High-performance double-gate α-InGaZnO ISFET pH sensor using a HfO₂ gate dielectric

In this paper, we present a high-performance double-gate (DG) amorphous indium-gallium-zinc-oxide (α-InGaZnO) ion-sensitive field-effect transistor (ISFET) using three HfO₂ gate dielectric thicknesses as a top gate (TG). The DG structure α-InGaZnO TFTs with a 40-nm TG HfO₂ dielectric exhibited a small threshold voltage of 50 mV, a low subthreshold swing of 144.1 mV/decade, and a high ION/IOFF current ratio of 4.5×10⁷. The α-InGaZnO ISFET prepared at the 40-nm HfO₂ sensing membrane in the DG mode showed a pH sensitivity of 937 mV/pH, which is far more than the Nernst limit. The hysteresis and drift behaviors of DG ISFET fabricated with the 40-nm condition also showed relatively better chemical stability compared with other conditions.