Ferroelectric Varactor Shunt Switches for Broadband Phase Control Applications

Abstract

Our research group has recently demonstrated the operation of ferroelectric varactor shunt switches. The device operation is based on the nonlinear dielectric tunability of a barium strontium titanate (BST 60/40) thin-film sandwiched between two metal layers in a parallel plate configuration. A 4:1 change in the varactor capacitance has been achieved with a bias voltage <10 V, through optimization of the nanostructured BST thinfilm fabrication process on high resistivity Si substrates. In this work, we investigated the varactor shunt switches for broadband phase control applications. Varactor shunt switches and cascaded shunt switches were used for phase control circuits, due to the large phase shifts possible as a result of the dc bias dependent tunability of the varactor. The differential phase shift can be defined as the difference between the phase of S 21 at a dc bias voltage and zero-bias. The measured differential phase shift for a single varactor shunt switch was 30 ± 2 degrees between 8 and 26 GHz at 8 V dc bias. A cascaded varactor shunt switch resulted in a differential phase shift of 110 degrees ± 10 degrees over the bandwidth of 20 and 31 GHz.