When polar compounds interact with this metal gate, the electric field, and thus the current flowing through the sensor, are modified. The recorded response corresponds to the change of voltage necessary to keep a constant preset drain current (Lundstrom et al., 1990). The selectivity and sensitivity of MOSFET sensors may be influenced by the operating temperature (50–200°C), the composition of the metal gate, and the microstructure of the catalytic metal. MOSFET sensors have a relatively low sensitivity to moisture and are thought to be very robust.