Bulk acoustic resonator

A bulk acoustic resonator includes a first electrode disposed on an upper side of a substrate, a piezoelectric layer disposed on an upper surface of the first electrode, and a second electrode disposed on an upper surface of the piezoelectric layer, wherein an upper surface of at least one of the first electrode and the second electrode has a recess region, wherein a depth of the recess region is D, a width of the recess region is W, and a resonance frequency is F, and ln is a natural logarithm, and wherein [{ln(D*W)}/(−0.59*F)] is [[ln{0.008 (μm)2}]/{−0.59*(3.5 GHz)}] or more and [[ln{0.022 (μm)2}]/{−0.59*(3.5 GHz)}] or less.