For RF and microwave applications, the quest for higher frequencies and higher power levels drives researchers to seek improvements in processes and device architectures.
Semiconductor technologies seem to race forward almost without impediment, especially in computer-related applications such as memory chips, logic, and processors. But those semiconductor processes geared for RF and microwave applications continue to advance steadily, even as the markets for these devices weaken, according to the latest set of presentations offered at the upcoming
2002 International Electron Devices Meeting (IEDM). Scheduled for December 8-11, 2002 at the Hilton San Francisco & Towers (San Francisco, CA), the conference features a host of quality presentations from around the world on emerging semiconductor processes of interest to RF and microwave designers, including silicon-germanium (SiGe), silicon-carbide (SiC), and gallium-nitride (GaN) device technologies.
SiGe first came to public attention approximately a decade ago at the IEDM, with announcements by Analog Devices (Norwood, MA, www.analog.com) and IBM Microelectronics (East Fishkill, NY, www.ibm.com) on high-speed data converters and other circuits fabricated with the process. Since that time, the IBM SiGe foundry has won more than dozens of believers (and customers) for its high-frequency SiGe process, which now boasts devices capable of transition frequencies (fTs) well in excess of 100 GHz.
This past May, IBM announced the shipment of its 100 millionth chip made with SiGe, produced at the company's Burlington, VT facility. The chip was delivered to test-equipment manufacturer Tektronix (Beaverton, OR, www.tektronix.com). This past summer, the company finalized the largest private-sector investment in New York State history with the completion of a $2.5 billion wafer-fabrication facility in East Fishkill, NY. Featuring 12-in. (300-mm) wafers using low-k dielectrics, copper (Cu) interconnects, and silicon-on-insulator (SOI)-based transistors, the facility is fully automated (requiring some 20,000 sensors to track wafers), but will also add approximately 1000 technical jobs to New York's Hudson Valley. The LINUX-based facility is managed by approximately 1700 microprocessors running at 1 GHz with about 600 TB of memory. An internally developed software program known as SiView controls the manufacturing operations. According to New York Governor George E. Pataki, "I've been to Silicon Valley. They don't have the trees we have. They have earthquakes and their lights go out. The Hudson Valley is a much better place to innovate and work."
In addition to IBM, companies deeply involved in the development of SiGe materials and semiconductors include Maxim Integrated Products (Sunnyvale, CA; www.maxim-ic.com) and Atmel (San Jose; CA, www.atmel.com). Most recently, Intel Corp. (Santa Clara, CA; www.intel.com) announced plans to develop SiGe-based mixed-signal circuits which, combined with the company's expertise in complementary metal-oxide semiconductor (CMOS), would result in high-frequency devices for wireless local-area networks (WLANs), as well as high-speed circuits for optical components operating in excess of 50 Gb/s. Intel's Chairman, Andy Grove, will address IEDM attendees at a luncheon presentation entitled "Changing Vectors of Moore's Law."
Although SiGe is usually associated with lower-power, lower-voltage applications, several companies have explored the use of the semiconductor material for power amplifiers (PAs) and devices. The WPTB48F2729C SiGe power heterojunction bipolar transistor (HBT) from Northrop Grumman Electronic Systems' Advanced Technology Center (Baltimore, MD; www.northrop-grumman.com/es/atc) is designed for pulsed radar applications from 2.7 to 2.9 GHz. Suitable for air-traffic-control (ATC) systems, the transistor is rated for maximum collector voltage of +55 VDC and peak collector current of 14 A. It is configured for common-base operation and will deliver 180-W Class C output power (with 32-W input power) when tested with 60-µs pulses at a 6-percent duty cycle at 2.8 GHz.
SiGe Semiconductor (Ottawa, Ontario, Canada; www.sige.com) also recently unveiled a SiGe PA integrated circuit (IC), the SE2522L, targeted at WLAN amplification. Capable of operating at 100-percent duty cycle, the amplifier delivers +20-dBm linear output power at 2.4 GHz with adjacent-channel power ratio (ACPR) of less than −20 dBm per 100 kHz. Unlike the high-voltage Northrop Grumman device, the SE2522L is designed for a supply voltage of +3.3 VDC.
Several presentations at the IEDM will highlight research advances in SiGe, including a report by J. Bock and associates from Infineon Technologies (Munich, Germany; www.infineon.com) revealing devices with gate delays of less than 5 ps. Designed for mixed digital and analog use, the SiGe bipolar technology achieves fT of 155 GHz at a breakdown voltage of +1.9 VDC, and a maximum frequency of oscillation (fmax) of 167 GHz with only 4.7-ps gate delay. The researchers will detail a 99-GHz digital frequency divider and a low-noise amplifier (LNA) with 2.2-dB noise figure at 19 GHz.
At even higher frequencies, researchers at IBM Microelectronics will announce a new SiGe process featuring a raised extrinsic base with low base resistance (for low noise and high gain). The novel architecture allows the process to achieve HBT devices with fT performance of 200 GHz and fmax performance of 285 GHz. The fabricated devices feature emitter stripe width of 0.12 µm with stripe length optimized to balance high perimeter-to-area ratio (and achieve the low base resistance). The technology implies useful LNAs to 26 GHz, with the researchers to report minimum noise figures of 0.4 and 1.5 dB, respectively, at 10 and 26 GHz. And if 200 GHz is not high enough, J.S. Rich and additional IBM researchers will also report on SiGe HBTs with cutoff frequencies approaching 300 GHz.
D. Knoll and associates from IHP (Frankfurt, Germany) will reveal their efforts at creating a simplified, low-cost SiGe:C bipolar-CMOS (BiCMOS) process capable of producing a one-mask HBT module. The technology is a 0.25-µm BiCMOS SiGe:C process with 19 lithographic steps offering four levels of aluminum (Al) and a variety of active and passive devices. Based on a standard CMOS approach, the process can be used to fabricate devices with fT performance to 75 GHz at +2.4 VDC. Takashi Hashimoto and associates from Hitachi Ltd. (Tokyo, Japan) will also unveil their work on the integration of a standard CMOS process platform with 0.13-µm SiGe. The approach yields HBT devices with cutoff frequency of 122 GHz and fmax of 178 GHz due to the low base resistance.
In the area of power-device technology, James Fiorenza and fellow researchers from the Massachusetts Institute of Technology (Cambridge, MA) will review new technology for RF-power laterally-diffused-metal-oxide-semiconductor (LDMOS) transistors above 2 GHz. The novel approach features a damascene gate with very low gate resistance to minimize RF gate-resistance losses. Using this topology, the researchers examined three approaches for reducing RF substrate losses: SOI technology, high-resistivity-bulk-silicon (HRS) technology, and high-resistivity SOI technology. The HRS technology was found to improve power-added efficiency (PAE) compared to standard bulk Si. By combining the low-loss substrates with metal/poly-Si damascene gates with low sheet resistance, the researchers created LDMOS devices with 0.5-µm gate length and 20-nm gate-oxide thickness on high-resistivity SOI substrates capable of +23-dBm output power at 1.9 GHz and +3.6 VDC and +27-dBm output power at 1.9 GHz and +6 VDC. The peak PAEs are 60 and 63 percent, respectively, with PAE of nearly 45 percent possible at 4 GHz.
SiC has long been a substrate of interest for high-power-device developers. Cree Microwave (Sunnyvale, CA), for example, recently announced its new LDMOS 8 process technology based on SiC. At present 30- and 60-W transistors have been released to production, with larger 85- and 125-W devices soon to follow. The devices are designed for cellular and personal-communications-services (PCS) frequency ranges for use in cell sites and base-station equipment.
Pushing the high-frequency limits of SiC by combining it with GaN technology, R. Quay and associates from the Fraunhofer-Institute of Applied Solid-State Physics (Freiburg, Germany) will describe their work on devices intended for frequencies through 40 GHz. They will describe AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on SiC substrates capable of breakdown voltages up to +60 VDC. For devices suitable for high-power use (with extended gate-drain spacing), the researchers achieved cutoff frequencies to 60 GHz and fmax up to 140 GHz at a drain-source voltage of +7 VDC, the highest reported frequencies for GaN-based HEMTs.
H. Kashahara and associates from the Photonic and Wireless Devices Research Labs of NEC Corp. (Shiga, Japan) will also detail their experiences with power GaN devices, notably at Ka-band frequencies. The researchers work on power AlGaN/GaN heterojunction field-effect transistors (FETs) has yielded more than 2-W output power at 30 GHz from a single chip with a gate width of 0.36 mm. By developing a device with a gate length of 0.25 µm, the researchers achieved linear gain of 8.8 dB at 30 GHz and 6.6 dB at 38 GHz, indicating the potential of this technology for millimeter-wave use.
Using gallium indium phosphide/gallium arsenide (GaInP/GaAs), researchers from the Ferdinand-Braun Institut fur Hochstfrequenztechnik (Berlin, Germany) and United Monolithic Semiconductors GmbH (Ulm, Germany) will offer HBT power cells operating to +32 VDC as an alternative to high-voltage Si LDMOS transistors. The power cells are capable of delivering 10 W of output power at 2 GHz with considerably higher output impedances than silicon LDMOS devices, making it significantly easier to combine the outputs of multiple devices in high-power applications.
Finally, lest CMOS be forgotten (see sidebar), J.Y. Yang and co-workers from Texas Instruments (Dallas, TX) will report at IEDM on a 0.1-µm RF CMOS process on high-resistivity substrates for system-on-chip (SoC) applications. The researchers will offer results for devices past 5 GHz, using a standard process approach that allows the use of standard library device models and structures. For more information about the 2002 IEDM, please contact Phyllis Mahoney, Conference Manager, Widerkehr & Associates, 16220 S. Frederick Ave., Suite 312, Gaithersburg, MD 20877; (301) 527-0900, or visit the IEDM website at www.ieee.org/conference/iedm.