CMOS And SOI Invade RF Front End

By Mark LaPedus
The next-generation 4G wireless standard known as long-term evolution (LTE) presents some new and difficult design choices for OEMs.

One of the more difficult choices involves the less glamorous, but arguably the most critical part in a handset—the radio-frequency (RF) front-end. Typically, the RF front-end often comes in a module and includes various key components, such as the power amplifier (PA), antenna switch and filter.

The latest RF front-ends are moving towards multi-mode, multi-band PAs, based on the traditional technology for PAs—gallium arsenide (GaAs). The new PAs handle more frequencies, but it’s still difficult to support all 40 LTE bands; the RF front-end would end up being too big and costly. So for practical purposes, a 4G handset generally is configured with a different RF front-end to support various bands in a specific region, a sometimes complex and cumbersome process for OEMs and carriers alike.

But now there are some new options in the mix, which could help solve the band fragmentation problem for LTE and also turn the RF market upside down. One vendor, Peregrine Semiconductor, has been sampling a PA based on a variant of silicon-on-insulator (SOI) technology called silicon-on-sapphire (SOS).

And looking to accelerate the deployment of LTE, Qualcomm recently unveiled an RF front-end device, based on a mix of bulk CMOS and SOI. Instead of using an RF module, Qualcomm’s solution is housed in a package-on-package (PoP) configuration, enabling OEMs to save board space and re-configure the device more easily for a given region.

Multiple sources indicate that Qualcomm’s RF front-end incorporates the industry’s first multi-band, multi-mode PA based on SOI. Qualcomm declined to comment, saying the company isn’t ready to break out the technologies within the device. But after dissecting Qualcomm’s device, analysts said the part poses as a potential threat to GaAs-based PA suppliers, such as RF Micro Devices, Skyworks, TriQuint and others. “Qualcomm fired the first shot across the bow,” said Eric Higham, an analyst at Strategy Analytics, a research firm. “The subsystem consists of an antenna tuning IC, an envelope tracking (ET) IC for Qualcomm’s PA and a multi-mode, multi-band CMOS PA fabricated using a silicon-on-insulator substrate.”

Christopher Taylor, an analyst with Strategy Analytics, added: “This does not mean the death of GaAs, but the Qualcomm announcement undoubtedly signals faster acceptance of CMOS PAs. To stay competitive, GaAs PA suppliers will have to continue to innovate, and they may also need to offer their own CMOS PAs for the most cost-sensitive phones, as Skyworks and RFMD have already done.”

All told, there are some dramatic changes taking place in the RF front-end, where CMOS, SOI, and SOS are making inroads at the expense of GaAs. “GaAs has been displaced by SOI in the switch,” said Rodd Novak, chief marketing officer of Peregrine. “The PA is the next thing to conquer. The stranglehold that GaAs has on the power amp will start to erode.”

RF complexities for LTE
The stakes are high in the RF front-end, a $5 billion business, according to Strategy Analytics. The big market is LTE, a technology that boasts data rates of up to 100 megabits per second, which is up to 10 times faster than 3G. In total, there were 88 million connections on LTE networks in 2012, but this number is projected to jump to 322 million in 2013 and 1.6 billion by 2017, according to the firm.

LTE could grow even faster, but in many respects the technology is being held back amid a slew of challenges, namely the band-fragmentation problem. Today, there are four frequency bands in 2G cellular networks and five for 3G. “Right now, there are about 40 cellular LTE bands in total when you add 2G, 3G and 4G worldwide,” said Peter Carson, senior director of marketing for Qualcomm. “And so the challenge in terms of getting to scale in an LTE device, meaning the ability to design one device and be able to ship it anywhere, is really a function of how many bands you have in LTE.”

The problem is that many countries support their own LTE frequencies, making it difficult for handsets to provide coverage for all 40 bands. “Each country has its own frequency challenges,” said Shane Smith, vice president of mobile devices global marketing at TriQuint. “So, you are dealing with multiple bands in each country at a 3G level. This proliferates in LTE. And then with global roaming, (the bands) can’t interfere with each other. And that’s where the RF complexity is significant.”

In another example of the complexities, AT&T uses Band 17 and bought some spectrum in Band 4 for LTE. Technically, the two bands are not contiguous. But AT&T has implemented carrier aggregation techniques to make them look contiguous. “That’s the benefit and advantage of carrier aggregation, but that causes the RF architecture to change (to meet) that new requirement,” Smith said.

Generally, the 2G and 3G cell phone is relatively simple. Chipmakers ship an RF front-end, which includes a discrete PA that would support a particular band. In contrast, OEMs face some difficult choices with LTE. In theory, OEMs could build a “universal” handset that could support all LTE bands, but that could be large and expensive due in part to the RF content, screen size and other features. “You would be paying a lot of extra cost for bands that may or may not be used,” Smith said.

In a more practical scenario, OEMs can develop “regional” phones that support limited bands in a given region. But still, the question is how much RF content does a “regional” handset require? It depends on the type of handset and price point. As a rule of thumb, Smith draws the line at four bands. A handset that requires four or more bands may need multi-mode, multi-band PAs, while cheaper discrete PAs are suitable for a phone with anything less than that.

“Of all the smartphones shipped this year, the average band count is actually still less than four. Some 60% to 70% of the market would probably lean towards a more discrete solution, whether that is a discrete PA or putting two power amps in one package,” he said. “Some 30% to 40% of the market would take advantage of multi-mode, multi-band PAs. The ones shipping today would probably (support) six to seven bands. Then, on top of that, they also have discrete PAs, which can be populated or de-populated depending on the region they want to support.”

OEMs face other complex choices. To date, the PA has been dominated by GaAs. Now there are some new and emerging PAs based on CMOS, SOI and SOS, all of which promise to provide more integration and have lower power than GaAs. What’s next? “The RF antenna switch is moving from III-V materials to SOI,” said Paul Boudre, chief operating officer at Soitec. “GaAs pHEMT will not disappear, but it will remain for more specific devices.”

Soitec sees a surge in its RF business, where the company develops substrates based on bonded silicon-on-sapphire (BSOS) and high-resistivity SOI. “Our technologies’ market penetration in smartphones and other RF-based communication devices proves that our engineered substrates are competitive,” Boudre said.

GaAs is still a better solution for the PA, TriQuint’s Smith contends, but SOI still has its place. “All of the traditional RF manufacturers have SOI designers and are making many of our switches in SOI,” Smith said.  “SOI has better insertion loss and some natural linearity aspects due to the materials that GaAs pHEMT switches could not meet very easily. The SOI performance actually meets or exceeds (GaAs pHEMT). And there is a cost advantage.’’

The new contender

Leveraging the benefits of bulk CMOS and SOI, Qualcomm recently rolled out the RF360, a front-end solution that combines a PA, antenna switch, antenna matching tuner and an envelope power tracker. Supporting all seven cellular modes, the RF360 also works in conjunction with Qualcomm’s digital cell-phone chipsets.

Qualcomm integrated the PA and antenna switch into one device. “What we tried to solve here is what we call the LTE band fragmentation problem,” Qualcomm’s Carson said. “The integration of the PA and antenna switch frees up the board area so you can have enough space for the filters, duplexers and additional switches to support roaming bands, and have a single design that can be shipped to any market.”

Another key is that the device comes in a PoP package, which cuts board space by 50%. “It allows (OEMs) to have a faster development cycle,” added Steve Brown, senior director of product management at Qualcomm. “By just changing the top of the PoP package, you can actually have a different set of characteristics in bands for a given region and phone.”

The RF360 is based on both CMOS and SOI. “It’s a mix-and-match of SOI and CMOS,” Brown said. “What we’ve done is look at each of the various areas and look at the best way to get to the highest levels of integration.”

For PAs, many argue that GaAs has a huge power-added-efficiency (PAE) advantage over CMOS. Brown dismissed that notion, saying CMOS and SOI are indeed ready for LTE. “You can actually use CMOS for very complicated RF front-end solutions. For example, we have a GSM, UMTS, CDMA and LTE front-end all on one piece of silicon,” he said.

Another key to Qualcomm’s PA is a technology called envelope tracking. In this approach, the voltage is constantly adjusted to make sure the PA is operating at peak efficiency. “PA efficiency is a challenge,” Carson said. “You don’t want to waste power and generate heat. Those two things are critical to smartphone design because you want to preserve battery life. If you don’t do something like envelope tracking, you actually waste power.”

Qualcomm’s rivals are keeping a close eye on the company’s new RF solution. “Do I think it’s a competitive threat long term? Sure,” said TriQuint’s Smith. “But I also think the CMOS solutions are not superior in performance to GaAs (for the PA).”

Qualcomm already dominates the cell-phone chipset business. Many OEMs may want to differentiate their RF front-ends and not get locked into using both Qualcomm’s chipset and RF solution, Peregrine’s Novak said. In any case, Qualcomm’s solution is a step towards bringing out the long-awaited single-chip, monolithic RF front-end. But it’s unlikely that OEMs will see a single-chip RF solution anytime soon due to cost. “The RF architectures are also changing so quickly,” Novak added.