We present a new MOSFET-based pressure sensor, incorporating an air gap that is a function of pressure and a permanently charged dielectric layer (electret) between the gate and bulk of the MOS structure. In this way we obtain a MOSFET with a pre-charged variable gate capacitance. The theory of this sensor and the NMOS-compatible process for its realization are given. We present also the first experimental results on this new sensor. We have determined the characteristics of sensors with outer dimensions of 1 mm × 2 mm × 0.3 mm and measured a maximum sensitivity of about 10 mA/A/100 mm Hg, which is about ten times higher than the sensitivity of piezoresistive pressure sensors with comparable dimensions. Drawbacks of the present sensor design are a relatively high temperature sensitivity and a rather complex fabrication process.