Lateral bulk acoustic wave filter

1. 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; 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 output electrodes are such that application of a radio frequency voltage between the input electrode and the counter electrode creates symmetric and antisymmetric 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 from a first end of the at least one extension to a second end of the at least one extension such that the width increases by between 10% and 250% from the first end to the second end.

2. The device of claim 1 , wherein the first end of the at least one extension is distal from the base of the corresponding electrode and the second end of the at least one extension is proximal to the base of the corresponding electrode.

3. The device of claim 1 , wherein the first end of the at least one extension is proximal to the base of an electrode corresponding to the at least one extension and the second end of the at least one extension is distal from the base of the electrode corresponding to the at least one extension.

4. The device of claim 3 , wherein an extension adjacent to the at least one extension has a width that increases from a third end of the adjacent extension to a fourth end of the adjacent extension, wherein the third end is distal from the base of an electrode corresponding to the adjacent extension, and the fourth end is proximal to the base of the electrode corresponding to the adjacent extension.

5. The device of claim 4 , wherein the at least one extension is an extension of the input electrode, and the adjacent extension is an extension of the output electrode.

6. The device of claim 1 , wherein the device is a laterally coupled bulk acoustic wave filter.

7. The device of claim 1 , wherein the gap width between extensions is constant.

8. The device of claim 1 , wherein the width of the at least one extension changes continuously from the first end of the at least one extension to the second end.

9. The device of claim 1 , wherein the input and output electrodes each have a comb structure with interdigitated extensions.

10. The device of claim 1 , further including a Bragg reflector in contact with a bottom surface of the counter electrode.

11. The device of claim 1 , wherein the input and output electrodes have at least 10 extensions in total.

12. The device of claim 1 , wherein the thickness of the piezoelectric layer is at least 0.5 μm.

13. The device of claim 1 , wherein a respective width of each extension of an electrode corresponding to the at least one electrode increases by between 10% and 250% from the first end to the second end.

14. The device of claim 1 , wherein a respective width of each extension of an electrode corresponding to the at least one electrode increases by between 25% and 100% from the first end to the second end.

15. 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 at least one extension extending from the base, the at least one extension 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 at least one extensions of 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 output electrodes are such that application of a radio frequency voltage between the input electrode and the counter electrode will create symmetric and antisymmetric acoustic thickness-extensional resonance modes in the piezoelectric layer, and

wherein the gap width between adjacent extensions of the input and output electrodes has a difference of 0.5 μm to 3 μm in width when moving in a direction from the base of the input electrode towards the base of the output electrode.

16. The device of claim 15 , further including a Bragg reflector in contact with a bottom surface of the counter electrode.

17. The device of claim 15 , wherein the gap width between adjacent extensions of the input and output electrodes increases when moving in the direction from the base of the input electrode towards the base of the output electrode.

18. The device of claim 17 , wherein the gap width between adjacent extensions of the input and output electrodes increases for 2 μm to 3 μm.

19. The device of claim 15 , wherein the gap width between adjacent extensions of the input and output electrodes decreases when moving in the direction from the base of the input electrode towards the base of the output electrode.

20. The device of claim 15 , wherein the input and output electrodes each have a comb structure with interdigitated extensions.