Smartphones Dial Up New RF Processes

By Mark LaPedus
The rapid shift towards smartphones and tablets is driving the need for new and low-power chips at finer geometries.

Today, the latest application processors, integrated basebands and other digital cell-phone chips are 28nm planar devices. And it won’t be long before OEMs incorporate 20nm planar and finFET devices in their systems as a means to reduce power and extend battery life.

The mobile revolution is also having a profound impact on radio frequency (RF) designs, processes and packaging. For example, there were four frequency bands in 2G cellular networks and five or so for 3G. In comparison, the next-generation, 4G wireless standard known as long-term evolution (LTE), ultimately could support 43 bands at multiple frequencies.

To support all 4G/LTE and legacy bands worldwide, the smartphone would incorporate a gigantic, power-hungry and expensive RF front-end. So, it’s difficult to envision a “world smartphone” that supports all 4G/LTE bands in every country. Instead, consumers may settle for “regional” smartphones that support some but not all 4G/LTE bands.

“The big challenge is to cover as many bands as possible in a region with an easy-to-use and low-cost system,” said Christopher Taylor, an analyst with Strategy Analytics, a market research house. “As long as the industry moves toward more bands and faster data rates, it puts more stress on the component guys.”

Responding to the demands, RF chipmakers are rolling out a new class of multi-mode, multi-band power amplifiers for 4G/LTE, based on traditional gallium arsenide (GaAs) technology. And suppliers also are ramping up new power amps based on CMOS and a silicon-on-insulator (SOI) variant called silicon-on-sapphire (SOS).

Other changes are taking place in the RF front-end. “The RF antenna switch is moving from III-V materials to SOI,” said Paul Boudre, chief operating officer at SOI wafer specialist Soitec. “GaAs pHEMT will not disappear, but it will remain for more specific devices.”

In 4G/LTE, there is also a need for a new class of diversity switches and tunable capacitors. All told, RF chipmakers are moving toward integrated RF front-end solutions in an effort to boost power efficiencies and battery life at lower costs.

Sea of RF change
In total, RF component sales are expected to grow from $22 billion in 2011 to more than $30 billion in 2016, according to Strategy Analytics. One of the big drivers for digital and analog chip makers is 4G/LTE, a technology that boasts data rates of up to 100-megabits-per-second, up to 10 times faster than 3G.

4G/LTE smartphone shipments are projected to triple from 90.9 million units in 2012 to 275 million in 2013, according to Strategy Analytics. However, the complexity of 4G/LTE smartphones is expected to come at the expense of power and battery life, thereby requiring a new class of low-power, multicore chipsets.

Broadcom, Intel, MediaTek, Qualcomm and others are shipping cell-phone chips based on bulk CMOS. Taking another approach to the problem, ST-Ericsson has rolled out an integrated cell-phone chipset based on 28nm, fully-depleted SOI (FD-SOI). The FD-SOI part is 30% faster than bulk devices, said Joel Hartmann, executive vice president of front-end manufacturing and process R&D at STMicroelectronics. “We have demonstrated a 50% power reduction,” he said.

The next breakthrough could occur by year’s end, when Intel hopes to ship its first 22nm finFET device for the mobile market. The foundries will enter the finFET market at 14nm. “Going to 14nm finFETs will help with the battery life,” said Ajit Manocha, chief executive of GlobalFoundries.

Like the digital market, there are also challenges for the RF front-end in 4G/LTE. “The current challenge is handling many of the worldwide LTE bands. The second challenge is MIMO involving multiple carriers. The third is handling the smart antennas for all bands and multiple input/output streams,” said Will Strauss, president of Forward Concepts, a research firm.

The classic RF front-end in cell phones includes three main parts: the power amplifier, antenna switch and filter. For years, the cell-phone power amp has incorporated GaAs-based heterojunction bipolar transistor (HBTs) technology. The power amp amplifies RF signals in the phone.

Typically, the RF front end is separate from the digital modem and transceiver, which are based on 28nm and 65nm/40nm CMOS or other processes, respectively. The interaction between the RF and digital blocks present some major challenges, including the ability to maintain the isolation between the frequency bands, said Thomas Richter, senior marketing director at RF specialist Skyworks Solutions.

Generally, a 2G or 3G cell phone required separate and discrete power amps to support the various bands. In 4G/LTE, cell phones must not only support LTE, but also the existing GSM, EDGE and WCDMA standards. To date, there are 17 bands in place for 4G/LTE worldwide. That list could grow to 43.

In any case, it’s impractical and too expensive to build a cell-phone that supports all 17 bands. “I don’t think anybody will fit 17 power amps in a phone,” Richter said.

The solution to the problem is the advent of a multi-band power amp, which is a single device that supports a wider frequency range. Multi-band power amps, however, suffer from a power-added-efficiency (PAE) drop, as compared to a discrete device. “Multi-band power amps make for cheaper RF solutions, but no current power amps can handle all the bands,” Forward Concepts’ Strauss said. “There are multi-band power amps in the market, but only for a few bands. Smartphones still require multiple power amp chips.”

In 4G/LTE, a smartphone may incorporate a mix-and-match of devices, possibly six discrete power amps and one multi-band power amp in the same system. Taking another approach to the problem, Skyworks recently rolled out SkyOne. This technology can combine several discrete devices, such as multi-band power amplifiers, switches, filters, and duplexing functions, in a small system-in-package (SIP). The power amp is based on GaAs, while the switch uses SOI. “With SkyOne, you can condense your PCB,” said Skyworks’ Richter.

Skyworks, RF Micro and others are also fielding CMOS-based power amps, which claim to have lower power consumptions than GaAs. “GaAs is still favored,” Forward Concepts’ Strauss said. “The only CMOS power amps have been used in low-end GSM phones in China. CMOS has not proved to be sufficient for high-performance power amps.”

The wild card is Peregrine Semiconductor, which is sampling a power amp based on its silicon-on-sapphire (SOS) technology. Peregrine’s 0.35-micron process, dubbed UltraCMOS, is a variant of SOI that makes use of an insulating dielectric sapphire substrate. SOI wafer provider Soitec provides the bonded silicon-on-sapphire (BSOS) substrates to Peregrine. “The challenge (for SOS) is getting the heat out of a power amp,” said Strategy Analytics’ Taylor.

Soitec itself is also offering a separate RF SOI technology, dubbed Wave, which are high-resistivity substrates. “These engineered substrates enable more functionality on smaller chips and lower power usage for longer battery life in portable electronics,” said Soitec’s Boudre.

Dialing up switches and tunable capacitors
IBM, TowerJazz and others provide RF SOI processes, as well. Generally, SOI, and its variants like SOS, promise the long-awaited integration of the RF front-end. But at least in the near term, however, the RF front-end will remain a collection of discrete devices.

“If you look at the front-end, you see an explosion of components,” said Rodd Novak, chief marketing officer of Peregrine. “The area for the RF front-end is shrinking. OEMs want to put more and more battery content in the system. Integration is really the driving force, as opposed to just price erosion. That’s a big change.”

Peregrine and others provide another key part of the RF front-end: the RF switch. RF switches route signals between the antenna and the handset core, through one or more signal paths. As the design of the mobile device becomes more complex, more signal paths are required.

The RF switch was once dominated by GaAs. As of late, Peregrine’s SOS technology has been winning RF switch sockets at the expense of GaAs. In response, the GaAs suppliers, RF Micro and Skyworks, are now pushing RF switches based on SOI. “We’ve displaced GaAs,” Novak said. “Now, the GaAs guys are using a highly insulated SOI substrate. But we believe sapphire is the highest insulating substrate.”

4G/LTE is also propelling a new and emerging component–tunable capacitors. These components tune the antennae to boost efficiencies. Peregrine is selling components based on SOS. Paratek, a subsidiary of Research in Motion (RIM), and STMicroelectronics, are selling components based on barium strontium titanate (BST). Another vendor, WiSpry, is offering a MEMS solution.

“There is such a huge amount of bandwidth to cover with a small antenna (in LTE),” Peregrine’s Novak said. “In fact, the industry is looking for multiple antennae now. We are trying to prevent those antennae from cross correlating. It’s almost impossible to do that without some level of tunability. ”

Tunable capacitors are now being integrated into the transceiver block. “In 2013, we are also going to see the entry of tunable networks. You will see up to three tunable components to provide a greater tuning range,” Novak said.

What’s next in RF? Some are looking at software-defined power amps. And in the distant future, tunable capacitors may end up being integrated in the power amp. “People are trying to develop multi-mode and multi-band power amps. But to do that efficiently, you need a tunable output match with tunable components,” he added.