Improved Sensitivity and Stability for SnO Ion-Sensitive Field-Effect Transistor-Based pH Sensor by Electrical Double Layer Gate and AlO Sensitive Film

Abstract

Hydrogen ion sensor (pH sensor) with high sensitivity and good stability has great potential in modern life, medicine, industry, and other fields. As a core of the pH sensor, an ion-sensitive field-effect transistor (ISFET) based on SnO2 is paired with a high selectivity Al2O3 inorganic insulating film, which enhances its pH sensitivity. In the presence of a voltage applied to the extended gate, the gate and channel act as capacitance plates, and the sensor is equivalent to an electrical double layer (EDL). The hydrogen ions in solution alter the solution capacitance, which changes the capacitance of the solid dielectric layer, regulating the source–drain current. As a result of this work, the sensitivity of the ISFET-based pH sensor with extended gate increased <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$15.8\times $ </tex-math></inline-formula> to 25.33 nA/pH <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot \text{V}$ </tex-math></inline-formula> when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{{\mathrm {GS}}}$ </tex-math></inline-formula> is 300 mV, compared to a SnO2-based sensor without the extended gate. Also, the stability of the sensor has been greatly improved, as well as the response time of 0.2 s. Our research provides an effective strategy for high-performance ISFET-based pH sensor, and the concept of using the pH-ISFET with an extended gate for direct quantification of pH values in solutions provides a dependable method for industrial applications.