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We report a thin-film piezoelectric microelectromechanical systems (MEMS) transducer suitable for in vivo implantation as audio prosthesis and a method for fabricating it. The transducer consists of a layered stack of thin films comprising a MEMS membrane made of an isolating oxide interface; two platinum (Pt) layers acting as electrodes, electrical paths, and contacting pads; a thin-film sputtered aluminum nitride (AlN) layer acting as acoustic active material of the transducer; a passivation layer for device protection; and a 3-D micromachined acoustic cavity fabricated on the bulk of a silicon-on-insulator supporting substrate. The transducer has a reduced size and a low cost associated with CMOS-like mass production. Finite-element modeling and experimental tests conducted at the audio and ultrasonic bands show that the device has the performance required in audio prosthesis applications. The technology offers possibilities for biocompatibility or biocompatible packaging and is integrable to microelectronic circuit technologies.