Nonlinear Response of Ba x Sr 1-x TiO 3 Planar and Parallel-Plate Capacitors to Microwave Power: Comparative Study of Power Handling Capability A.Kozyrev 1 , A.Ivanov 1 , T.Samoilova 1 , O.Soldatenkov 1 , M.Zelner 2 , I.P.Koutsaroff 2 , T.A.Bernacki 2 , A.Cervin-Lawry 2 1 Electrotechnical University of St-Petersburg, Prof. Popov Str. 5, 197376 Russia, Phone/fax +7 (812) 234 48 09 2 Gennum Corporation, 970 Fraser Drive, Burlington, Ontario L7L 5P5 Canada, Phone: (905) 632 29 96 Abstract — The power handling capability of tunable microwave devices employing planar and parallel-plate BaxSr1-xTiO3 film-based capacitors was experimentally measured and analyzed. A microstrip resonator, excited by either harmonic or two-tone microwave signals of elevated power, was selected as an example of tunable test fixture. I. INTRODUCTION High microwave (MW) power handling is one of the advantages of paraelectric film-based tunable devices. Traditionally planar structures incorporating paraelectric films (Fig. 1) are used for MW applications. Such structures provide relatively high MW power handling capability (PHC) limited by paraelectric film overheating due to MW power dissipation [1]. However planar structures require a high tuning voltage (up to hundreds of Volts), that limits their use in low voltage applications. On the other hand, due to the small electrode spacing parallel-plate (sandwich) capacitors (Fig. 1) with thin (100-150 nm) paraelectric films [2] require a tuning voltage of 1-10V, which falls into the same range as for semiconductor devices. However, tuning under low voltage can lead to nonlinear distortions, whereas the decrease in the volume of the paraelectric film active area can result in capacitor overheating. Consequently, the PHC of sandwich capacitors is a critical parameter and requires special study. This paper presents an experimental study of the nonlinear responses to elevated MW power for microstrip resonators loaded with planar and parallel- plate Ba x Sr 1-x TiO 3 (BSTO) film-based capacitors of the same capacitance and loss tangent. A comparative analysis of the PHC of the resonators tested is presented as well. II. RESULTS AND ANALYSIS Two measurement techniques were used in experiments. The first technique allows the study of the interference from two MW signals due to BSTO capacitor nonlinearity [3]. The second technique allows the recording of the resonant curve distortion due to averaged capacitance variation under increasing pulsed MW power [4]. Fig.2 presents the results of measurements and simulations of the resonator output power at a fundamental frequency (f 1 , f 2 ) and at the 3-rd order intermodulation distortion (IMD) product frequency (f 3 =2f 2 -f 1 ) as a two-tone MW signal with the same power per tone is fed to the resonator input. The results were obtained for a resonator employed with sandwich or planar paraelectric capacitors. Fig.3 shows the results of measurements of the resonant curve of the sandwich capacitor-loaded resonator with increasing incident MW power, P inc . One can see that the difference between resonant curves measured at the leading and trailing edges of the MW pulse enhances with P inc increasing. Photos illustrate the output pulse shape corresponding to different MW signal frequencies. Two effects can be responsible for BSTO capacitance behavior under elevated MW power. They are MW electric field (E) and thermal (T) effects on BSTO film dielectric permittivity. To analyze the E-field effect we used the description of the capacitance-voltage curves (CVC) as ( )                             + - + = 2 0 U U 1 1 K 1 K ) 0 ( C ) U ( C , (1) top metal electrodes ferroelectric film substrate bottom metal electrode sandwich capacitor planar capacitor Fig. 1. Schematic structures of sandwich and planar ferroelectric film capacitors.