Ion selective FET

In ion-selective FETs (ISFETs), an ion-selective membrane replaces the gate electrode. When an enzyme-loaded gel is combined with the membrane, the device can be used to measure substrates which enzymically generate charged species. 

The electrical current flows from the source, via the channel, to the drain. However, the channel resistance depends on the electric field perpendicular to the direction of the current and the potential difference over the gate oxide. Should this surface be in contact with an aqueous solution, any interactions between the silicon oxide gate and ions in solution will affect the gate potential. Therefore, the source-drain current is influenced by the potential at the Si02/aqueous solution interface. This results in a change in electron density within the inversion layer and a measurable change in the drain current. This means we have an ion-selective FET (an ISFET), since the drain current can be related to ion concentration. Usually these are operated in feedback mode, so that the drain current is kept constant and the change of potential compared to a reference electrode is measured. 

Because of its particular technological characteristics, the discussion of the ICD shall remain confined to those made of c-Si therefore it will not be considered in this section. On the other hand, only those structures having a configuration adaptable to a-Si H thin-film technology will be taken into consideration. Two kinds of FETs are referred to in the literature the ion-selective FET (ISFET) and the gas-sensitive FET (CHEMFET). 

Progress in analytical potentiometry in recent years has brought a miniaturization of detection systems via the hyphenation of the sensor with a large extent of integration electronics. In this context, chemically sensitive layers are placed directly onto field effect transistors (FETs) to produce ion-selective FETs (ISFETs). The use of two identical ISFETs in a flow-through cell offers the possibility of differential measurements without the influence of the liquid junction potential. 

The membrane potentials can also be measured indirectly, if the membrane is fixed on the surface of a field-effect transistor (FET) from which the metallic gate has been removed. The electric current passing between the source and drain of the EET is then controlled by the membrane potential. The first sensor of this type [ion-selective FET (ISFET) or chemically sensitive FET (CHEMFET, CSFET)] was described by Bergveld in 1970 and the field has been developed primarily by Janata and co-workers and Japanese researchers. 

Semiconductor processing technologies have often been used to produce ISEs, particularly as field-effect transistors (FETs) with ion-selective layers like silicon oxide over the gate region. Such ion-selective FETs (ISFETs) are, in principle, solid ISEs, although sometimes the dielectric over the gate is covered with a second, liquid membrane-type layer to achieve different selectivities. 

Many IS Es based on solid or Kquid membranes and ion-selective FETs possess sufficiently small Kij values (Eq. 1) toward common interferents, and have been developed into commercial benchtop analyzers that allow the measurement of Na+, K+, Ca +, Cl , HC03, and other ions, in instruments that are designed solely for IS E measurements or in combination with other integrated devices (such as amper-ometric sensors and spectrophotometers) to measure other parameters [7]. Applications of these devices to blood gas analysis have been reviewed recently [81]. Companies that produce this instrumentation for clinical chemistry applications include Abbott Diagnostics, AVL, Bayer Diagnostics, Beckman Coulter, Nova Biomedical, Ortho-Clinical Diagnostics, Boehringer-Mannheim/Roche, and Radiometer [82]. Many instruments allow measurements to... 

The chemically sensitive MISFET, called CHEMFET (insulator coated with a sensitive membrane) or ISFET (ion-selective FET, usually with only an ion-active insulator), is arranged in such a way that the reference electrode, measured solution, and sensitive membrane or insulator surface are inserted between the gate of the original MISFET and the gate dielectric (see Fig. 7A). The metallic ISFET gate is now represented by a contact of the reference electrode and the voltage Uqs is identical to a constant or controlled reference voltage, Ur, applied to the reference electrode ... 

Enzyme-immobilized FETs (ENFETs), 22 269. See also Enzyme ion-selective field-effect transistor (ENFET) Field effect transistors (FETs)... 

ISFETs), 3 799 9 585 74 24. See also Field effect transistors (FETs) Ion-selective membranes, cavities in, 9 584 Ion size, cluster glass transition and, 74 469, 470... 

These measure the potential difference between the transducing electrode and a reference electrode under conditions of zero current. Three types of potentiometric detectors are commonly employed ion-selective electrodes (ISE), gas-sensing electrodes and field effect transistors (FET). 

Fig. 6.19 Evolution of ion-sensitive field-effect transistor (ISFET) from insulated gate FET and ion-selective electrode (ISE)...

The enzyme-containing FET, called ENFET, is fabricated from an ISFET by casting a thin membrane containing the enzyme over an ion-selective membrane... 

Another approach is the use of the potentiometric principle with planar thin film electrodes on a separate chip but in close vicinity to a FET input amplifier. Glucose and urea chips are now on the market commercialized by the company i-stat. These sensors are based on ion selective electrodes. The problems of stability are circumvented by a simple on chip calibration procedure and by the use of such microelectronic electrodes as disposable single shot probes for measuring Na, K, Cl, BUN, Glucose,iCa,pH,pC02 and Hct [56,57]. 

Chemisorption. A key step in the development of a successful membrane modified FET is the adherence and longevity of the membrane and the success of the encapsulation procedure. Sudholter et al (35) have proposed a method for the attachment of the ion selective membrane to a silylated SiC>2 gate oxide. An organofunctional silane, for example methacryloxypropyl trimethoxysilane, forms a bond between the surface and a photocrosslinkable polymer (eg polybutadiene). The ionophore would be directly bound to the polymer backbone, thus eliminating the need for plasticiser. Without any further modification with ionophore the membrane is sensitive to pH and shows a long life time (some months). Similar approaches have also been proposed by other workers, eg (36)... 

The recently developed field-effect transistors (FETs)41 have also been used as biosensors. The ion-selective field-effect transistor (ISFET) uses ion-selective membranes, identical to those used in ion-selective electrodes, over the gate. 

Joining of an ion-selective electrode and a field-effect transistor (FET) in a single device has made it possible to create a new type of ion concentration detector called the ion-selective field-effect transistor (ISFET.) High sensitivity and selectivity, as well as the very small size of this device, have resulted in its constantly growing applications in recent years in industry, medicine, biology, scientific research, etc. 

This feature of the FET is transformed into high sensitivity in the ISFET. That is, in order to go over from the FET to the ISFET it is sufficient to replace the insulating layer by an ion-selective membrane permeable only to one sort of iont and the metal gate by an electrolyte solution, which contains a reference electrode needed to short circuit the electric circuit (Fig. 19b). The potential, which develops across the membrane in the presence in the solution of that sort of ions for which the membrane is selectively permeable, acts here as an external voltage on the gate. This potential can be determined from Nernst s equation [cf. Eq. (4)]... 

Polymeric films of immobilized or entrapped ionophores have been widely applied in the fabrication of small, coated wire ion selective electrodes [127] and in the fabrication of ISFETs [128-130] where the ionophore is immobilized on the gate of the FET structure to give an ion selective response. 

Ion, molecule Carrier Protein Change of concentration or (redox) potential Ion selective electrode or FET, basing on ionophores... 

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