Thin film barium strontium titanate (BST) for a new class of tunable RF components

Thin film ferroelectrics (TFFE) are receiving increased attention because of their potential in producing tunable RF and microwave circuits. Tunable circuits, including filters, matching networks and phase shifters offer the flexibility to adapt to changes in operating conditions, such as frequency, impedance environment or RF drive level, a property that is highly sought after in the wireless communications industry. Components using alternative technologies, such as GaAs varactor diodes, tend to have low values, implying high RF loss, and generally do not handle high RF power levels. MEMS-based devices have unproven reliability at this point, often have difficult biasing requirements and have stringent packaging needs. As a result, much emphasis has been placed on thin film ferroelectric material barium strontium titanate, (BaSr)TiO3, or BST, because of its promising properties related to capacitance tuning range, dielectric loss, breakdown voltage, power handling and component integration. This article reviews the material and electrical properties of thin film BST, contrasts them with other forms of BST, reviews basic tunable capacitor operation and describes some useful circuit applications that result. Simulation results and measurements from such circuits are also given.