Improved pH Sensitivity and Reliability for Extended Gate Field-Effect Transistor Sensors Using High-k Sensing Membranes.

Abstract

In this study, we fabricated extended-gate (EG) field-effect transistor (FET) pH sensors with dualgate (DG) structures, using a range of dielectric sensing membranes (SiO₂, Si₃N₄, HfO₂ and Ta₂O5) to vary their sensitivity. The fabricated EGFETs consisted of a silicon-on-insulator (SOI)-based metal-oxide semiconductor field-effect transistor (MOSFET) transducer and an EG sensor. We amplified the sensitivity of the device far beyond the Nernst limit (59 mV/pH), which is the theoretical maximum of conventional ion-selective FET (ISFET) sensing, by applying capacitive coupling. Among the evaluated dielectric sensing membranes, we obtained the highest sensitivity (478 mV/pH), low hysteresis (100.2 mV) and drift rate (24.6 mV/h) from the pH sensor with a Ta₂O5 membrane. Hence, we expect DG FET configurations using Ta₂O5 films as EG sensing membranes to be useful for high performance biosensor applications, as they satisfy the requirements for sensitivity, stability and reliability.