Study on electro-mechanical transduction employing piezoelectricity on the gate of a field-effect transistor and its application to hydrophones

Abstract

In contrast to the induced charge or voltage in conventional piezoelectric sensors, changes in the bound surface charge density and the corresponding changes in the electric field from a piezoelectric body do not depend on the size of the piezoelectric material. In this thesis, we describe a new transduction mechanism, the piezoelectric gate on a field-effect transistor (PiGoFET), focusing on the applications in micro-hydrophones. In the PiGoFET transduction mechanism, a piezoelectric body is combined directly on the gate of a field-effect transistor (FET) to decouple the sensitivity from the dimensions of the piezoelectric body, enabling the miniaturization of hydrophones. We describe a theoretical model of the PiGoFET, which shows that high sensitivity can be achieved with a small hydrophone due to the dimensionless characteristics of the PiGoFET transduction mechanism. The operating principle of the PiGoFET was analyzed using the theoretical model, which was experimentally verified in macro-scale device to within 2 dB. Using the verified theoretical framework for the PiGoFET, a micro-machined PiGoFET (micro-PiGoFET) hydrophone was designed and fabricated via hybrid bonding integration in a CMOS-compatible manner. The hybrid bonding integration employs separate wafers for the piezoelectric MEMS and CMOS processes, which are combined via eutectic wafer bonding to complete the micro-PiGoFET device. The resulting micro-PiGoFET hydrophone was characterized as an underwater acoustic receiver for frequencies in the range 50−1000 Hz. The internal pre-stress and polarization of PZT in the micro-PiGoFET were exploited to improve the sensitivity and signal-to-noise ratio (SNR). The measured sensitivity −156 ± 1 dB (Ref. V/μPa) of the micro-PiGoFET hydrophone was comparable to that of commercial reference hydrophone (B&K 8103) coupled with a charge amplifier (B&K 2692), provided that the noise floor of those sensors are similar. These results demonstrate the potential for high-performance miniaturized hydrophone systems for wide-band and low-frequency applications, as well as system-on-chip functionality.