Bulk acoustic wave devices with tuned acoustic impedance

A bulk acoustic wave device comprising:

• a first electrode;
• a second electrode;
• a piezoelectric layer between the first electrode and the second electrode, the bulk acoustic wave device configured to have a main resonant frequency for an acoustic wave having a resonant wavelength λ;
• and
• a raised frame structure including a first raised frame layer below a second raised frame layer that extend into an active region of the piezoelectric layer, the first raised frame layer having a first non-gradient region and a first gradient region, the first raised frame layer having a lower acoustic impedance than at least one of the first electrode, the second electrode, and the piezoelectric layer, and the first gradient region of the first raised frame layer extends into the active region to a lesser extent than the second raised frame layer extends into the active region.