Do we really need ferroelectrics in paraelectric state only in electrically controlled microwave devices?

摘要

A brief historical overview on electrically tunable ferroelectric microwave devices will be given. Typical paraelectric materials (e.g. SrTiO/sub 3/, KTaO/sub 3/, Ba/sub x/Sr/sub 1-x/TiO/sub 3/, x>0.5) and electrically tunable microwave devices based on these materials will be reviewed. The analysis show that in spite of the recent year's extensive efforts no considerable improvement in the microwave losses in thin paraelectric films has been achieved. Typically, loss tangent is of the order of 0.01 (at zero DC bias field) or higher at room temperature and at about 10.0 GHz. A question then arises whether this is a theoretical or practical limit or some improvement is possible in the nearest future? The other question concerns the possibilities of using ferroelectrics in polar phase in electrically controllable microwave devices. Resent experimental results relating to microwave properties and examples of using ferroelectric in polar state (e.g. Na/sub 0.5/K/sub 0.5/NbO/sub 3/, SrTiO/sub 3/ in antiferroelectric phase) in electrically tunable devices will be presented in this report. Examples of digitally (non-analogue) controlled microwave devices based on ferroelectrics, such as switchable filters will also be presented.