I. Introduction
Topical developments of wireless electronics led to an urgent need in compact and low power consumption tunable systems. Indeed, high-performance semiconductor-based tunable devices are already integrated in new generation wireless systems. However, this solution presents some disadvantages in terms of mobility because of high levels of power consumption of semiconductor devices. Thus, research efforts are now focused on tunable components aimed at reducing power consumption, with several technologies being investigated. Despite some persistent problems of process dispersions, microelectromechanical systems (MEMS) components proved to be a reliable solution [1], [2]. Liquid crystals were also used to design microwave tunable phase shifters and filters [3], [4]. Performance of such devices is however weakened by very low switching times. In another effort where correlated materials such as VO2 are used, it has been demonstrated that high contrast of conductivity can be obtained because of the material's semiconductor-metal transition [5]. Very low switching times, quasi-null power consumption, and the simple fabrication process of ferroelectric materials are clear advantages for addressing tunability problems. Therefore, a significant amount of work has been carried out during the last decade to integrate ferroelectrics into microwave devices. Among these materials, (BST) compounds appear today to be the most suitable ferroelectrics for microwave applications because of their adjusting Curie temperature, low dielectric losses, and high tuning performance. Very promising tunable devices were manufactured using BST materials. For example, highly tunable ferroelectric capacitors with Q factor of 50 up to 30 GHz were recently demonstrated with both planar [6] and parallel-plate [7] structures. Good performance figures were also obtained with phase shifters [8], [9] or filters [10], [11].