Compact Intrinsically Switchable FBAR Filters Utilizing Ferroelectric BST

Intrinsically switchable thin film bulk acoustic resonator (FBAR) filters based on ferroelectric barium strontium titanate (BST) are presented. A 1.5-stage π-network intrinsically switchable FBAR filter unit cell with a 3-dB fractional bandwidth of 3% at 2 GHz is systematically designed, simulated, and fabricated. The minimum insertion loss (IL) for the filter unit cell is 2.25 dB, representing the lowest IL reported for BST bulk acoustic wave filters to date, which is mainly due to its high Q × K
_t
²
BST resonators. Two 1.5-stage filter unit cells are connected in series to form a 2.5-stage filter, providing more than 25 dB of out-of-band rejection and OFF-state isolation between the input and the output ports. The measured input thirdorder intercept point (IIP3) of the 2.5-stage filter is 47 dBm. Furthermore, the footprint of the fabricated filters is notably small, due to the high permittivity of BST.