Ion-sensitive field-effect transistors (ISFET) are sensors for chemical solutions. They are based on the fact that the electrical conductivity of the field effect transistor (FET) changes due to the pH value of chemical solutions.
In general, the current in a field effect transistor depends on the potential difference between the source and drain and the voltage applied to the gate. This current can be affected by charge carriers on the gate electrode, although it does not matter whether these are electrical or electrochemical charge carriers. In the ISFET, an ion-sensitive layer is applied to the gate electrode, which is immersed in the chemical liquid to be measured. The resulting potential depends on the ion concentration and provides current flow via the ion-sensitive layer in the ISFET. Aluminum oxide (Al2O3) or silicon compounds are used as ion-sensitive material.