Ion-sensitive field effect transistor
The ion-sensitive field effect transistor (ISFET, also ion-selective or ion-sensitive field effect transistor ) is a special field effect transistor (FET) that can measure the pH value of a solution through the electrical conductivity of the transistor .
functionality
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. Instead of the electrical contact on the gate, an ion-sensitive layer (e.g. Al 2 O 3 , Si 3 N 4 , or Ta 2 O 5 as a pH-sensitive layer) is applied, which is brought into direct contact with the liquid to be measured . The operating point of the ISFET can be set via a bias voltage , which is applied via a reference electrode, which is also located in the solution (analogous to setting the operating point of an FET). Depending on the concentration of the ions in the solution to be examined, an additional surface potential is created at the contact between the liquid and the ion-sensitive layer ( Nernst equation ). This potential adds up to the constantly applied bias voltage and thus influences the space charge zone between 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 in the analyte concentration. A calibration can be used to calculate back to the analyte concentration with the help of the measured current. From an electrical point of view, the ISFET is a transimpedance converter . An electrical potential is measured potentiometrically without a significant current flowing and converted into a measurable source-drain current. As a first approximation, this is independent of subsequent circuits and can therefore be loaded as a high-resistance power source .