Infrared spectra of the OH stretching vibrations of the jet-cooled salicylic acid (SA) monomer and dimer were measured for both their electronic ground (S 0 ) and first excited (S 1 ) states. For the SA monomer, conformations of two rotational isomers (rotamers) which have been identified by the electronic transitions were determined on the basis of the infrared spectra with the help of theoretical calculations. The intramolecular hydrogen-bonded OH stretching vibrations of the monomer showed a drastic change upon the electronic excitation, and it was consistent with the distortion of the H-chelate ring in the S 1 state predicted by theoretical calculations of Sobolewski and Domcke (Chem. Phys. 1998, 232, 257). Structures of the dimer in the S 0 and S 1 states were also determined by the observation of the OH stretching vibrations. No evidence was found for intermolecular double proton-transfer both in the S 0 and S 1 states of the dimer.

We discuss the sensitivity enhancement of bottom-gate type amorphous InGaZnO 4 thin-film transistor (a-InGaZnO TFT) pH sensors from the viewpoint of top-gate effects. Comparing the top-gate effects in a-InGaZnO TFTs having TaO x and SiO x ion-sensitive insulators, we draw an analogy between the operations of dual-gate TFTs and TFT pH sensors. Our new concept for enhancing pH sensitivity is characterized by a high capacitance ratio of the ion-sensitive insulator to the bottom-gate insulator and pH sensing utilizing threshold-voltage shifts in bottom-gate transfer characteristics. The close similarity between top-gate effects and pH sensitivity strongly suggests that a common mechanism underlies the phenomena. We discuss the mechanism on the basis of the material properties of a-InGaZnO and the silicon-on-insulator (SOI) model that relates bottom-and top-gate electric fields in fully depleted operations. We believe that the pH-sensitivity enhancement utilizing top-gate effects is one of the potential applications that would make the most of the intrinsic features of a-InGaZnO TFTs.

We evaluate pH sensitivity in bottom-gate amorphous InGaZnO4 thin-film transistor (a-InGaZnO TFT) pH sensors having various capacitances of the ion-sensitive insulator corresponding to the top-gate insulator in the TFT and the bottom-gate insulator. We change the capacitance values by varying the material and thickness of these insulators. Our results confirm the previously reported concept on the sensitivity enhancement exceeding the Nernst theoretical maximum in various ISFETs, which is based on capacitive coupling of bottom-gate and ion-sensitive insulator capacitances. The results also suggest that the pH sensitivity is determined by the coupling ratio irrespective of the surface material of the ion-sensitive insulator. We believe that the results would be helpful in further ensuring the validity of the sensitivity-enhancement concept from a viewpoint of a-InGaZnO TFT pH sensors having a wide variety of bottom-gate and ion-sensitive insulator capacitances.

We investigate the dependence of bottom-gate transfer characteristics on top-gate voltage, which we call “top-gate effects”, using amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) having a dual-gate structure. We found that the positive top-gate effect varies depending on the density of electron traps at the top-channel interface, while the negative top-gate effect has a similar impact on the bottom-gate transfer characteristics irrespective of the top-channel property. OFF-current increase due to the positive top-gate effect, which is one of the undesirable behaviors for practical use, was found to be effectively suppressed by sacrificing the subthreshold performance. These behaviors were described in terms of mutual interactions between the bottom-gate and top-gate electric fields. In comparison with conventional hydrogenated amorphous silicon (a-Si:H) TFTs, a-IGZO TFTs showed more significant top-gate effects. We consider this result to be due to the intrinsic material nature of a-IGZO, i.e., high electron mobility and nonexistence of hole accumulation in a-IGZO.

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