Lateral bulk acoustic wave filter

An acoustic wave filter device comprising:

• a piezoelectric layer;
• an input electrode and an output electrode located on a top surface of the piezoelectric layer and physically separated from one another, the input and the output electrodes each including a base and one or more extensions extending from the base, the one or more extensions of the input electrode extending alongside and in a generally opposite direction to and separated by a gap width from an adjacent extension of the one or more extensions of the output electrode,
• each of the one or more extensions of the input and the output electrodes having a respective length that extends from the base of one of the input and the output electrodes towards the base of the other one of the input and the output electrode;
• and
• a counter electrode having a top surface connected to a bottom surface of the piezoelectric layer,
• wherein a thickness of the piezoelectric layer and the gap width between adjacent extensions of the input and the output electrodes are such that application of a radio frequency voltage between the input electrode and the counter electrode creates acoustic thickness-extensional resonance modes in the piezoelectric layer, and
• wherein at least one extension in the one or more extensions of the input or the output electrodes has a width that changes non-linearly from a first end of the at least one extension to a second end of the at least one extension when moving along the length of the at least one extension.