Closed-form analysis of high-resistivity thin-film electrode in the hybrid varactor structure

Hybrid ferroelectric thin-film varactor device architecture is presented that allows for integrating the features of both structures of coplanar and parallel-plate varactors. In this structure, a high-resistivity oxide thin film is used as dc bias bottom electrode, while the existence of the high-resistivity oxide thin-film electrode is considered as disturbance to the original electric field in the coplanar varactor. Using analytic solution of electromagnetic field, the disturbance caused by high-resistivity thin film in microwave TEM wave is derived. The result shows that this disturbance could be modeled as serial or shunt resistance in the transmission line equivalent circuit and the magnitude of it is proportional to the conductivity and thickness of the conductor thin film. With conductivity and thickness low enough, the conductor film will be “transparent” to microwave field. Using high-resistance dc bias electrode “transparent” to microwave is expected to be able to bring much more flexibility in the developing of microwave ferroelectric tunable devices.