Tunable microwave device: status and perspective

State-of-the-art frequency-agile technologies such as semiconductors, micro-electro-mechanical systems (MEMS) and ferrites are briefly overviewed and compared with the emerging ferroelectric techniques such as Ba_xSr_1-xTiO₃ (BST) that are becoming an attractive candidate for designing low-cost and miniaturized tunable integrated microwave devices such as filters and phase shifters. Particularities of ferroelectric materials are summarized and difficulties in connection with the use of BST thin-films in RF monolithic circuit design are addressed. On the basis of a commercially available sol-gel process, BST thin-films have successfully been deposited on alumina substrate to fabricate metal-insulator-metal (MIM) tunable capacitors. Early results of such capacitors at the room temperature indicate that a tunable ratio (C_max/C_min) as high as 3.5 can be obtained over 1-6 GHz with 20 biasing voltage (34 V/μm) even though the ohmic loss remains high. This research also shows a 7% weak effect of hysteresis.