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

• Published in: Japanese Journal of Applied Physics 2014-07, Vol.53 (7)
• Main Authors: Takechi, Kazushige, Iwamatsu, Shinnosuke, Yahagi, Toru, Abe, Yutaka, Kobayashi, Seiya, Tanabe, Hiroshi
• Format: Article
• Language: English
• ISSN: 0021-4922; 1347-4065
• DOI: 10.7567/JJAP.53.076702

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.