ISFET

The ion-sensitive field effect transistor ( ISFET also ion-selective or ion- sensitive field effect transistor ) is a specific field effect transistor ( FET) that can measure the pH of a solution by the conductivity of the transistor.

Operation

The measuring principle is based just like the field effect transistor on a change of the field effect (formation of a space charge region ), which is formed between source and drain. In place of the electrical contact to the gate of an ion-sensitive layer (e.g., Al2O3, Si3N4, or Ta2O5 as a pH-sensitive layer) is applied, which is brought into direct contact with the liquid to be measured. About a bias voltage, which is a reference electrode, which is also located in the solution applied, the operating point of the ISFET can be set (analogous to set the operating point of a FET ). Depending on the concentration of ions in the solution to be tested is an additional potential to the contact surface between the liquid and ion-sensitive layer is formed from ( Nernst equation). This potential is added to the constant bias voltage applied, thus influencing the space charge region between the source and drain. This leads to a change in the source-drain current which can be measured. The changes are therefore directly proportional to the change of the analyte concentration. About a calibration can be calculated back using the measured current to the analyte concentration. The ISFET is electrically regards a transimpedance converter. An electrical potential is without an appreciable current flows potentiometrically measured and converted into a measurable source-drain current. This is independent of subsequent circuits and thus loads than high-impedance current source to a first approximation.