In 2008, RF power transistors experienced remarkable advancements. Both silicon-based bipolar and CMOS technologies saw significant breakthroughs. Notably, L-band field-effect transistors became widely available with integrated circuits capable of delivering up to 1000W of peak power. This made it easier to construct kilowatt-class solid-state amplifiers for radar and avionics applications. A decade earlier, the semiconductor industry could only offer RF transistors with a maximum peak power of 100W. To achieve 1000W, the driver stage of the final amplifier required a power splitter, with 10 transistors arranged into five independent push-pull circuits. The power output was then synthesized through an output synthesizer. However, this approach led to challenges in power supply design, board space utilization, and heat dissipation, increasing both the size and cost of the system. Additionally, the reliability of such systems was lower due to the increased number of components, which raised the failure rate. Overall, the cost-to-performance ratio of high-power RF solid-state amplifiers still needed improvement.
Since the early 2000s, there have been numerous innovations in the materials, processes, design, testing, and packaging of integrated circuits. For example, silicon wafer sizes expanded from 150mm to 800mm, and unit defect rates decreased significantly. Improvements in ion implantation, diffusion, epitaxial growth, and metal wiring enhanced device reliability. The minimum feature size of devices dropped from 1 micrometer to below 60nm. Design automation now spans the entire production chain, and measurement techniques have become more sophisticated, enabling wafer-level parameter testing. Chip-level three-dimensional interconnect packaging has replaced traditional multi-chip or wafer-level interconnects, boosting the efficiency of RF power transistors and accelerating integrated circuit development.
Recent progress in measuring power transistor parameters has had a major impact. Power transistors often operate in nonlinear conditions, and previous testing methods—such as characteristic curve tracers, network analyzers, and spectrum analyzers—were limited in their ability to capture DC and AC characteristics under these conditions. As a result, it was difficult to build accurate high-frequency amplifier models, which hindered the development of high-frequency power transistors. Fortunately, new instruments have emerged that can measure multiple parameters under nonlinear conditions, solving key challenges in modeling, design automation, and performance enhancement. Moreover, many high-frequency transistors now include internal matching networks between input and output terminals, simplifying external circuit design and improving frequency response, efficiency, and power output. Combined with other IC innovations, today’s RF power transistors have reached a new level of kW-class performance.
**Bipolar RF Power Transistors**
In the current RF power transistor landscape, conventional Si-based homojunction bipolar devices and GaAs-based heterojunction bipolar devices are widely available. Si-based bipolar transistors offer mature manufacturing processes and low costs but have relatively lower frequency performance. GaAs-based transistors, on the other hand, provide better frequency characteristics, although they come at a higher cost due to the superior electron/hole mobility in heterojunctions. Both Si and GaAs have their own advantages and disadvantages in RF power amplifier applications. In recent years, both materials have seen continuous improvements, with traditional materials being re-engineered to meet next-generation demands and new materials continuing to evolve.
While Si and GaAs have made significant strides in field-effect transistors, Si bipolar transistors remain competitive. In 2008, Microsemi introduced the TAN500 Si Bipolar Transistor, capable of generating 500W of pulse power in the 960–1215MHz band, primarily used in the TACAN tactical air navigation system. The device operates at +50V and is driven by a 70W, 10μs pulse-modulated signal in Class C mode, achieving a peak output of 500W with a minimum gain of 10dB and a collector efficiency of 40%.
The final stage of the Si bipolar RF power amplifier uses a common base configuration, with Au thin film metallization forming the internal wiring. The base region epitaxial diffusion and emitter ballast diffusion enhance the device's amplification and stability. The TAN500 features a high average failure rate, along with wideband matching circuits at the input and output, and a low thermal resistance between the heat sink and the base, which supports higher RF output power. Its maximum ratings include a power dissipation of 2500W at 25°C, a collector breakdown voltage of 65V, an emitter breakdown voltage of 3V, a collector current of 50A, a storage temperature range of -65°C to 200°C, and an operating junction temperature of +200°C.
At 25°C ambient temperature, the TAN500 exhibits excellent operational characteristics, as shown in Table 1. Typical input/output characteristics and collector efficiency curves are illustrated in Figures 1 and 2. At frequencies of 90MHz, 1090MHz, and 1215MHz, the input and output power show a strong linear relationship, with collector efficiency exceeding 40% at a rated peak power of 500W. The TAN500 represents one of the best-performing Si bipolar transistors in RF terminal power amplifiers. Currently, the TAN series includes models like TAN300, TAN350, and TAN500, offering 300W, 350W, and 500W output power, respectively. Models with over 1000W output are expected soon.
Microsemi’s MDS series, introduced in 2005, includes the MDS1100, which is used in avionics applications. It delivers over 1000W of peak power at +50V and 1030MHz, with a maximum power consumption of 8750W at 20°C ambient temperature, a maximum operating temperature of +200°C, and a minimum collector efficiency of 45%. The design concept of the TAN series mirrors that of the MDS series, but with further optimization to achieve superior performance metrics.
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