In order to examine the applicability to electromechanical transducers, fundamental characteristics of an MOS field-effect structure with a movable gate electrode, or a ’’movable-gate–air-gap–oxide–semiconductor’’ system, are investigated. In this system, a channel current suffers field-effect modulations by a mechanical displacement of a gate electrode. As a movable-gate electrode, a metal plate, and a foil electret are used. Equations derived from an approximate treatment of MOS–FET equations are found to explain the experimental results. Based on these equations and experimental facts, design considerations for electromechanical transducers using this structure are made. As an example of applications, microphones utilizing foil electrets are constructed and their performances are discussed.
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Yoshikawa et al. (1978) studied this question.