Tunable BAW resonator with ion-conductible structure

A tunable Bulk Acoustic Wave (BAW) resonator comprising:

• a top electrode;
• a bottom electrode;
• a piezoelectric layer sandwiched between the top electrode and the bottom electrode;
• and
• a reflection region residing underneath the bottom electrode and comprising a reflection layer and an ion-conductible structure between the bottom electrode and the reflection layer, wherein:
• the ion-conductible structure includes a first terminal layer coupled to the bottom electrode, a second terminal layer coupled to the reflection layer, and an ion conductor between the first terminal layer and the second terminal layer;
• the ion conductor is capable of transporting ions between the first terminal layer and the second terminal layer, so as to achieve a mass-loading shift between the first terminal layer and the second terminal layer, and consequently, to tune a resonance frequency of the tunable BAW resonator;
• and
• the reflection layer has a greater density than a density of the ion conductor, and at least one of the first terminal layer and the second terminal layer has a greater density than the density of the ion conductor.