Sensitivity Evaluation on Amorphous InGaZnO₄Thin-Film Transistor pH Sensors Having Various Capacitances of Ion-Sensitive and Bottom-Gate Insulators

Abstract

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.