ISFET

An ISFET, or ion-sensitive field-effect transistor, is a type of field-effect transistor in which the traditional gate electrode is replaced by an ion-sensitive membrane that interfaces with an electrolyte solution. The device is typically a metal-oxide-semiconductor transistor, and the drain–source current is modulated by the electrostatic potential at the interface between the sensing membrane and the electrolyte. Changes in ion activity in the solution, such as hydrogen ion concentration, alter the surface charge at the interface and shift the gate potential, thereby changing the channel conductance.

In operation, a reference electrode provides a stable potential against which the ISFET gate is measured. The sensing membrane can be a glass, polymer, or metal-oxide layer selected for sensitivity to particular ions. A common variation is the extended-gate ISFET, in which the ion-sensitive membrane is physically separated from the transistor by an extending conductor, allowing the sensitive layer to be protected while the transistor remains in a separate package.

Membranes confer specific ion selectivity, enabling a range of sensors including pH, sodium, potassium, and calcium ion sensors. pH-sensitive ISFETs often approach the Nernstian response (about 59 mV per pH at room temperature), though practical performance depends on surface chemistry, temperature, and other factors. Devices can be built as single sensors or arrayed on a chip for multiplexed measurements.

Applications span chemical and biological analysis, environmental monitoring, agriculture, and medical diagnostics. Benefits include solid-state form, compactness, compatibility with CMOS readout, and potential for large-scale integration; challenges include drift, temperature dependence, fouling, and the need for a stable reference electrode.