Investigation of Ferroelectric Thick-Film Varactors for Microwave Phase Shifters

This paper presents a novel cost-effective microwave phase-shifter technology using thick-film ferroelectric varactors as the tuning component. The devices are fabricated by conventional screen-printing techniques with silver/platinum as the conducting layers and Ba_0.7 Sr_0.3TiO₃ as the tunable dielectric layer. The microwave performance of the ferroelectric varactors is optimized in terms of both material process parameters and the varactor gate area dimensions. S-parameter measurements on a tunable reflective circuit are used to evaluate the performance of the varactor at around 2 GHz. The varactor parameters such as RF capacitance, tunability, and the Q factor of the varactor are studied as a function of the gate dimension. At 100-V dc-biasing voltage, the BSTO varactor loaded tunable reflective circuit exhibits a differential phase shift of 70deg with a return loss of -1.2 dB. This is equal to a figure-of-merit value of 58deg/dB. Full analog reflection-type and all-pass network phase shifters are also implemented in order to explore the monolithic integration of phase control devices into the RF system based on low-cost ceramic thick-film technology