Inside view

Abstract

Sebastian Preis of the Ferdinand-Braun-Institute in Germany tells us about his group's work on high-electron mobility-transistors (HEMT) and varactors. The work was in collaboration with researchers at Darmstadt University of Technology. The transistor module mounted to a test fixture A high-electron mobility-transistor (HEMT), also known as heterojunction FET (HFET), is a type of transistor especially suited for high frequency power and low-noise applications. The gallium nitride (GaN) HEMT used here is a specially designed version. Based on the high bandgap of the GaN material used, as well as a high electron velocity, its benefits are high breakdown voltage, high output power, high maximum transition and robustness in terms of channel temperature. Varactors are variable capacitors – they can be tuned by applying a certain voltage. Depending on the technology, this can be some volts, as in case of silicon based semiconductor varactor diodes, but also kV for thick barium-strontium-titanate (BST) ceramic varactors. Already the concepts of realising a varactor differ a lot. Established are varactor diodes based on silicon but also compound semiconductors, such as SiGe, GaAs and GaN. Inherently, diodes suffer from their nonlinear characteristic. Micro-electro-mechanical-systems (MEMS) are introduced as very linear counterpart, but are limited in switching speed and power handling capability. Ceramic based varactors are similar to common multilayer capacitors with high dielectric constant ceramic material. BST varactors make use of the usually unwanted property of electric field dependent dielectric constant. Furthermore, prior to the present generation of metal-insulator-metal (MIM) BST varactors, the power handling capability of ceramic varactors for RF and microwave operation was limited to some watts, but this has been greatly improved now. Another problem is the thermal capacitance drift of BST varactors. The BST ceramic does not only change its dielectric constant in an electric field, but also if the temperature changes. Some other groups use the BST material with different doping as temperature sensors. The deployed MIM BST varactor generation is capable of handling power levels of at least 50 W. In fact, the power handling capability is a function of the surface area and is limited by the increasing capacitance of larger varactors. The whole module, thanks to the varactors, provides an electronically tuneable transistor with more than 20 W output power. The module can be used like conventional transistors, but it can be adapted to varying situations – such as changing load conditions or different operation frequency – by simply adapting the varactors’ control voltages. Thick-film metal-insulator-metal barium-strontium-titanate varactors in a GelPak Such a transistor can be used in a load modulated amplifier, which adapts its optimum load line to the actual required output power and therefore operates always in a very energy efficient mode. Thanks to the BST technology, the varactors power consumption is negligible. Besides this, base station amplifiers can be built, which can be operated at different frequencies. Also, space-born modules can be envisaged. The operator just needs to adjust the varactor control voltage to switch the frequency band. In the frame of the project, the European space agency (ESA) will test the modules, manufactured at Ferdinand-Braun-Institute (FBH). Further, the varactors and modules will be improved in terms of performance and maturity. BST varactors can be used to realise tuneable oscillators, mixers, and also passive circuits like phase shifters and filters. In general, this could link the analogue RF to the digital signal processing world. By simply changing a voltage, an RF system can be switched to operate in a different frequency band, on a different power level, while maintaining efficiency. Analogue RF systems have to become not only smaller but also smarter, i.e., easier to adjust remotely. Electronically tuneable capacitors, such as BST based varactors, are ideal for this purpose and will gain importance in the near future. First, big suppliers, such as ST Microelectronics, will start to distribute BST varactors commercially, together with GaN HEMT technology high performance transmitters. Space applications might change to this combination. Even in the telecom market, some applications such as point to point links could benefit from GaN and BST. I would like to see an integration of BST on wafer level into GaN transistor processes. This would solve several problems like the heat removal from the BST varactors and the parasitic circuit elements introduced due to varactor mounting. GaN MMICs incorporating BST varactors would allow smart devices, such as frequency agile MMIC amplifiers, adjustable mixers, oscillators, etc.