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In our previous two blog posts, How Ultrasonic Fingerprint Sensing Works and Why it is Important and Which transducer type is best for ultrasonic fingerprint sensing: CMUT, PMUT or PZT?, we explained the fingerprint sensing principles and the different ultrasound transducers. Micromachined ultrasound transducers (MUTs) can be fabricated in a small size in an array suitable for relatively large frequency applications compared to bulk piezoelectric transducers, and are then suitable to perform beamforming to generate ultrasound images. Piezoelectric micromachined ultrasonic transducers (PMUTs) are a better candidate for fingerprint sensing compared to CMUTs, though there is no need for DC bias voltage for both Tx and Rx operation. In fact, PMUTs AC only working regime reduces the charging effect in the dielectric/piezo improving the reliability of the device.
Ultrasound Transducers: Modalities and Operation During the last several decades, ultrasound devices have become ubiquitous in daily life for various airborne and immersion applications such as automobile, parking sensors, and medical imaging. Traditional piezoelectric transducers were previously used mainly in ultrasound applications however, in the past two decades, micromachined ultrasound transducers (MUTs) have been developed and used in several medical imaging and consumer electronics applications such as handheld/catheter-based medical devices and fingerprint sensors. In general, MUTs operate in 2 different mechanisms, capacitive force (CMUT) or piezoelectric (PMUT) sensing-actuation. See figure 1 and ref.  .
Human fingerprints are detailed, unique and more importantly, invariant over time, making them useful and reliable markers of human identity. Fingerprint sensors are used to capture an image of a human fingerprint, and can be realized through different technologies such as optical, capacitive and acoustic mechanisms    . Ultrasonic sensing has started to make headway into much wider applications as new ultrasonic transducer technologies have reduced the power, size, and cost of the technology. With significant use in the medical and industrial markets, consumer electronics is also starting to adopt this technology.