Passive components on AlN for application in AlGaN/GaN power amplifiers
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Passive components on AlN for application in AlGaN/GaN power amplifiers B. Jacobs1, B.van Straaten1, M. Kramer1, F. Karouta1 P. De Hek2, E. Suijker2, R. Van Dijk2 1 Eindhoven University of Technology, Department of Electrical Engineering, Opto-electronic devices group, PO box 513, NL-5600 MB, Eindhoven, The Netherlands 2 TNO Physics and Electronics Laboratory, 2509 JG, The Hague, The Netherlands ABSTRACT We have investigated Coplanar Waveguide (CPW) elements on AlN for use in future AlGaN/GaN based power amplifiers. This technology becomes crucial if a via-hole technology is not available. Lines, discontinuities, metal-insulator-metal (MIM) capacitors and resistors were measured and modelled. These elements are embedded between two adaptors for RF probing. A technique was developed to de-embed the adaptors from the overall measurement and hence correctly determine the properties of the element itself. Measurements on elements containing multiple ports with right angles can best be carried out using standard calibration techniques followed by carefully reorienting the probes. It is shown that for accurate design of matching networks operating at 10 GHz each element has to be carefully modelled. The method presented in this paper can be a useful contribution tackling some of the problems related to the design of these networks. INTRODUCTION In the early stages of developing a new material system like the AlGaN/GaN system most research efforts are spent on realizing active components like high electron mobility transistors (HEMTs). However, combining these elements requires some sort of matching. The choice of components needed for this matching depends largely on the substrate used and whether a via-hole technology is available or not. In the case of the AlGaN/GaN material system two substrates are commonly used; the relatively cheap sapphire and the much more expensive SiC that has a much higher heat conductivity. In the case of SiC, a via-hole technique can be developed although this is far from trivial. If sapphire is used this technique is not available. The absence of such a technique implicates that microstrip technology cannot be used. In this case one has to resort to CPW technology that is less developed and not so implemented in commercial simulators as microstrip. Therefore, we have chosen to develop our own library of CPW elements. DE-EMBEDDING CPW LINES The general layout of a typical CPW line is illustrated in Figure 1. The structure consists of a line connected to an adaptor on either side. This adaptor is necessary because not every line is suitable for probing. Also, because the elements are on a different substrate than the calibration substrate and the metal geometry around the probe is different, one cannot assume a perfect RF connection between probe and line. The adaptor accounts for all the effects mentioned above.
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Figure 1: Layout of a CPW line (left); Equivalent circuit of an adaptor (right).
The addition of the two adaptors makes it impossib
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