Process Improvements Drive Device Advances

Jan. 26, 1998
High-frequency devices and ICs continue to achieve new levels of performance thanks to enhancements in basic materials and semiconductor process technologies.

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 ( 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.

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Cree Microwave, a supplier of SiC wafers and SiC MMIC foundry services, also offers SiC power transistors, such as its MESFET model CRF-24010 which is available in pill- and flange-style packages. The SiC power transistor boasts 10-W minimum output power at 1-dB compression with 15-dB typical small-signal gain at 2 GHz. The drain efficiency is typically 45 percent at +48 VDC and 250 mA, and the third-order intermodulation distortion is a respectable −31 dBc. In spite of its healthy output power, the device also achieves a minimum noise figure of 3.1 dB. (Cree also offers 600-V SiC Schottky diodes with fast switching speeds for reduction in size of EMI filters, for high-power electronic systems.)

In contrast, GaN devices are still largely in the research phase. With a strong push by the Office of Naval Research (ONR,, research on GaN high-power devices is being pursued by more than 100 laboratories at present, including major defense contractors such as the Information and Electronic Warfare Systems division of BAE Systems ( and the Electronic Sensors and Systems Sector of Northrop Grumman. While the material holds great promise for extremely high-power discrete transistors at microwave frequencies, GaN wafers are still expensive and generally only 2 inches in diameter compared to wafers as large as 6 and 12 inches in diameter, respectively, for GaAs and silicon wafers. Recently, however, TriQuint Semiconductor and defense contractor Lockheed Martin announced major strides in the development of a power GaN high-electron-mobility-transistor (HEMT) device. At an undisclosed frequency, the firms noted a power density of 11.7 W/mm, output power of +34 dBm, small-signal gain of 9.83 dB, and power-added efficiency of better than 50 percent (see figure). According to Dr. Mahesh Kumar, director of Research and Technology for Lockheed Martin Maritime Systems and Sensors business (Moorestown, NJ), "Gallium nitride will redefine what is possible by providing our customers the reliable, compact, high-powered technology they need to field solid-state phased-array radar, space systems, and missiles to protect against emerging threats."

One semiconductor company now offering GaN devices is relative newcomer Nitronex Corp. (Raleigh, NC). Founded in 1999 by Dr. Kevin Linthicum and three other graduate students from NC State University, the company is one of the best-funded private firms in North Carolina, having raised more than $45 million in funding. The company's prototype discrete devices include the +28-VDC models N10 and N20 RF power transistors for WCDMA applications. The former delivers more than 10 W output power while the latter is designed for more than 20 W of WCDMA output power; both flange-package-mounted transistors exhibit 11.5-dB gain from 1800 to 2200 MHz with 25-percent typical efficiency. The company plans on releasing a 36-W WCDMA power transistor later this year.

At higher microwave and millimeter-wave frequencies, power generation is still a challenge, and more mature semiconductor materials such as GaAs and InP are the substrates of choice for most devices. Velocium, formerly a part of TRW and now a Northrop Grumman company, offers both GaAs and InP foundry services, with one-tenth-micron HEMT GaAs and InP devices featuring cutoff frequencies as high as 120 and 180 GHz, respectively. The company recently announced the model APH462 two-stage 15-to-27-GHz GaAs amplifier for point-to-point digital radios at 18, 23, and 26 GHz. It features 17-dB gain and more than 1-W saturated output power over the operating band.

Even at lower frequencies, GaAs MMICs still comprise a large portion of the RF active devices used in cellular handsets, a factor that motivated Fairchild Semiconductor's acquisition of the RF Components Div. of Raytheon Company last October. The move, which strengthened Fairchild's presence in the wireless-communications market, provides the semiconductor pioneer and leading supplier of power semiconductors with a strong foothold in a growing market. According to research firm Strategy Analytics, the total market for GaAs power amplifiers is projected to grow to between $0.77 and $1.2 billion by 2006, at a compound annual growth rate of 16 percent. Fairchild also acquired Raytheon's foundry partnership and an equity stake in WIN Semiconductor.

In addition, silicon discrete devices, whether as bipolars, MOSFETS, or LDMOS devices, still dominate applications requiring tube-like power through about 1200 MHz. In pulsed avionics applications, for example, LDMOS transistors from Advanced Power Technology provide output power to 300 W at 1090 MHz. The company also offers MODE-S transistors for pulsed outputs to 1100 W at 1090 MHz.

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