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SiGe steps on the GaAs

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Radio-frequency chips for mobiles and other wireless products have long relied on the speed, low signal distortion and power-handling capabilities of gallium arsenide (GaAs) chips. Yet silicon germanium (SiGe) chips are increasingly proving cheaper, less power-hungry and easier to integrate with other chip functions. And that’s helping SiGe chip makers make inroads into markets once dominated by GaAs.

The Information Network research firm forecasts that worldwide SiGe sales will increase nearly 40 percent, to US$1.6 billion ($2.1 billion), this year whereas GaAs sales will grow less than 10 percent, to US$3.5 billion ($4.6 billion). Although GaAs sales figures are still twice those of SiGe sales, SiGe sales are growing much faster.

GaAs, a brittle gray compound made from gallium and arsenic, carries electrons six times as fast as silicon, the base material used in most of today’s chips. GaAs also can be made to emit light, making it useful in lasers and light-emitting diodes. But GaAs chips require specialised fabrication plants. SiGe chips, made from conventional silicon wafers, can be fabricated on conventional production lines with only a few extra process steps. When doped with germanium, silicon can nearly match the speed, signal integrity and power handling of GaAs chips, at a lower cost.

“I definitely see gallium arsenide’s days as being numbered,” says Teddy O’Connell, business development manager for IBM, the leading SiGe foundry. Besides offering affordability, he says, SiGe chips can be designed to use significantly less power than GaAs chips, a key advantage in battery-powered devices. “SiGe gives you great performance with much lower power consumption,” he says.

Frost & Sullivan analyst Deepa Doraiswamy says SiGe’s impressive performance and low cost explain the technology’s popularity, especially in price-sensitive devices such as signal amplifiers for disk drives and Wi-Fi wireless networks. SiGe wafers cost almost 70 percent less than GaAs wafers, she says, and offer better manufacturing yields. Those savings are partly offset, though, by the higher cost of the masks that etch circuits onto the SiGe chips, which typically use much finer geometries than comparable GaAs chips.

GaAs chips can also handle higher power levels than SiGe chips without breaking down or losing their high-speed capabilities. That’s a big reason why GaAs is used in many high-power applications whereas SiGe is limited mostly to products that use less power.

Nonetheless, SiGe already is supplanting GaAs chips in TV tuners, satellite and cable set-top boxes and mobile transceivers, analysts say. The next battleground between the two technologies is likely to be mobile phone power amplifiers (PAs), most of which currently use GaAs. The worldwide market for mobile phone PAs was US$1.64 billion ($2.13 billion) last year, according to iSuppli.

Aditya Gupta, vice president of device technology for GaAs chip maker Anadigics (distributed by Richardson Electronics ), notes that PAs in mobile phones using the GSM standard require about 4 Watts of power, compared with 1 Watt for CDMA-based phones and 100 milliWatts for Wi-Fi transmitters. Wi-Fi’s lower power needs have allowed SiGe PAs to take over much of that market, whereas SiGe mobile phone PAs are just being introduced, starting first with CDMA phones. SiGe Semiconductor, for instance, claims to serve about 45 percent of the market for Wi-Fi PAs but is still trying to attract customers for its new CDMA SiGe PAs. Another company, Parkervision, has also announced plans for a line of SiGe PAs.

Probably the biggest attraction of SiGe technology is its ability, unlike GaAs, to integrate easily with conventional digital silicon chips. That’s becoming more important, as mobile phone makers demand new power-management capabilities in their PAs. As a result, says IBM’s O’Connell, “next-generation power amps are going to require a substantial amount of digital logic”.

Ultimately, chip makers hope to integrate numerous mobile phone components, including PAs, filters, switches and perhaps even transceivers, onto the same chip. “Nobody’s quite there yet,” says Paul Kempf, chief technology officer for SiGe manufacturer Jazz Semiconductor, “but the drive is there to look at all of the pieces and create a path for their integration.”

Jerry Neal, executive vice president of strategic development for leading GaAs chip maker RF Micro Devices (RFMD), concedes that SiGe has a bright future but warns against writing off GaAs prematurely, especially in specialised markets such as microwave-frequency chips for radar and satellite communications. “My friends in the silicon area have been pronouncing the obituary of gallium arsenide for a long time,” says Neal. “Eventually they may be right, but it’ll be a while.”

Neal expects GaAs to continue dominating mobile phone PAs for at least the next few years. But when the industry starts to integrate large power-control chips with PAs, he says, “then the game starts to change.” By then, though, RFMD intends to be leading the way in silicon-based PAs. Says Neal, “If gallium arsenide is going to be made obsolete, we’re going to be the one that does it.”

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