RF SOI Shines for 5G Power Amps

By: Dave Lammers

Using GF’s 45RFSOI technology, UCSD Prof. Peter Asbeck recently developed a power amplifier operating at 28 GHz with output power of 22dBm and more than 40 percent power-added efficiency (PAE).  When backed off for the 64QAM OFDM signals used in 5G, the amplifier achieves an average output power of 13 dBm at 10 dB backoff, with 17 percent PAE, even without digital predistortion.“This is the right level of power and efficiency for the majority of the 5G 28 GHz applications,” Asbeck said.

Power amplifiers (PAs) are a different breed of cat from most other chips, and the PAs needed for the 5G wireless solutions are likely to be much different than those used in today’s 4G smart phones and base stations. Most 5G wireless applications will use phased array antennas to focus and steer multiple beams, and it is this ability to divide the transmission task among multiple beams, which gives 5G the ability to achieve what, to many, seem improbable performance targets.

While early 5G systems will use the sub-6 GHz frequency range, the real promise of 5G comes from using bandwidth in the 24, 28, and 39 GHz millimeter-wave ranges. There, phased array antennas, such as a 4×4 array, will be deployed, with each PA operating at much lower power than those needed to amplify the single-beam, omnidirectional signals used now.

Ned Cahoon, RF business development director at GF, said in the 4G wireless generation, gallium arsenide (GaAs) has been a leading technology in the power amplifier sector. “We believe we are moving from a sub-6-GHz regime, where gallium arsenide has dominated, to the millimeter-wave market, where most front-end solutions will be in silicon.”

Already pervasive in handset switches and antenna tuners, Cahoon said RF SOI technology – in production for over a decade and now extended by GF to 300mm diameter wafers at the 45nm node – is ideal for the integrated front-end devices needed for the millimeter-wave 5G handsets, access points, and base stations.

Cahoon bases that belief partly on work being done by several of the key professors working in the power amplifier field, notably Peter Asbeck at the University of California at San Diego (UCSD). Asbeck earned his doctorate at M.I.T., spent 15 years in industry developing high-frequency wireless technologies, and became the Skyworks Professor in High Performance Communications Devices and Circuits at UCSD’s Jacobs School of Engineering, and is a Member of the National Academy of Engineering for his development of the GaAs HBT device.

State of the Art

Using GF’s 45RFSOI technology, Asbeck recently developed power amplifiers operating at 28 GHz that can provide up to 22dBm output power and peak PAE of more than 40%. In a 5G application, the transmitted waveform requires substantial backoff from peak power, and excellent linearity.  The 45RFSOI circuit provides an average output power of 13 dBm, with 17 percent backoff PAE for the 5G case. “This is the right level of power and efficiency for the majority of the 5G 28 GHz applications,” Asbeck said, adding that the PA was used to transmit the standard 64 QAM OFDM signals, without using expensive digital pre-distortion (DPD) filtering techniques. While noting that other research labs are approaching similar results, he described his lab’s 45RFSOI-based power amplifier as “pretty much state-of-the-art.”

“There is really intense competition now between different technologies for the emerging 5G system slots.  45RFSOI is very close to being the ideal technology for configuring the RF front-end modules for 5G at 28 and 39 GHz. I think it is likely to emerge as the winner for very many 5G systems,” Asbeck said.

That statement is particularly interesting because during his career much of Asbeck’s work has been in gallium arsenide (GaAs), which is able to support the high voltages required for power amplifiers more easily. Since moving to UCSD in 1991, Asbeck has pioneered the ability to put silicon-based transistors in a series to achieve higher voltages, with these “stacked” transistors together providing the required output power. Four transistors in a serial arrangement are sufficient to produce the voltages required for most PA’s, he said. (Cahoon said, simply put, RF power equals the voltage times the current, with higher voltages needed for optimal linear circuits; basically the opposite of digital ICs.)

Asbeck is quick to note there are competing technologies to 45RFSOI, including gallium arsenide (GaAs), gallium nitride (GaN), and silicon germanium (SiGe). Another contender is the 22FDX® technology from GF. In a Foundry File blog last year, another professor at UCSD, Gabriel Rebeiz, described the work he is doing to develop low-noise amplifiers (LNAs) in the 22FDX technology.

Rebeiz, in an interview, said he believes the power levels in 22FDX-based amplifiers can be increased so that integrated 5G front-end solutions can be developed. But Rebeiz tipped his hat toward Asbeck’s work in 45RFSOI, saying “it is essential that the functions be integrated together, without that you do not have a (marketable) part. With RF SOI, besides the stacked-transistor PAs, you can also stack the switches. My group, together with GF, has shown 45RFSOI-based switches with only 0.8 dB of insertion loss. So yes, 45RFSOI is an ideal front-end-module technology.”

Besides the power amplifiers, RF front-end solutions need to integrate the low-noise amplifiers and switches, as well as phase shifters and variable gain amplifiers. RF SOI, Asbeck said, has proven to be “the world’s best approach” for millimeter-wave switches, which “outshine” switches available in SiGe HBT or GaAs technologies.

“There are some applications in 5G, to be sure, that require higher output power than has been demonstrated so far in 45RFSOI. The first generation 5G deployments may employ PAs in other technologies such as SiGe HBT or GaAs or GaN.  But we think there is a great opportunity to further increase the output power achievable with 45RFSOI into the Watt range for peak power, and to take over these slots as well as the lower-power ones,” he said.

Minimizing Parasitics

What gives RF SOI an edge? Asbeck said to reach the required levels of output power, transistor stacking “needs to be done – that is, placing a number of FETs in series, so that the overall voltage handling is increased.” The silicon-on-insulator structure of 45RFSOI removes all the parasitics associated with body-effect and substrate capacitance that hamper circuits made in bulk-CMOS.

Also, 45RFSOI has a high-resistivity substrate that “minimizes the capacitances of interconnects as well as of the devices. The three thick metal layers make the losses of matching networks the lowest of pretty much any IC technology. And the high Ft and Fmax values provide lots of gain at 28 GHz,” he said.

Battery life, so important in handsets, relates to the efficiency of the power amplifiers. Asbeck said, “45RFSOI ​has exceptional efficiency for the 28 GHz amplifiers, and it is pretty close to the best achieved in GaAs or GaN. I think that the high substrate resistivity and thick metals by themselves add about 5 percent to the PAE (power added efficiency) of the 28 GHz power amplifiers that we have made. Our record is 47 percent PAE, with saturated output power of 19.5 dBm in a simple 2-stack PA. A couple of other laboratories are reporting PAEs in this range too, using 45RFSOI.”

Cahoon said Asbeck’s 45RFSOI work has demonstrated the value of high-speed pFETs, along with the nFETs. Asbeck said the fast pFETs will enable the use of complementary circuits, with the pFETs improving the AM-PM characteristics of mostly nFET circuits. “We are also optimistic about mostly pFET circuits, because these transistors actually have potentially even better voltage handling than the nFETs,” he said. (The AM-PM conversion of an amplifier is a measure of the amount of undesired phase deviation (PM) that is caused by amplitude variations (AM) inherent in the system.)

For me, I took away two thoughts. One is that GF’s “pivot” away from leading-edge bulk CMOS in order to support technologies such as RF SOI was a smart move.

And listening to Asbeck, Cahoon and Rebeiz, one senses a confidence that millimeter-wave 5G wireless is readily achievable, providing the world with unbelievably fast wireless connections based on affordable, highly integrated ICs.