The Ion-Selective Field-Effect Transistor

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. 

Since the ISFET is based on the field-effect transistor, let us recall briefly how the latter operates (see, e.g.. Ref. 98). The field-effect transistor (Fig. 19a) represents the so-called MIS (metal-insulator-semiconductor) structure (hence the abbreviation MIS-FET), i.e., a semiconductor base, onto which an insulating layer and a metal electrode (gate) are deposited. The base usually is a p-type silicon plate and the insulator, a Si02 or Si3N4 layer. With a thickness of 100-200 nm, the resistance of this layer is of the order of 10 fl. Two regions are produced in the base by local 

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)] 

With this mechanism of the formation of potential across the membrane, the current in the measuring circuit depends on the ion activity a, which makes the ISFET a very sensitive ion detector. 

Particular types of the ISFET are very diverse, since there is a possibility of widely varying the membrane composition, the type of reference electrode, the mode of operation (e.g., an additional polarizing voltage can be used), etc. 

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The ion-selective field-effect transistor (ISFET) represents a remarkable new construction principle [7, 63], Inverse potentiometry with ion-selective electrodes is the electrolysis at the interface between two immiscible electrolyte solutions (ITIES) [28, 55],... 

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. 

Bergveld introduced the ion selective field effect transistor (ISFET) [105[. 

For pesticide analysis, the potential of enzyme biosensors has been tested. In this field, biosensors based on the inhibition of acetylcholinesterases, acylcholinesterases, or butylrylchol-inesterases by organophosphorus compounds are widely used. Their specific activity can be monitored by electrochemical methods such as the ion-selective electrode and the ion-selective field effect transistor (ISFET). 

The ion-selective field-effect transistor (ISFET) is derived from the well-established MOSFET device familiar in silicon integrated circuits. Field effect devices have three major advantages to offer (see Chapter 10 of this volume). 

These problems might be resolved by the ion-selective field-effect transistor (ISFET). In an IS-FET the ion-selective membrane is placed directly on the gate insulator of the field-effect transistor alternatively, the gate insulator itself, acting as a pH-selective membrane, may be exposed to the analyte solution (Fig. 28). If one compares an ISFET with the conventional ISE measurement system, the gate metal, connecting leads, and internal reference system have all been eliminated. An ISFET is a small, physically robust, fast potentio-metric sensor, and it can be produced by microelectronic methods, with the future prospect of low-cost bulk production. More than one sensor can be placed within an area of a few square millimeters. The first ISFETs were described independently by Bergveld [142] and Matsuo, Esashi,... 

Bergveld P (1972) Development, operation and application of the ion-selective field-effect transistor as a tool for electrophysiology. IEEE Trans Biomed Eng 19 340-351... 

The working principle of LAPS resembles that of an ion-selective field effect transistor (ISFET). In both cases the ion concentration affects the surface potential and therefore the properties of the depletion layer. Many of the technologies developed for ISFETs, such as forming of ion-selective layers on the insulator surface, have been applied to LAPS without significant modification. 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

Ion-Selective Field Effect Transistors [22b,c,d] An ion-selective field effect transistor (ISFET) is a hybrid of an ion-selective electrode and a metal-oxide semiconductor field effect transistor (MOSFET), the metal gate of the MOSFET being replaced by or contacted with a thin film of a solid or liquid ion-sensitive material. The ISFET and a reference electrode are immersed in the solution containing ion i, to which the ISFET is sensitive, and electrically connected as in Fig. 5.37. A potential which varies with the activity of ion i, o(i), as in Eq. (5.38), is developed at the ion-sensitive film ... 

Successful operation of potentiometric chemosensors opened up the possibility for the fabrication of chemical field-effect transistors (chemFETs) and ion-selective field-effect transistors (ISFETs). A sensing element in these devices, i.e. the MIP film loaded with the molecular, neutral or ionic, respectively, imprinted substance is used to modify surface of the transistor gate area. Apparently, any change in the potential of the film due to its interactions with the analyte alters the current flowing between the source and drain. 

This interface is also known as the perm-selective interface (Fig. 6.1a). It is found in ion-selective sensors, such as ion-selective electrodes and ion-selective field-effect transistors. It is the site of the Nernst potential, which we now derive from the thermodynamic point of view. Because the zero-current axis in Fig. 5.1 represents the electrochemical cell at equilibrium, the partitioning of charged species between the two phases is described by the Gibbs equation (A.20), from which it follows that the electrochemical potential of the species i in the sample phase (S) and in the electrode phase (m) must be equal. 

The first representative of a potentiometric sensor was the pH-glass electrode invented in 1906 [35]. Decades of development resulted in the invention of many more ion-selective electrodes including more recently those based on neutral carrier membranes [36] and of the microelectronic fabricated ion selective field effect transistor (ISFET) [37]. 

Ion-selective field-effect transistors (ISFETs) are ion sensors that combine the electric properties of gate-insulator field-effect transistors and the electrochemical properties of ion-selective electrodes (ISEs). ISFETs have attracted much attention for clinical and biomedical fields because they could contain miniaturized multiple sensors and could be routinely used for continuous in vivo monitoring of biological fluid electrolytes (e.g., Na+, K+, Ca +, Cl", etc.) during surgical procedures or at the bedside of the patients in clinical cate unit (2). 

There are also new methods suggested recently, but they are not yet commonly employed.5 One of them involves ion-selective field effect transistor as a sensor, which requires smaller measurement area compared to the glass electrode.10 Other possible methods are electron spin resonance imaging and confocal microscopy.5 11 12 They require treatment of the skin with an indicator substance, which penetrates into the epidermis and allows the pH to be detected in several layers simultaneously.12... 

The integration of chemically sensitive membranes with solid-state electronics has led to the evolution of miniaturized, mass-produced potentiometric probes known as ion-selective field effect transistors (ISFETs). The development of ISFETs is considered as a logical extension of coated-wire electrodes (described in Section 5.2.4). The construction of ISFETs is based on the tech-... 

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