A bulk acoustic wave (BAW) resonator includes: an acoustic reflector disposed in a substrate; a lower electrode disposed over the acoustic reflector; a piezoelectric layer disposed over the lower electrode; and an upper electrode disposed over the piezoelectric layer. A contacting overlap of the lower electrode, the piezoelectric layer and the upper electrode over the acoustic reflector comprising an active area of the BAW resonator. An opening exists in the upper electrode in a region of the BAW resonator susceptible to unacceptable overheating.