5.8 The ISFET as a Sensor

The ability of changing the electrolyte surface potential in an ISFET device can be exploited for various sensor applications. The possible sensor types can be chemically, e.g. a $CO_{2}$ sensor, as well as biologically, e.g. detecting the pH change due to cell metabolism, motivated. Depending on the type of sensor the mechanism changing the electrolytes surface potential varies. Most methods exploit a change in the charge population in the electrolyte. Commonly, depending on the nature of the experiment, reactants, or mechanisms leading to the modification of the charge distribution, ISFET devices are addressed as Chemically sensitive Field-Effect Transistor (CHEMFET), Enzyme Field-Effect Transistor (ENFET), DNA sensitive Field-Effect Transistor (DNAFET), or BioFET as a generic term.

There are two major concerns for efficient sensor designs: sensitivity and specifity. Specifity, is meant in the context of only responding to the selected species without any unwanted cross interactions. For instance, an ISFET sensor for $Na^{+}$ must not react to any of $M\!g^{++}$ or $C\!a^{++}$ ions. However, as explained in the previous sections, an ISFET gate stack does not meet these requirements ad hoc: due to the electrostatic interaction any charge carrier will be sensed by the FET. There are ways to circumvent this obstacle as will be explained later. Sensitivity, used here in a chemical and biological context, is the minimum concentration of the target species required to generate a fair signal level at the sensor's output. Sensors designed for the same species, but exploiting different physical phenomena, can show different sensitivities. For instance, mass spectroscopy with a detection limit of $\sim10^{-8}\frac{\mathrm{g}}{\mathrm{ml}}$ is less sensitive than electrochemical sensors $\sim10^{-13}\frac{\mathrm{g}}{\mathrm{ml}}$ [207].

The manifold different techniques for permitting an ISFET gate stack to change its surface potential, can be divided for two distinct applications: affinity based sensors and catalytic sensors [199]. For affinity based sensors, the reaction at the surface of the insulator is caused by chemical and thermodynamic affinity, while for catalytic sensors the reaction is driven by certain locally constrained catalysts at the surface [208].