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CMOS takes a stab at deflating GaAs' RF dominance

Posted: 01 Aug 2005 ?? ?Print Version ?Bookmark and Share

Keywords:gallium arsenide? rf module? rf cmos? bicmos? mtt-s 2005?

Is gallium arsenide losing its hegemony among designers of RF modules? The threat from RF CMOS and BiCMOS with silicon germanium implants was a lively topic of conversation at the MTT-S 2005 International Microwave Symposium.

Many participants in a panel session agreed that RF CMOS may be useful for low-cost, short-distance applications like WLANs and Bluetooth transceivers, but not for cell phone power amplifiers (PAs). GaAs will continue to dominate in that slot, some suggested.

Roughly 664.5 million cell phones shipped in 2004. At two or three power amps per phone, the total approached 2 billion PAs last year alone, said panel organizer Fazal Ali, manager of RF engineering and technology at phone maker Nokia. How many of those PAs were CMOS? "Not very many."

Co-organizer and moderator Mike Golio of HVVi Inc. (Phoenix) pondered the potential for displacing GaAs in cell phone PAs. Might CMOS PAs eventually dominate the mobile space? Would they coexist with III-V compound semiconductor processes or silicon bipolar junction transistors? Or would they remain the plaything of some adventurous designers, never capturing much market share?

CMOS has had no commercial successes in the handset PA market, said panelist Aditya Gupta, vice president of technology for GaAs transistor maker Anadigics Inc. Gupta's sentiments were echoed by Pete Zampard of Skyworks Solutions Inc. (Woburn, Mass.), which builds modules with both GaAs and silicon discretes.

CMOS, Zampard quipped, "stands for 'can't meet our specs.' "

But Ali Hajimari, a professor of electrical engineering at the California Institute of Technology and co-founder of Axiom Microdevices Inc. (Orange, Calif.), said CMOS will be a formidable opponent for III-V PAs. Axiom has successfully demonstrated a single-chip, fully integrated quadband PA for GSM/GPRS applications in 0.13-micron CMOS, he claimed.

Hajimari cited 55 percent efficiency for the PA, a level that he believes is comparable to GaAs specs. The part includes matching I/Os and meets cellular designers' concerns for performance and ruggedness. The use of integrated CMOS PAs will pave the way for entire radios on the same chip, he said.

It is nonetheless likely that GaAs, RF CMOS and BiCMOS will coexist in many systems, panelists said.

"We are not striving to build an SoC [system-on-chip]," Rik Jos, a fellow in RF device technology for Philips Semiconductors (Nijmegen, Netherlands), told EE Times. The problem for integration, he said, is in building radios that operate on multiple frequencies and embody multiple standards. "Every generation will need a new PA, and there are only limited windows for accomplishing this on a CMOS SoC," Jos said.

Rather, Philips is building multichip modules for performance-oriented RF applications. "We integrate system-in-package, with the best or cheapest technology for each component," Jos said.

Like an express train

The persistent use of GaAs in RF designs, however, hasn't stopped manufacturers from pushing performance thresholds with CMOS devices. Peregrine Semiconductor Corp. has tweaked CMOS for use as an antenna switch and as a power amplifier, said Rodd Novak, VP of marketing.

Peregrine's Ultra CMOS process uses dielectric isolation, ensuring the performance of each MOS device in a complementary pair by isolating it from cross-coupling effects of the substrate. The technology is effectively silicon-on-insulator. Peregrine claims its 0.13?m Ultra CMOS devices are less complicated to manufacture than GaAs and perform better than others' 90nm CMOS at extended frequencies.

"Module design using GaAs transistors has been the traditional sweet spot for engineers," acknowledged Ted Miracco, vice president of Applied Wave Research Inc., an RF EDA tool maker. "But people are finding clever ways to do things in CMOS. We see that as the futurecoming at us like an express train."

- Stephan Ohr

EE Times

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