A Theoretical Analysis of the Electret Air-gap Field- Effect Structure for Sensor Applications

In order to develop a capacitive solid state sensor that makes use of an electret, a theoretical analysts 1s given of an electret air-gap held-effect structure This structure 1s basically an MOS transistor with a movable gate and can thus be considered as a pressure-sensltlve field effect transistor It 1s shown that the addition of a metal layer on top of the semlconductor-oxide increases the sensltlvlty due to charge density multlphcatlon All calculations are based upon the displacement sensltwlty S, which IS independent of the mechanical properties of the diaphragm and thus mdependent of a specific apphcatlon Based upon the calculated sensltlvltles of the several confrguratlons, a well-considered decision can be made as to which conflguratlon 1s best suited for a specific application In this paper this has been done for a solid state microphone and a pressure sensor as examples