While the silicon CMOS process has made significant progress in terms of integrating RF functions on-chip and delivering complete system-on-a-chip (SoC) solutions for cellular handset and Bluetooth applications, the frequency covered is limited to the lower echelon of the microwave spectrum. But, by combining SiGe bipolar transistors with conventional CMOS, designers are now tapping the benefits of BiCMOS to extend the reach of silicon RFICs from the upper end of the microwave region into the millimeter-wave space. By merging transistors with 155 GHz f_T capability with high-quality passive elements, the SiGe BiCMOS process is enabling designers to integrate bipolar-based microwave and millimeter-wave functions with CMOS-based mixed-signal and digital control blocks on a single die.

In fact, SiGe BiCMOS is now taking on challenges that were once dominated by the gallium arsenide (GaAs) and other compound semiconductor technologies. A good example of this trend is a recent development of an eight-element 6 GHz to 18 GHz phased-array chip by researchers at the University of California, San Diego (UCSD). Combining accurate models and design with Jazz Semiconductor's 0.18-micron SiGe BiCMOS process, UCSD designers have developed an eight-element RFIC phased array receiver covering the 6 GHz to18 GHz frequency range. According to the researchers, the SiGe BiCMOS chip is only 2.2 mm × 2.3 mm, replaces at least 16 GaAs chips, consumes 20x less power than traditional phased array implementations. It will also allow a new generation of miniature and low-cost phased arrays for X-band to Ku-band applications.

“This is the first demonstration, ever, of a single silicon chip with eight complicated 6 GHz to 18 GHz phased array receivers together with all the necessary CMOS controlling circuits,” stated professor Gabriel M. Rebeiz of the department of electrical and computing engineering (ECU) at UCSD, and a co-developer of this chip.

By developing this chip, UCSD has successfully demonstrated independent amplitude and phase control at 6 GHz to18 GHz of eight different antenna elements with at least four-bit of phase resolution. Thus, it paves the way for commercial feasibility of highly integrated RFICs for X-band and Ku-band phased array applications.

While this SiGe BiCMOS chip was designed and tested by ECU graduate student Kwangjin Koh, it was sponsored by the DARPA's scalable millimeter-wave array technology (SMART) program. It was subcontracted to UCSD from Teledyne Scientific Corporation in Thousand Oaks, Calif.

On chip, it integrates eight silicon low-noise amplifiers (LNAs) operating at 6 GHz to 18 GHz, eight-phase shifters with at least four-bit of phase control, and an 8:1 active power combiner with very wide bandwidth. Plus, all the digital functions needed to control the chip such as the address decoders for the individual eight elements, the memory latches for the phase settings, the clock enable functions to load the information on the chip, and power regulators. It consumes 140 mA to 200 mA of dc current from a 3.3 V power supply to provide a RF gain from 12 dB to 24 dB with a noise figure of 6 dB. It can be integrated directly with eight planar antennas on a standard PC board.

Phased arrays have been in use since the 1950s in defense applications and have seen limited use in commercial systems due to their relatively high cost. Hence, this new development at UCSD can be viewed as a dramatic improvement in the design of low-cost phased arrays and is likely to push this technology into the commercial sector as defense systems continue to benefit from these advances.

However, before such a development can be migrated to the production phase, it must tackle issues like operating temperature range, supply voltage deviations and process variations. This chip was tested at room temperature. Thus, to make it useful for commercial and defense applications, it must be tested over a wide temperature range. Plus, it must handle a wide supply voltage range — 2.5 V to 4.0 V. And, it must be tweaked to tackle foundry process variations. Meanwhile, the researchers are also planning to push the frequency range to 30 GHz to 50 GHz by summer.