ISFETs

An ISFET, or Ion-Sensitive Field-Effect Transistor, is a type of field-effect transistor in which the gate electrode is replaced by an ion-sensitive membrane in contact with an electrolyte solution. The device translates ion activity, particularly hydrogen ion activity (pH), into a change in the transistor's channel conductance. In operation, a reference electrode provides a stable gate potential; changes in the ion concentration at the membrane surface alter the surface potential, which modulates the threshold voltage and therefore the drain current at a given drain-source voltage.

Most ISFETs use a MOSFET with a chemically sensitive gate dielectric or an extended gate configuration where a separate ion-sensitive membrane is connected to the transistor. Common sensing layers include silicon nitride, aluminum oxide, tantalum oxide, or layered polymer membranes designed for specific ions.

Performance: At room temperature, an ideal pH response would be near the Nernst limit of about 59 mV per pH, but real devices show smaller slopes due to surface states, ionic strength, and topology. Practical devices exhibit drift, hysteresis, temperature sensitivity, and require periodic calibration and impermeable packaging to limit fouling.

Applications include pH sensing, detection of monovalent/divalent ions, and integration into lab-on-a-chip systems, biosensors for enzyme-coupled measurements, and environmental monitoring. Advantages include CMOS compatibility, small size, and direct electrical readout; disadvantages include drift, sensitivity to ionic strength, and dependence on a stable reference electrode.