Speakers at CS International reveal how the introduction of envelope tracking will help silicon CMOS to wrestle a share of the power amplifier market from GaAs

RICHARD STEVENSON REPORTS



If you go to conferencesthat encompass the entire silicon industry, you may have been taunted by the saying: "˜If it can be done in silicon, it will be done in silicon'.

Like many sweeping statements, there is more than an element of truth in this − but there are also discrepancies. While everyone accepts that silicon is a very mature, low-cost semiconductor technology, there are times when it fails to deliver the performance that engineers are looking for. This accounts for the inferiority in its bang-per-buck in some types of device, and explains why the power electronics market, which was once utterly dominated by silicon, is starting to witness growing sales of wide bandgap devices.

The performance-cost equation also explains why mobiles phones are packed full of GaAs power amplifiers, which combine substantial gain with high linearity. But in this market the dominance of GaAs is under threat, due to engineers within the silicon industry getting more out of their power amplifiers with every passing year. Leading the charge are industry heavyweights Qualcomm and Peregrine, which have recently unveiled silicon CMOS components with comparable levels of performance to the incumbent GaAs technology.

Differing perspectives

How the ensuing battle between GaAs and silicon CMOS will unfold was a hot topic at the CS International conference held at the Frankfurt Sheraton Airport Hotel on 18th and 19th March. At this gathering Eric Higham, Director of the GaAs and Com pound Semiconductor Technology service at Strategy Analytics predicted growth for silicon CMOS power amplifier shipments over the next few years; Thomas Meier, Vice President of Engineering at TriQuint Semiconductor gave his reasons for why GaAs and silicon can co-exist in the wireless sector; and Jeremy Hendy, Vice President of Sales and Marketing at Nujira, explained how the introduction of envelope tracking is helping silicon CMOS to close its performance gap with GaAs.

Higham began his presentation by reviewing sales in the GaAs market since the turn of the millenium. Worth $2.5-2.6 billion in 1999, this market shot up in value the following year by the largest growth rate ever to top $3.5 billion, thanks to the deployment of fibre networks. According to Higham, the pursuit of greater bandwidth fuelled this growth in GaAs sales, but a "killer application" was never uncovered, leading to lots of "dark fibre".

When the bubble associated with dot.com boom burst in 2001, the GaAs market entered a period of weaker revenues that persisted throughout the first half of the last decade. "But 2006 saw the dawn of the wireless era," said Higham, and since then sales have climbed every year, apart from 2009, when revenues were brought down by the global economic crisis. This growth is behind a GaAs device market that was worth $5.3 billion in 2012, and tipped to have grown another 8 percent the following year, according to Higham. Some of this production is performed in house, and some is carried out in pure-play foundries, which have seen a drop in market share by around 9 percent in 2013.

In recent times, GaAs device revenue has been driven by sales to handsets manufacturers: In 2011 this accounted for 55 percent of the market, rising to 59 percent in 2012. And there is no sign of this trend petering out. Although shipments of handsets are rising slowly and should hit 1.8 billion this year, the key statistic is that smartphone sales are increasing and should be in the range 1.1-1.2 billion in 2014. GaAs is in many entry-level, mid-price and top-of-the-range handsets, but its strong credentials are particularly valued in smartphones "“ and this is good for the industry, because these devices require a higher GaAs content.

Higham also discussed Qualcomm's RF360 Front End Solution, which was unveiled at the 2013 Mobile World Congress in Barcelona. The RF360 covers all bands and all modes from 700 MHz to 2700 MHz, while boasting features such as an antenna-matching tuner, envelope power tracker and power amp and antenna switch, all in a package with a relatively small footprint.

When Qualcomm unveiled the RF360, share prices of the leading GaAs power amplifier makers nosedived. However, according to Higham, the promise of this all-CMOS front-end is yet to fully live up to its billing. He pointed out that although Qualcomm's product is impressive, it does not cover every band and there are challenges with duplexers. That is not stopping this front-end module from getting tracking in the market place, but its impact is not as fast as one might have expected.

Possibly even stiffer competition for the GaAs power amplifier will come from the Peregine UltraCMOS Global 1. Compared to the Skyworks SKY77729 GaAs amplifier, this device, which features envelope tracking, delivers very similar levels of performance, equipping it with the capability to support high-end smartphones.

However, the Strategy Analytics analyst does not expect CMOS devices to lead to the extinction of the GaAs power amplifier any time soon. "The history of GaAs shows that it is nimble and resilient," argued Higham, who pointed out that with mask sets for silicon costing between $0.5 million and $1 million, it can be expensive to develop CMOS products. What's more, he argued that the move toward higher spectral efficiency favours GaAs. In his view, this combination of factors will enable silicon CMOS to start to erode the market share for GaAs in the handset business, but it will only capture about 10-15 percent of the business in the next five years.

Peregrine Semiconductor claims that its UltraCMOS Global 1 power amplifier is the first of its kind to match the performance of a GaAs power amplifier.

Moving to modules

Meier from TriQuint offered a similar vision to Higham for the future of the power amplifier market. He argued that GaAs should dominate for at least the next three-to-five years, and could even be the incumbent technology in the middle of the next decade. But he believed that CMOS will grab some market share, due to its ease of integration and recent improvements to its performance.

According to Meier, for transmission at 1950 MHz using WCDMA modulation, a GaAs power amplifier is 5-10 percent more efficient than one based on silicon CMOS. Although this gap can be shrunk by turning to envelop tracking, which can boost silicon efficiency by 5-10 percent "“ it can be re-opened by applying envelope tracking to the GaAs amplifier.

Meier expects the cellular market to shift to more modules incorporating various materials. For example, a module could contain a GaAs power amplifier, a silicon-on-insulator switch, a CMOS power management chip, and filters based on surface acoustic wave and bulk acoustic wave technologies. If that happens, handset manufacturers will not concern themselves with what material is in the amplifier. "Silicon verses gallium arsenide will not be a question, and we will just use the technology for the best performance."

Tracking perforfmance

A more detailed insight into envelope tracking was provided by Hendy, who described the technology as "simple, but difficult." It enables more efficient amplification by replacing a fixed supply voltage with one that varies with the input signal, so that the ideal voltage is used all the time, trimming heat generated by the amplifier.

These gains in efficiency are more important than ever before. Increases in data rates are driven by the introduction of new transmission technologies sporting greater spectral efficiency, thanks to not only using the phase, but also the amplitude, to encode information (in 4G transmission the peak-to-average ratio can be as high as 7 to 1).

This trend has meant that the migration from 2G to 3G, 4G and beyond is pegging back the efficiency of the power amplifier, which in the absence of envelope tracking could be less than 10 percent in a 5G phone with a GaAs power amplifier. Such a low efficiency would result in a very short battery life.

With envelope tracking, this weakness can be addressed: "You can get 2G battery life with 4G performance," said Hendy. But this is only possible by tracking the waveform accurately, which requires fast modulation and thus a bandwidth of 30-40 MHz.

Linearity also improves with the introduction of envelope tracking. One hallmark of a GaAs power amplifier is its high level of linear gain, which can occur for output powers approaching 30 dBm, before sharpcompression kicks in.

In comparison, CMOS power amplifiers are plagued by "soggy" compression "“ gain can be reasonably linear up to 15 dBm, before rapidly declining at higher output powers. This non-linearity is highly undesirable, leading to high levels of distortion.

It is possible to minimise this non-linearity with Nujira's IsoGain envelope tracking linearization. This has been applied to a 4G CMOS power amplifier developed by Peregrine that delivers a mean efficiency of 57 percent "“ and according to Hendy, more recent designs can be even more efficient.

Increases in gain also result from Nujira's envelope tracking. This pays dividends with a range of data transfer technologies, such as LTE QAM, LTE QPSK, HSUPA and WCDMA, with the latter delivering almost 3 dB more gain thanks to envelope tracking.

In short, Hendy believes that envelope tracking enables CMOS power amplifiers to be competitive with their GaAs cousins. However, based on the presentations given at CS International, it seems that GaAs will remain the dominant technology for some time to come.