BAWFET: A Monolithically Integrated RF FET Amplifier + BAW Filter on a Multifunctional AlN Platform

Aluminum nitride (AlN) is widely used as the piezoelectric layer for RF acoustic filters for its high electromechanical coupling coefficient, high acoustic phase velocity, as well as low acoustic and dielectric loss. In addition to being an excellent piezoelectric, AlN is also an ultra-wide bandgap semiconductor used in UV photonics and RF transistor amplifiers. An exciting opportunity enabled by Bulk Acoustic Wave (BAW) filters is the monolithic integration with active devices such as high-electron-mobility-transistor (HEMT) amplifiers. While BAWs can be fabricated by sputtering, nitride transistor amplifiers require epitaxial growth. Growing on SiC substrates with high thermal conductivity and mechanical toughness, the convergence of AlN BAW and AlN HEMT material layers by epitaxy can enable a unique RF front end. Here in this talk, I will talk about the BAWFET, a compact monolithic integration of an epitaxial AlN FBAR and an AlN/GaN/AlN quantum well HEMT for active filtering. This device represents the first proof-of-concept demonstration of such an integration. In the AlN/GaN/AlN quantum well HEMTs, the conventional AlGaN barrier layer and the AlGaN back barrier layer were replaced with unstrained AlN epitaxial layers. By utilizing the AlN buffer layer of the HEMT, BAWs were fabricated on the same SiC substrate by fully suspending the piezoelectric cavity for higher quality factor using deep through-SiC-vias (TSVs). The single crystalline AlN allows for a smarter, highly-scaled heterostructure design that improves the output power and thermal management of III-nitride semiconductor amplifiers.