Abstract: An ultrasonic transducer multilayer structure, including: a semiconductor layer stack defining a diode, a micro-machined ultrasonic transducer, MUT, layer stack being electrically in series with said diode and comprising a first electrically conductive layer disposed at least partly on the diode, a cavity extending over a region comprising at least a portion of the semiconductor layer stack and the first electrically conductive layer, wherein the MUT layer stack comprises a membrane extending at least partly over said region. The ultrasonic transducer multilayer structure may be used in a sensor apparatus for measuring at least one characteristic of an object. The sensor apparatus is used for medical imaging, such as cardiac imaging, obstetrics, gynaecology, abdominal imaging, intravascular imaging, and mammography, or non-destructive testing (NDA), fingerprint sensors, range finders, gesture recognition, ultrasonic haptic feedback, ultrasonic communication or MEMs speakers.