  This article presents a bulk acoustic wave (BAW) resonator with complementary
switchable operation in the first and second thickness extensional modes (TE1
and TE2) at 7.04 and 13.4 GHz. Two ferroelectric scandium aluminum nitride
(Sc0.28Al0.72N) layers are alternatively stacked with three molybdenum
electrodes, creating a laminated BAW resonator with independent switchability
of polarization in constituent transducers. This enables intrinsic
switchability of the resonator in TE1 and TE2 modes, when the ferroelectric
Sc0.28Al0.72N layers are poled in the same or opposite directions,
respectively. A generalized analytical proof of complementary switchable
operation, extended to laminated BAW resonators consisting of arbitrary number
of ScxAl1-xN layers, is presented. For the demonstrated prototype,
electromechanical coupling coefficients ( k2t ) of 10.1% and 10.7%, and quality
factors ( Q ) of 115 and 151, are measured for TE1 and TE2 modes, respectively,
when the resonator is configured in the corresponding operation states. Besides
showing intrinsically configurable operation in super-high-frequency regime
with high k2t and Q , a laminated Sc0.28Al0.72N BAW resonator exhibits
repeatable operation under switching cycles.
