Ion Sensitive Field-Effect, Transistors ISFETs

Explain the main mechanistic differences between a glass membrane electrode and an ion-sensitive field effect transistor (ISFET). 

The measurement of changes of the surface potential Vo at the interface between an insulator and a solution is made possible by incorporating a thin film of that insulator in an electrolyte/insulator/silicon (EIS) structure. The surface potential of the silicon can be determined either by measuring the capacitance of the structure, or by fabricating a field effect transistor to measure the lateral current flow. In the latter case, the device is called an ion-sensitive field effect transistor (ISFET). Figure 1 shows a schematic representation of an ISFET structure. The first authors to suggest the application of ISFETs or EIS capacitors as a measurement tool to determine the surface potential of insulators were Schenck (15) and Cichos and Geidel (16). 

ION-SELECTIVE ELECTRODES (ISEs) AND ION-SENSITIVE FIELD-EFFECT TRANSISTORS (ISFETs)... 

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

Fig. 6.22 Analog circuits for operation of ion-sensitive field-effect transistor (ISFET) (a) in constant applied voltage mode ((6.62) and (6.63)) and (b) in (source-follower) constant current feedback mode ((6.65) and (6.69))...

Fig. 6.26 Individual and differential response of the NaCI ion-sensitive field-effect transistor (ISFET) sensor (a) response of Na-ISFET and (b) Cl ISFET both measured against regular reference electrode (c) differential current measurement of concentration of NaClin 0.01MMgSO4 solution (adapted from Bezegh et al., 1987)...

Our choice for an Ion Sensitive Field Effect Transistor (ISFET) as a transducing element was based on the fact that the SiO surface contains reactive SiOH groups for the covalent attachment of organic molecules and polymers. In addition the FET has fast response times and can be made very small with existing planar IC technology. FIGURE 1... 

Ion-sensitive field effect transistor (ISFET) — In a semiconductor device based on the principle of the field effect transistor (FET) the current between two - semiconductor electrodes (designated source and drain) is controlled by a third electrode, the gate. In an ISFET this gate is modified on its surface in a way which makes the surface ion-responsive (-selective and -sensitive). Changes in the concentration of the species in the solution in contact with the gate surface thus control the current between source and drain. In order to establish proper working conditions a reference electrode (e.g., a -+ REFET) is needed. See also - CHEM-FET. 

The acid-base behavior of proteins can reveal some important properties with respect to both their composition (selectivity) and their concentration (sensitivity). The most direct way to exploit these acid-base properties is to make use of acid-base titration, Titrant should be added somehow and the resulting change in pH should be measured. Since the ion-sensitive field-effect transistor (ISFET) is suitable for fast (and local) pH detection, an ISFET can be used for protein titration if the protein to be detected can be immobilized in a membrane, deposited on top of the device. Advantages are the small amount of protein necessary for the characterization owing to the small membrane volume, and the relatively short time needed to perform a full titration. 

The ion sensitive field-effect transistor (ISFET) is a special member of the family of potentiometric chemical sensors [6,7. Like the other members of this family, it transduces information from the chemical into the electrical domain. Unlike the common potentiometric sensors, however, the principle of operation of the ISFET cannot be listed on the usual table of operation principles of potentiometric sensors. These principles, e.g., the determination of the redox potential at an inert electrode, or of the electrode potential of an electrode immersed in a solution of its own ions (electrode of the first kind), all have in common that a galvanic contact exists between the electrode and the solution, allowing a faradaic current to flow, even when this is only a very small measuring current. 

The first example of chemically modified field-effect transistors (CHEMFETs) was reported by Janata et al. [15] in 1978 for ion-sensitive field-effect transistors (ISFETs) in which the gate oxide was covered with a PVC membrane containing... 

The basic building block of all CHEMFETs is the ion-sensitive field effect transistor (ISFET), introduced in 1970 by Bergveld [4]. In an ISFET the gate metal of a metal oxide semiconductor field effect transistor (MOSFET) is removed and the resulting gate oxide surface (2) is directly exposed to the electrolyte solution (Figure 1). 

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