and 5 GHz. The fabless semiconductor company's 802.11b/g power amplifiers were chosen by Broadcom for that supplier's WLAN reference designs.

Companies currently offering SiGe-based semiconductors constitute a long and ever-growing list that includes Atmel, Hittite Microwave, IceFyre, Infineon Technologies, Inphi, Intersil, Maxim Integrated Products, SiGe Semiconductor, Sirenza Microdevices, and RF Micro Devices. Infineon's models BFP640 and BFP650 NPN transistors, for example, are based on the company's own 70-GHz SiGe process. The devices, developed for use in WLANs, feature noise figures rivaling those of low-noise GaAs MESFETs, with a noise figure of 0.65 dB at 1.8 GHz and 1.3 dB at 6 GHz for the BFP640 device.

IBM, one of the best-known merchant foundries for SiGe processing services, currently offers a variety of technologies geared to different applications. The BiCMOS 7HP SiGe process, for example, features self-aligned emitters, shallow and deep trench isolation, and high-speed transistors capable of cutoff frequencies to 120 GHz. The 180-nm-feature process is ideal for fabricating the high-gain heterojunction-bipolar transistors (HBTs) common to many wireless circuits. Similarly, Taiwan Semiconductor Manufacturing Company (TSMC) is a well-known merchant foundry of Si and SiGe processes.

At present, power devices based on SiGe are rare, although the Electronic Sensors and Systems Sector of Northrop Grumman (www.es.northropgrumman.com) has developed a SiGe power transistor for air-traffic-control radar applications. The company's model WPTB48F2729C achieves better than 7 dB typical gain from 2.7 to 2.9 GHz with typical collector efficiency of 46 percent. Under Class C conditions, the SiGe HBT can generate more than 180 W output power when fed with pulsed (60-µs pulses at a 6-percent duty cycle) input signals.

Although SiGe can support high-power devices, it is the thermal characteristics of SiC that have made that material so attractive to developers of high-power transistors. As part of The SiC Program, NASA's Glenn Research Center has been one of the chief driving forces behind the development of high-power SiC device technology. For example, Rockwell Scientific now offers its model T4200 SiC power MESFET for applications to 3.6 GHz. The rugged transistor, with 12-dB typical gain and 25-W minimum output power at 2 GHz, achieves 40-percent drain efficiency at +50 VDC and 1200 mA. The device features a third-order intermodulation product of typically −30 dBc, making it well suited for applications in CDMA and WCDMA systems.