Bottom-gate amorphous InGaZnO4 thin-film transistor pH sensors utilizing top-gate effects

We discuss the sensitivity enhancement of bottom-gate type amorphous InGaZnO4 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 TaOx, and SiOx 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. (C) 2014 The Japan Society of Applied Physics