High Tunability Ba 0.5 Sr 0.5 TiO ₃ Thin‐Film Varactor Under Low Electric Fields for Reconfigurable RF Applications

This paper presents the design and experimental demonstration of a compact metal‐insulator‐metal (MIM) varactor with high tunability under low electric fields, integrated on a coplanar waveguide (CPW) platform. The device utilizes a (BST5) thin film deposited by pulsed laser deposition (PLD) on a platinum‐coated sapphire substrate. RF characterization was performed from 1–10 GHz under varying bias conditions. The proposed varactor achieves a high capacitance tunability of 82% at 1 GHz under an electric field of 250 kV/cm. Compared with conventional floating metal varactor structures, it delivers similar tunability with nearly half the applied bias field by providing direct access to the bottom electrode, thereby eliminating series capacitance and improving electric field efficiency. Finite element simulations using ANSYS high frequency structure simulator (HFSS) were performed to analyze and compare the electric field distributions in the floating and non‐floating configurations. With enhanced tunability, compact structure, and low bias operation, the proposed varactor is a promising candidate for integration into tunable microwave components such as phase shifters, filters, and antennas.