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What Future For The PA?

Will ease of integration spur eventual dominance of the CMOS power amplifier, or will GaAs continue to reign, thanks to superior performance?

RICHARD STEVENSON INVESTIGATES

If you had happened to be following the stock market on Thursday 21 February 2013, you would have witnessed a bad day for the compound semiconductor industry. Shares in two of the world's leading makers of GaAs microelectronics, Skyworks and RFMD, plummeted by more than 10 percent, and valuations of global epiwafer supplier IQE, Taiwanese GaAs foundry WIN Semiconductors and III-V substrate maker AXT also took a knock.

What was the reason for this? Was it breaking news of a supply shortage for gallium? Or a report of an impending decline in smartphone sales? No, neither were the cause "“ instead, a shot had been fired across the bows of the III-V industry by a leading maker of silicon chips for handsets, Qualcomm. On that day, during Mobile World Congress 2013, this company launched the RF 360, a single chip claimed to provide all of the functionality required for the front-ends of leading smartphones.

In hindsight it is clear that the stock market over-reacted. Qualcomm's release didn't deliver an instant death to the GaAs power amplifier. Instead, in recent times GaAs chipmakers have actually had considerable success, with share prices of the biggest players trading much higher than they were after the hit they took two year's ago. Shares in Skyworks are now hovering at around $100, about five times that on black Thursday, and the valuation of RFMD has also climbed "“ although it's not possible to know how much of that rise has been driven by positive expectations relating to the merger with TriQuint.

While this indicates that GaAs is in a healthy state, with sales on the up, it would be careless to ignore the threat faced by the silicon PA. And it maybe that it is not Qualcomm, but Peregrine, a pioneer of silicon-on-insulator technology, that is going to be the toughest opponent.

Speaking to a packed hall, president and CEO of Peregrine, Jim Cable, outlined the virtues of a CMOS front-end during the opening session of CS International, held in Frankfurt on 12 and 13 March, 2015. As one might expect, Cable didn't have it all his own way, however, with Skyworks' Ed Anthony, Vice President, Engineering, delivering a compelling case for the inclusion of GaAs in handsets sporting a variety of technologies. Qorvo "“ the company that formed this January from the TriQuint-RFMD merger "“ supported this view, with Sean Riley, Vice President, Strategic Marketing, revealing how fast the value of GaAs and filter content will rise in the coming years. Meanwhile Eric Higham, market analyst at Strategy Analytics, argued that although GaAs will continue to dominate amplification in cell phones, silicon CMOS will steadily increase its presence there.

Cable is convinced that Peregrine's technology will play a major role in handsets, arguing: "CMOS wins in every market. It's because of integration." He is not singling out GaAs when expressing this view, and points out that integration is the reason for CMOS success against PMOS, NMOS and bipolar technologies.

CMOS does not guarantee success, however. Cable believes that if a CMOS PA that to match the performance provided by GaAs, it will not win significant sales, because handset makers are not prepared to compromise performance. Products by Peregrine are claimed to meet that demanding benchmark of a CMOS product delivering GaAs performance.

Founded in 1990, Peregrine has focused on what it describes as "intelligent integration". What this means is to use one technology "“ silicon-on-insulator (SOI) CMOS "“ to produce highly capable, flexible single die that trim the overall footprint. Cable admits that the development costs for these chips can "skyrocket", but the flipside is that the production costs for the components are very low.

To illustrate the potential of CMOS within the front-end of handsets, Cable described the evolution of the company's switch products. Initially, these switches made with an UltraCMOS technology lagged those built with GaAs in the key figure of merit, the product of on-resistance and off-capacitance. But in 2011, UltraCMOS caught up, and has since overtaken by a meaningful margin, leading to a hike in sales. Shipments have more than doubled between 2011 and 2013.

At Mobile World Congress 2015, held in early March, Peregrine launched the UltraCMOS Global 1 Initiative. This is claimed to integrate the PE56500 all-CMOS front-end solution, unveiled 12 months earlier, with filter's made by Murata, which recently acquired Peregrine. The Global 1 includes a three-path multi-mode, multi-band PA, post PA switching and an antennae switch and tuner. "Any band can do any frequency, all controlled by tuning on the chip," explains Cable.

The head of Peregine drove home the benefit of the CMOS PA by comparing the Ultra CMOS Global 1 PE56500 with an equivalent GaAs-based product made by Skyworks. "The differences are pretty clear", remarked Cable, pointing out that while his company's single-package product just had two flip-chip ICs, 17 surface-mount technology passives and no wire bonds, the two-package rival had 9 ICs, 43 surface-mount technology passives and 136 wirebonds. Reduced component count is claimed to lead to far shorter design times, with the design-time-per-band plummeting from 2-4 days to less than an hour.

Upper hand for GaAs?

Whether CMOS PAs can actually deliver the same performance as those based on GaAs was questioned by Anthony, who offered a mixture of simulated results and measured performance.

By both metrics, GaAs PAs are quite a way ahead. Highlighting results presented at the 2014 RF-SOI Workshop, simulated and measured power-added efficiency (PAE) for the GaAs HBT was 71 percent and 66 percent, respectively, while for the SOI equivalent, the simulated figure was 60 percent, and performance, measured by Peregrine, 57 percent.

HBT efficiency continues to improve, with the PAE for Skyworks 2015 products hitting 78 percent. "This resets the bar on what is possible," argued Anthony, who added that GaAs also holds the upper hand over CMOS in terms of linearity, size of footprint and cost.

Skyworks philosophy is to combine the GaAs HBT with other components that are built from the best technology for the part, because this is argued to lead to a system-in-a-package that is outstanding in terms of cost, performance and time to market. The company has various in-house technologies, including SOI capability, having shipped more than 6.5 million devices "“ it views this as the best option for making an RF switch. Anthony explained that SOI offered the best bang-per-buck, with MEMS technology providing a far lower insertion loss, but being far too pricey to be considered today.

The dollar content for this switch, plus that for filters and PAs, was given in the talk by Riley, who compared a typical 2G and a 3G phone with those offering regional and global LTE capability. These figures, given in the table below, show that the advancing sophistication of phones is good news for makers of PAs "“ and even better news for those that make filters.

The surge is wireless traffic is helping to drive up revenues for GaAs microelectronics towards $7 billion, with just over half that figure associated with power amplifier sales.

Everyone a winner?

Implications of the emergence of the silicon PA, and the evolution in wireless networks, were discussed by Higham, who offered his take on the GaAs RF market. He argued that the backdrop to what is happening in this industry is the rapid rise in mobile data consumption, which is increasing at a staggering compound annual growth rate of 78 percent, according to data from Cisco.

Driving the explosion in mobile data consumption is the rising sales in cellular terminals. Shipments of tablets and E-readers are outpacing handsets, although the latter will continue to occupy the lion's share of cellular terminal sales for the foreseeable future. Sales of handsets within the handset market will continue to rise, but increasingly slowly. "Smartphones are starting to saturate "“ some areas can't support the technology," argued Higham.

The analyst went on to underline the importance of cellular revenue to the GaAs microelectronics industry: in 2013, it was worth 53 percent of the market, which totalled just under $6.6 billion. And in the wireless era, defined as 2004 and beyond, revenue has climbed by an average of 11 percent per year. An initial estimate for total revenue for 2014 is $ 6.6 billion, but this could be revised upwards. "It could be $7 billion," admitted Higham.

This makes the current market for the PA, which needs to sport multi-band, multi-mode capability, worth $ 3.5 billion. The majority of this will continue to go to GaAs for the foreseeable future, but silicon PAs will have an impact, increasing in market share from just a few percent in 2013 to 20 percent in 2018.

The view that the CMOS PA will not have substantial success overnight is shared by Cable. "Adoption will take time, but numbers will be much higher than 20 percent," claims the Peregrine CEO.

It is not just this threat from silicon that could squeeze margins of those in the GaAs RF business. The move to multi-band, multi-mode PAs makes the GaAs offering more competitive, but the size of the chip is much smaller than a family of single-band PAs, leading to changes in epitaxial volumes and revenue. With the exception of 2009, recent growth in revenue has outpaced production, implying that the revenue per square inch of epitaxial material is falling, making it tough for makers of substrates and epiwafers.

The one issue that wasn't discussed in detail by any of those speakers is the emergence of 5G. This might be good for GaAs "“ and it might even be good for GaN. To hear the arguments for and against, get along to next year's CS International, where it may well be one of the biggest talking points of the conference.

Handsets capable of operating at higher data transfer rates have a higher bill of materials for front-end components. Source: Qorvo.



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