Micromachined film bulk acoustic resonator

The present invention provides an FBAR device with a means for internally varying a capacitance. The device comprises a bottom electrode and a top electrode, said bottom electrode and said top electrode have an overlap area A, the overlap being defined by the projection of the top electrode onto the bottom electrode in a direction substantially perpendicular to the plane of the bottom electrode. Furthermore, the device comprises a piezoelectric layer in between said bottom electrode and said top electrode and a dielectric layer in between the piezoelectric layer and the bottom electrode. The device also comprises means for reversibly varying an internal impedance of the device. The capacitance may be tuned by either varying the thickness of the dielectric layer, by changing the overlap between the bottom electrode and the top electrode or by shifting a different dielectric material between the electrodes.