Amplifying structure for the development of field-effect capacitive sensors

An electronic structure associating a field-effect based sensor and a MOSFET amplifier is presented, evidencing an amplification of the sensor detection properties. This structure is applied to a field-effect capacitor (FEC) sensor, leading to the bias of the MOSFET transistor by a FEC/capacitor bridge. The static behaviour of the FEC/MOSFET amplifier is studied and a good agreement is obtained between theoretic calculations, simulations and experiments. The different limitations and optimisations of the proposed structure are given according to the FEC and MOSFET characteristics. In order to further improve the FEC/MOSFET amplification properties, the influences of the FEC serie and parallel resistors must be controlled. A solution is proposed by adding a resistive bridge in parallel with the FEC/capacitor one. The dynamic behaviour of such R–C electronic structure is presented, evidencing a good agreement between theoretical calculations, simulations and experiments. Thus, the influences of the different resistive elements on the gate and drain voltages are analysed, evidencing competition phenomena between the resistive and the capacitive bridges.