A new class of electronic devices, micromechanical membrane switches, has been developed. These switches have operating characteristics that fill the gap between conventional silicon transistors and mechanical electromagnetic relays. Although they are batch fabricated on silicon using conventional photolithographic and integrated circuit processing techniques, their unique properties allow them to perform functions not ordinarily associated with silicon. The devices are basically extremely small, electrostatically controlled mechanical relays, typically less than 100 µm long. Their high off- to on-state impedance ratio and all-metal conduction paths make them ideally suited for ac signal switching arrays. This paper describes the design, fabrication, operating behavior, and potential applications of these voltage-controlled, micromechanical switches.