High-Sensitive Extended-Gate FET Type pH Sensors Utilizing Ga₂O₃ Thin Film Prepared by Chemical Solution Deposition

Abstract

This study aimed to develop a high-sensitivity pH sensor exceeding the Nernst limit (∼59 mV/pH) for biosensing applications. We fabricated gallium oxide (Ga₂O₃) thin films on Ti/glass substrates via a cost-effective and vacuum-free Chemical Solution Deposition (CSD) method. Structural analysis confirmed the formation of a monoclinic β-Ga₂O₃ crystalline structure with a nanostructured surface morphology exhibiting an RMS roughness of 28.7 nm. An Extended-Gate Field-Effect Transistor (EGFET) sensor was constructed using a Ga₂O₃ thin film as the sensing extended-gate electrode, and its pH response characteristics were evaluated. The sensor demonstrated a super-Nernstian sensitivity of 80.1 mV/pH in the neutral to weakly alkaline region (pH 7.0-8.0), significantly exceeding the theoretical limit. We infer that this enhanced sensitivity results from the increased effective surface area due to the CSD-derived nanostructured and porous film, which allows proton diffusion not only onto the surface but also into the internal hydrated layer, contributing to the change in potential. These results indicate that CSD-derived Ga₂O₃ thin films are promising transducer materials for highly sensitive biosensing in the neutral pH region.