Field effect transistors possesses many advantages for chemical detection. As their electrical characteristics strongly depend on electrical charges located on their surface, they are able to measure with good sensitivity and very precisely any variation of charges concentration of the surrounded media. They can be easily integrated, are low-cost and provide a rapid and reliable result. The main application developed here is the pH measurement. In this case, the gate contact is removed and replaced by a liquid polarized by a reference electrode, leading to the ISFET technology. Those structures can also be developed by using a thin-film technology. The technology we developed is based on LPCVD polysilicon, silicon oxide and nitride deposited by CVD.

The sensitivity of ISFET or ISTFT is limited by Nernst equation, but can be enhanced by using dual-gate structures. In this technology, capacitive enhancement can be optimized by choosing specific thicknesses and materials for back and top gates. In this case, the sensitivity enhancement is theoretically given by the ratio of the top gate capacitance to the back-gate capacitance. This technology has been developed also by using thin-film transistors from polysilicon material.

Several structures have been developed in order to obtain good electrical performances for field-effect transistors, as low threshold voltage and good electrical characterization for back and top gates. Those characteristics are controlled by a good choice of insulator, and a thickness optimization.

Transistors are characterized with different polarizations; in back-gate, top-gate and dual-gate operating modes. Capacitive enhancement in determined for the different cases and compared to theory.

Transistors are then used for chemical detection, especially with measurement of pH. ISFET is already highly used for pH measurement, but its sensitivity is limited to 59 mV/pH. By using a dual-gate structure, this sensitivity can be substantially enhanced. The developed structures, based on previous studies of dual-gate TFTs, includes a back-gate contact, and an integrated electrode at the surface for the top-gate liquid polarization. Threshold voltage are measured and optimized. A slight doping of the active layer induces a decrease of the threshold voltage, necessary in the case of measurement in liquids. Sensitivities for pH measurement are compared with theory and show the capacitance enhancement.