Slowly, but steadily, gallium nitride (GaN) power transistors are moving forward. Every year the progress report looks better and commercial viability seems inevitable. Consequently, at this month's IEEE MTT-S International Microwave Symposium in San Francisco, Calif., proponents demonstrated new levels of power and performance for wireless infrastructure and WiMAX base station applications.

Pioneers like Cree and Eudyna were showcasing much higher power levels than previously reported by these manufacturers. Cree, for instance, disclosed a high-power GaN transistor with 400 W of peak pulsed power at 3.3 GHz with 10.6 dB associated power gain and 62% drain efficiency when operated at 40 V. “We are pleased to announce that we have achieved this 400 W milestone, stated Jim Milligan, Cree's product manager for wide bandgap radio-frequency products. “Upcoming mobile WiMAX applications are expected to require average orthogonal frequency-division multiplexing (OFDM) output power between 10 W and 25 W with peak-to-average ratios (PAR) as high as 12 dB. This will require transistors that are capable of delivering up to 400 W of peak RF output power.” For now, Cree has begun sampling its 15 W and 30 W wideband gap GaN versions with plans to sample 120 W GaN HEMTs by September. It is working toward qualifying its 28 V GaN process in a few months.

Likewise, Eudyna boasted some 500 W of peak pulsed power capability for its GaN devices. Concurrently, using Eudyna's 100 W GaN transistors, EADS Astrium of England is close to releasing a high-power solid-state PA to the mobile and navigation space. More details are in a paper given by EADS Astrium at the International Communications Satellite Systems Conference (ICSSC) in San Diego, Calif., June 11-14.

On the other hand, RF Micro Devices displayed a digitally predistorted linearized four-stage power amplifier based on its GaN high electron mobility transistors (HEMTs) for WCDMA. This four-stage amplifier comprised two pre-drivers, one driver and one high-power amplifier (HPA) with total out power rated at 12.5 W at 2.1 GHz. According to RFMD, its high power 28 V/48 V 0.5 µm GaN process will undergo qualification in September, and is expected to be completed by March 2007.

Meanwhile, TriQuint Semiconductor, which has been awarded a multiyear contract from the Defense Advanced Research Projects Agency (DARPA) to develop high-power wideband amplifiers in GaN, continues to tackle some reliability, material and design issues. Foreseeing this emerging trend in the PA space, Royal Philips Electronics has begun a GaN HEMT development program in partnership with Germany's Fraunhofer Institute.

Although at the moment the debut of GaN HEMTs and FETs is not a threat to entrenched LDMOS in the wireless infrastructure space, a few years down the road these devices will begin to make a dent in the marketplace as wireless base stations adopt new 3G and next-generation standards demanding higher power levels and higher efficiency with density at the transmitter output. Interestingly, at a press event hosted by an agency to kick off IMS 2006, Freescale Semiconductor's Jim Norling, marketing director for the RF division, presented a market projection for silicon LDMOS transistors that was reported by ABI Research. By comparison, silicon LDMOS transistors will continue to dominate the scene for the next couple of years. While GaN is shown to grab some insignificant market share starting next year, it is not before the end of this decade that GaN power transistors will begin to grab some sockets from LDMOS transistors.

Thus, Freescale continues to expand its portfolio of LDMOS devices, while Infineon unveiled its next-generation LDMOS in plastic packaging for wireless infrastructure PAs. Philips also released a new generation of LDMOS devices for wireless and WiMAX applications. And, last but not least, a new generation of gallium arsenide (GaAs) heterojunction bipolars (HBTs) was also released at the show as a competitor to LDMOS. WJ Communications disclosed a new line of 28 V InGaP/GaAs HBTs for mobile infrastructure applications. The developer flaunted its improved ACPR/ACLR performance at equivalent or higher efficiencies than LDMOS. Other HBT backers to display new devices included TriQuint.

With this evolution, PA designers have multiple choices of power transistor technologies to choose from. A healthy competition is good for the developers as well as the users of technology. It keeps the momentum going.

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