ISFET

It is a special type of MOSFET (metal–oxide–semiconductor field-effect transistor),[1] and shares the same basic structure, but with the metal gate replaced by an ion-sensitive membrane, electrolyte solution and reference electrode.

The hydroxyl groups coating an oxide surface such as that of SiO2 can donate or accept a proton and thus behave in an amphoteric way as illustrated by the following acid-base reactions occurring at the oxide-electrolyte interface: An ISFET's source and drain are constructed as for a MOSFET.

For this reason, since more than 20 years many research efforts have been dedicated to on-chip embedded tiny reference field effect transistors (REFET).

Semi-conductor modified surfaces required for REFET are also not always in thermodynamical equilibrium with the test solution and can be sensitive to aggressive or interfering dissolved species or not well characterized aging phenomena.

This is not a real problem if the electrode can be frequently re-calibrated at regular time interval and is easily maintained during its service life.

However, this may be an issue if the electrode has to remain immersed on-line for prolonged period of time, or is inaccessible for particular constrains related to the nature of the measurements itself (geochemical measurements under elevated water pressure in harsh environments or under anoxic or reducing conditions easily disturbed by atmospheric oxygen ingress or pressure changes).

For example, to suppress the external noise, we can integrate a bipolar junction transistor with ISFET to realize immediate the internal amplification of drain current.

When the intrinsic response of the pH-ISFET is completed, the output voltage of the ISFET still vary with time gradually and monotonically, and this drift behavior exists during the entire measurement process It has been one of the serious obstacles in developing commercially viable, ISFET-based biomedical sensors.

Dutch engineer Piet Bergveld, at the University of Twente studied the MOSFET and realized it could be adapted into a sensor for electrochemical and biological applications.