Interface mechanisms involved in a-IGZO based dual gate ISFET pH sensor using Al2O3 as the top gate dielectric

Abstract

An a-IGZO based dual gate ion sensitive field effect transistor (DG ISFET) was fabricated utilizing a simple and cost effective method. Sputtered Al2O3 was used as the top gate dielectric to obtain a greater surface site density and thus a higher sensitivity due to higher surface roughness. Deteriorated characteristics of the bottom gate thin film transistor (BG TFT) caused by deposition of Al2O3, attributed to the induced oxygen vacancies at the Al2O3/a-IGZO interface were recovered by optimized annealing at 350 °C in presence of oxygen. The increased value of the OFF current (~nA) in the BG TFT upon exposure to the electrolyte (pH-4), was attributed to the formation of double channels in the active (a-IGZO) layer. The conduction in the channel formed at the Al2O3/a-IGZO interface dominated over the bottom gate channel (a-IGZO/SiO2 interface) in the turn OFF region while being comparable in the turn ON region. The fabricated DG ISFET showed ~6-fold enhancement in pH sensitivity (347.6 mV/pH) from the Nernst's limit (59 mV/pH). To validate the experimental findings, a T-CAD based simulation model was proposed by incorporating three dielectric layers to represent the electric double layer formed at the electrolyte/insulator interface and at the reference electrode.