Industry Power Transistors
NXP has built a 2.7 GHz Doherty
demo using three small, unmatched GaN devices that is claimed to achieve a decent power density and efficiency performance when
compared to silicon LDMOS
The first released type will be the CLF1G0530-50, hardly a catchy name but one with meaning: the ‘C’ is our code for GaN technology; ‘F’ denotes a ceramic package type; ‘1G’ stands for first-generation technology; 0530 describes the optimal frequency range of 500-3000 MHz; and ‘50’ is short-form for a nominal P1dB of 50 W. This is the naming convention we will apply to our portfolio: 100W and 150W, unmatched broadband versions will follow before we start on a few frequency-specific matched types.
The biggest market segment for RF power transistors is telecom base-station infrastructure equipment. Typical operating frequencies range from less than 1000 MHz for various GSM, WCDMA and LTE standards to 2700 MHz for other LTE use and to 3800 MHz for WiMAX. In recent times the power amplifier (PA) architecture of choice has changed from the classic class AB to the more exotic Doherty configuration. A Doherty is a hybrid amplifier, with one portion for the main signal and one for the peak power. The Doherty concept sacrifices linearity in favour of efficiency. Combined with system improvements in the signal handling – digital pre-distortion (DPD) – base-station designs can be made with much higher efficiencies whilst retaining linearity.
As a small step in the right direction, we have produced a neat 2.7 GHz Doherty demo using three small, unmatched GaN devices that achieve a decent power density and efficiency performance when compared to silicon LDMOS – this drew much interest at the recent International Microwave Symposium (IMS/MTTS) show in Baltimore, June 2011. Commercial devices for base- station applications will require the next process version and improved matching designs; these will start to become available during the first half of 2012.
From analogue to digital
As well as being a complement to LDMOS in existing linear topologies, GaN offers a much more exciting prospect – it is an enabling technology for digital
26
www.compoundsemiconductor.net August / September 2011
NXP has developed its power transistors on 3-inch SiC substrates in partnership with United Monolithic Semiconductors (UMS) and the Fraunhofer Freiburg Institute
transmission. This is another key research and development project for us, which illustrates how our focus in the high-performance RF domain allows us to demonstrate our abilities as an innovator of improved systems. The digital transmitter rationalises the digital signal chain and culminates in a switched mode PA (SMPA) – a concept impossible without GaN. The thinking behind the SMPA is to make a break from the current family of linear designs of PA, with their inherent current and voltage losses, to a true switching design, with near-zero switching losses. The SMPA will follow the class E/F topology, taking the efficiency up to the 70-80 percent mark. The whole concept is aimed at producing far smaller, cheaper, cooler, and hence greener base-stations.
Our application insight for system concepts like the digital transmitter is complemented by a deep appreciation of knowing how to get perfect device partitioning. In this respect, we are not constrained by a single material and adopt the right technology for the required functionality.
Here it is important to mention SiGe, another compound semiconductor receiving much of our attention. The benefits of SiGe are distinct but quite different to GaN. Whereas the benefits of GaN are as the optimum RF power transistor material, SiGe’s benefits are as a very cost-effective technology for mixed signal RF solutions compared to GaAs.
We will continue to adopt an agnostic approach to materials throughout this year and beyond. 2011 could well go down as a year in the evolution of GaN with great strides in becoming a material that is available, workable and reproducible in significant volumes.
© 2011 Angel Business Communications. Permission required.
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