It is always desirable to fabricate low-cost, highly sensitive and miniaturized hydrophones for applications in sonar technology. In this project, silicon based integrated hydrophone devices were designed, fabricated and tested. A theoretical model was also developed. A piezoelectric polymer, polyvinylidene difluoride (PVDF), was employed as the sensing material. Acoustic impedance possessed by this piezoelectric material provides a reasonable match to that of water, which makes it very attractive for underwater applications. A MOSFET with extended gate electrode fabricated on a silicon wafer using standard NMOS process technology was designed as the interface circuit. The electrical signal generated by the PVDF film was directly coupled to the gate of the hence improve the device sensitivity, a thick photoresist, SU-8, was used as the interconnecting dielectric layer under the extended gate electrode. With an 11 micrometers thick SU-8 layer, a sensitivity of the hydrophone was improved dramatically. For underwater operation, the hydrophone was encapsulated by a Rho-C rubber. To improve the reliability of the hydrophone, the active device was passivated by a silicon nitride layer, which is a good barrier material to most mobile ions and solvents. The device after passivation also shows a lower noise level. Further improvement is also achieved by using a differential amplifier as the interface circuit in the electronics. The differential amplifier is also less susceptible to noisy environments and thus improves the signal to noise ratio.