A comprehensive review of FET‐based pH sensors: materials, fabrication technologies, and modeling

Abstract The demand for miniaturized point‐of‐care chemical/biochemical sensors has driven the development of field‐effect transistors (FETs) based pH sensors over the last 50 years. This paper aims to review the fabrication technologies, device structures, sensing film materials, and modeling techniques utilized for FET‐based pH sensors. We present the governing principles of potentiometric sensors, with major focus on the working principles of ion‐sensitive FETs (ISFETs). We extensively review different sensing film materials deposited by various techniques, which is critical to the sensing performance of ISFETs. The popular fabrication technologies have been presented, with special emphasis on state‐of‐the‐art silicon‐on‐insulator based technology, which can achieve high sensitivity by utilizing the dual‐gate effect. Furthermore, recent advancements in nano‐ISFETs has been elucidated. We also discuss the adoption of unmodified complementary metal‐oxide semiconductor (CMOS) ISFETs using standard CMOS processes, which has enabled the fabrication of integrated ISFET arrays, which are especially suited for ion‐imaging applications. Moreover, recent developments in extended‐gate FETs has been discussed, which have gained lot of attention due to their design flexibility and ease of fabrication, which is desirable for wearable sensing applications. In addition, recently there have been efforts to utilize nonsilicon channel materials for pH‐sensing application to obtain superior performance and various channel materials have been reviewed. Finally, we have extensively reviewed the ISFET device modeling and simulation techniques using various computer‐aided design tools, which aid in sensor design and characterization.