In 2008, RF power transistors saw significant advancements. Both silicon-based bipolar and CMOS technologies achieved major breakthroughs. Particularly in L-band field-effect transistors, a wide range of integrated circuits became available with peak output powers reaching up to 1000W, making it easier to build kilowatt-class solid-state amplifiers for radar and avionics systems. A decade earlier, the semiconductor industry could only offer RF transistors with peak power outputs of around 100W. To achieve 1000W, the driver stage of the final amplifier required a power splitter, with ten power transistors arranged into five independent push-pull circuits. The power output was then synthesized through an output synthesizer. This setup led to challenges such as increased power supply demands, larger board space requirements, and more complex thermal management, which in turn raised the overall cost of the output module. Moreover, the reliability of the system was lower due to the increased number of components, leading to higher failure rates. Overall, the price-to-performance ratio of high-power RF solid-state amplifiers still needed improvement.
Since the early 2000s, numerous innovations have taken place in the materials, processes, design, measurement, and packaging of integrated circuits. For instance, silicon wafers expanded from 150mm to 800mm in diameter, while unit defect density decreased significantly. Improvements in ion implantation, diffusion, epitaxial growth, and metal wiring enhanced the reliability of devices. The device line width reduced from 1 micrometer to below 60nm. Design automation now covers the entire production chain, and measurement techniques have become more sophisticated, allowing for wafer-level parameter testing. Chip-level three-dimensional interconnect packaging has replaced traditional multi-chip or wafer-level interconnects, boosting both the performance and efficiency of RF power transistors.
In recent years, progress in measuring power transistor parameters has been crucial. Power transistors often operate in nonlinear conditions, and previously, only low-power characteristic curve testers or network and spectrum analyzers were used. However, these tools couldn’t capture DC and AC parameters under nonlinear operation, making it difficult to develop accurate high-frequency amplifier models. Fortunately, new measurement instruments have emerged, enabling comprehensive parameter analysis under nonlinear conditions. These advancements have resolved challenges in modeling, design automation, and performance optimization for high-frequency power transistors. Additionally, many high-frequency transistors now include internal matching networks, 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 kilowatt-class performance.
**Bipolar RF Power Transistor**
In the current RF power transistor market, conventional Si-based homogenous bipolar devices and GaAs-based heterojunction bipolar devices are widely available. Si-based bipolar transistors benefit from mature manufacturing processes and lower costs, but their frequency response is relatively limited. On the other hand, GaAs-based bipolar transistors offer superior frequency performance, though they come at a higher cost due to the higher 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 refined to meet next-generation needs and new materials driving further innovation.
While Si and GaAs have made great strides in field-effect transistors, Si bipolar transistors still perform well. In 2008, Microsemi introduced the TAN500 Si Bipolar Transistor, capable of delivering 500W of pulse power in the 960MHz to 1215MHz band, primarily for the TACAN tactical air navigation system. Powered by a +50V supply, it uses a 10μs pulse-modulated 70W signal in Class C operation to achieve a peak power 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. Internal wiring is formed using Au thin film metallization. The epitaxial diffusion of the base region and emitter ballast diffusion improve the device's amplification and stability. The TAN500 features a high average failure rate, but its wideband matching circuit between the emitter input and collector output, along with a low thermal resistance between the heat sink and base, provides a strong foundation for increasing RF power output. The maximum ratings for the TAN500 include a power dissipation of 2500W at 25°C ambient temperature and pulsed operation, 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 operating characteristics of the TAN500 are detailed in Table 1. Typical input/output characteristics and collector efficiency curves are shown in Figures 1 and 2. As seen, at 90MHz, 1090MHz, and 1215MHz, the input and output power show a good linear relationship, with collector efficiency exceeding 40% at a rated peak output of 500W. The TAN500 represents one of the best-performing Si bipolar transistors in this category. 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, a Si bipolar RF power transistor used in the final stage of avionics systems. 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 even better performance parameters.
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