Design Considerations for a Novel Coplanar Waveguide Based Ferroelectric Varactor Shunt Switch

Abstract

Novel ferroelectric variable capacitor (varactor) shunt switches have been designed, and experimentally verified recently. The varactor shunt switches are designed for microwave and millimeterwave switching applications, based on the nonlinear dielectric tunability of the Ba 0.6 Sr 0.4 TiO 3 (BST) thin-films. Nanostructured BST thin-films with dielectric tunability as high as 350% were used in this study. The device consists of a coplanar waveguide line loaded by a ferroelectric varactor. The large capacitance of the varactor at zero bias shunts the input signal to ground, thus isolating the output port, resulting in the OFF state of the device. When one applies a bias voltage corresponding of approximately 10 V (a dc electric field of ∼250 kV/cm), the varactor's capacitance is reduced to a minimum, allowing maximum transmission to the output, thus resulting in the ON state of the device. The microwave switching performance of the varactor shunt switch is compared with the RF MEMS switches for potential applications at microwave and millimeterwave frequencies.

Microwave characterization of nanostructured ferroelectric Ba0.6Sr0.4TiO3thin films fabricated by pulsed laser deposition

“…Amongst the samples selected for measurement, the loss tangent was lowest for the 75 mTorr sample (figure 13). Note that these loss tangents are higher than our other recent samples: 0.04 at 20 GHz and zero bias would be more typical than the 0.064 obtained here [40].…”

“…Figure 14 shows the dielectric properties of the 75 mTorr processed thin film as a function of bias voltage, extracted from scattering parameter measurements of a varactor shunt switch (5 μm × 15 μm varactor area). The dielectric constant is close to 500 at zero bias and tunes 4:1 with an applied bias voltage less than 10 V. The electrical circuit model for the varactor shunt switch used to determine these properties is shown in figure 15 [40].…”