A question re: piezoelectric materials and resonators

[Dishsoap]

TL;DR

Why do bulk acoustic resonators have higher resonant frequencies than PMUTs?

I'm new to the world of acoustics, and I've been reading up on various methods for wireless communication/sensing through ultrasound, especially with piezoelectric materials such as AlN. Fundamentally, I can't seem to find a difference between thin-film bulk acoustic resonators (1 um thick) and PMUTs, which are also about 1 um thick in AlN. They have similar structures (top and bottom electrode, sometimes suspended), and yet the resonance frequency of a PMUT is about 0.5-1 MHz and 10-100 for a TFBAR. I think I am missing something fundamental but I cannot figure out what.

 

[Baluncore]

PMUT = Piezoelectric micromachined Ultrasound Transducer. 0.5-1 MHz.
TFBAR = thin-film bulk acoustic resonators. 10-100 MHz. Both 1 um thick.

Is the device being specified as a resonator or a transducer?
Maybe the resonant frequency of TFBAR is being compared with the bandwidth of the loaded PMUT.

 

[Dishsoap]

Is the device being specified as a resonator or a transducer?

I think that is where I am misunderstanding - I don't see why it matters. The geometry, thickness etc. can be the same, but why would the resonance frequency change depending on whether it is being used as a resonator or a transducer? In both cases, an acoustic wave is being converted into an AC voltage, right?

 

[Baluncore]

In both cases, an acoustic wave is being converted into an AC voltage, right?

Yes, or vice versa.
It is also possible that the mode of operation might be different.
See; http://mems.usc.edu/fbar.htm
"A bulk-micromachined FBAR with Thickness Field Excitation () uses a z-directed electric field to generate z-propagating longitudinal or compressive wave. In an LFE-FBAR, the applied electric field is in y-direction, and the shear acoustic wave (excited by the lateral electric field) propagates in z-direction, as illustrated ()"

 

[Dishsoap]

Yes, or vice versa.
It is also possible that the mode of operation might be different.
See; http://mems.usc.edu/fbar.htm
"A bulk-micromachined FBAR with Thickness Field Excitation () uses a z-directed electric field to generate z-propagating longitudinal or compressive wave. In an LFE-FBAR, the applied electric field is in y-direction, and the shear acoustic wave (excited by the lateral electric field) propagates in z-direction, as illustrated ()"

Oh, that could be - I'll have to examine these papers further. Thank you!