The Ion Sensitive Field Effect Transistor (ISFET)

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The Ion Sensitive Field Effect Transistor (ISFET) is a chemical sensor with the same basic structure as a MOSFET. However, there are a few significant differences. The gate of an ISFET is replaced by the solution being monitored and an electrochemical reference electrode. The gate dielectric, generally SiO2, operates as an ion-sensitive membrane, with the pH of the solution controlling the surface potential. Furthermore, the electrode potential is defined by the Nernst equation. Compared with Ion Sensitive Electrodes (ISE), ISFETs are smaller, consume less power, and without the glass membrane, they are also more stable and less prone to ageing.Most electronic components are developed through design, manufacture, and testing processes. Computer-aided tools can play an essential role in the design steps of ISFETs. Although initially developed for the design and simulation of electronic circuits, these programmes can be used to design silicon-based chemical and biological sensors. Standard CMOS usually uses a self-aligned process during the microfabrication process, where the gate electrode is fabricated before the source and drain regions. However, for an ISFET, removing the gate electrode afterwards without damaging the thin gate oxide seems impractical. In terms of testing, characteristics such as sensitivity and linearity range require repeated calibration as a sensing element. Finally, although the ISFET was originally developed for pH measurement, it has many applications, such as biosensing and environmental monitoring.