Heavy boron diffusion followed by selective etching has been used to produce uniform edge‐supported Si diaphragms with thickness in the 1–3‐&mgr; range and areas up to 5 cm2. These diaphragms have numerous applications in the fabrication of rugged reliable electromechanical devices which are compatible with IC technology. In this paper we describe the performance of an electrically tunable resonant cavity which operates in the 10–12‐kHz range withQas high as 23 000. Devices based on this cavity structure include pressure transducers, microphones, speakers, tunable filters, and oscillators.