Improved Performance in BST-Based Tunable Circuits Employing Low-Loss Nontunable Dielectric

Ba _xSr _1-xTiO ₃ (BST) materials are being developed for tunable microwave applications. A figure of merit "K" defined as K = ^εmax- ^εmin/ ^εmax tan ^δmax is frequently used to evaluate tunable dielectrics. Based on a simple equivalent-circuit model, multilayer structures consisting of BST films and low-loss non-tunable dielectric should have a higher K than BST alone. The maximum improvement achievable by this approach is predicted to increase with tunability of the BST material. For a BST film with 80% tunability and maximum loss tangent of 0.07, such as those typically deposited by pulsed laser deposition, the maximum predicted improvement is close to a factor of five for K. A four-electrode chip capacitor was fabricated for experimental validation of the modeling predictions. A 25% increase in K was demonstrated for a multilayer four-electrode capacitor using low tuning voltages. An alternative figure of merit the Commutation Quality Factor (CQF) was also examined, it is reported to be invariable with the respect to series and parallel secondary low loss inclusions and is therefore potentially more universal quality parameter for characterization of tunable dielectrics than the widely-used K. We present experimental results for a multilayer structure and evaluate its impact on the two alternative figures of merits.