In order to develop a capacitive solid state sensor that makes use of an electret, a theoretical analysis is given of an electret air-gap field-effect structure. This structure is basically an MOS transistor with a movable gate and can thus be considered as a pressure-sensitive field effect transistor. It is shown that the addition of a metal layer on top of the semiconductor-oxide increases the sensitivity due to charge density multiplication. All calculations are based upon the displacement sensitivity S, which is independent of the mechanical properties of the diaphragm and thus independent of a specific application. Based upon the calculated sensitivities of the several configuration, a well-considered decision can be made as to which configuration is best suited for a specific application. In this paper this has been done for a solid state microphone and a pressure sensor as examples.