(Invited) Flexible Sensors Based on Low-Temperature Polycrystalline Silicon Thin Film Transistor Technology
The tactile sensors were designed according to the Piezoelectric Oxide Semiconductor Field Effect Transistor (POSFET) structure, enabling sensing and signal conditioning at same site. In this way, it is possible to improve signal to noise ratio and hence the force sensitivity. In our LTPS POSFET devices we adopted as piezoelectric film the polyvinyledenedifluoride –trifluoroethylene P(VDF-TrFE) polymer. The piezoelectric polymer generates a charge on application of a mechanical force which influences the gate bias of the transistor. Gate bias variations, in turn, are directly reflected into drain current variations, that can be further processed by electronic circuits. The measured response to applied forces is linear, as reported in Fig 1a, with a piezoelectric coefficient up to 47 pC/N. The frequency response was tested in the range 30 - 1200 Hz and presented a high-pass behavior (see Fig.1b), according to the adopted common-source bias configuration.
The pH sensor was fabricated according to the extended gate structure, as this structure offers many advantages over the conventional ISFET, such as the low cost, simple passivation and package, insensitivity of temperature and light, flexibility of shape of the extended-gate structure, and better long-term stability. The sensitive layer of the extended gate is a nanostructured ZnO film selectively deposited on the Al-gate electrode in a chemical bath at low-temperature. The pH-sensitivity was tested by exposing the extended gate to different pH-solutions and using as a reference electrode an Ag/AgCl electrode. In Fig. 2 the transfer characteristics, measured at low Vds and for different pH-solutions, are reported. In the inset of Fig.2, the threshold voltage shift induced by the pH variation is shown, giving a slope of 59 mV/pH for the pH range 1 – 9, close the ideal Nernstian response.