The effect of physical design parameters on the RF and microwave performance of the BST thin film planar interdigitated varactors

In this paper, we report the detailed RF and microwave characterization of planar interdigitated (PID) varactors fabricated on Ba0.6Sr0.4TiO3/Ba0.4Sr0.6TiO3 double-layer thin film deposited by pulsed laser deposition (PLD) on the MgO substrate. In particular, the dependences of the PID varactors’ performance (i.e. tunability and quality factor) on the physical design parameters, such as finger spacing and width, are investigated. It was found that both the capacitance and tunability of the PID varactors show strong dependence upon the finger spacing. This dependence can be divided into the linear part (when the finger spacing is large) and non-linear part (when the finger spacing is small). The finger width also affects the capacitance and tunability, although to a much lesser degree. The relations between the physical design parameters and the varactors’ performance provide useful scaling information for accurate and optimized varactor design. The relation between the quality-factor (Q-factor) and the bias voltage is also revealed and explained using an equivalent circuit model.