Interesting, thanks for posting
We were fortunate to work directly with Dr. Ken Lakin of TFR Technologies in Bend, Or. with our first silicon fully integrated MW/MMW receivers and later transceivers back in early 90s.
Ken was one of the pioneers of early BAW technology, and patented the Back Side Passivated BAW devices which eliminated the need for the chemical etching for standard BAW devices.
We wanted to develop a fully integrated silicon receiver with the BAW preselector filter integrated directly onto the active silicon at the wafer level. AlN which was one of the BAW active materials is directly compatible with silicon, so this deemed plausible.
For our 1st step Ken developed separate AiN BAW filters which worked quite well, and very successful. However we never got the chance to develop the fully integrated versions as funding became scarce and more effort and $ were put towards the first MW/MMW SoC Transceivers.
Later TRF was acquired by Sawtek, which was then acquired by RFMD, then Qorvo I believe.
Recall there was also some SAW work done with GaAs back in 80s and 90s with a version which had tunable characteristics.
Anyway, very interesting technologies that you don't hear much about today, thanks for posting
Best,