Substrate ISFET

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. 

For correct function of the ISFET, a sufficiently large gate voltage, Vq, must be applied between the leads to the reference electrode and to the substrate, so that a sufficiently large potential difference is formed between the surface and the interior of the substrate for formation of the n-type conductive channel at the insulator/substrate interface. This channel conductively connects drain 1 and source 2, which are connected with the substrate by a p-n transition. On application of voltage Vj between the drain and the source, drain current /p begins to pass. Under certain conditions the drain current is a linear function of the difference between Vq and the Volta potential difference between the substrate and the membrane. 

Fig. 5.37 An ion-selective field-effect transistor (ISFET). 1, drain 2, source 3, substrate 4, insulator 5, metal lead 6, reference electrode 7, solution 8, membrane 9, encapsulant [22b].

Local changes in the ion concentration as a result of substrate conversion by the immobilized enzymes have been measured using pH [41] or fluoride sensitive field effect transistors (pF-ISFET) [42] or ammonia-sensitive FETs [43]. 

The ISFET is deduced from the MOSFET, which is a well-known electronic device. Cross section of the MOSFET and the ISFET are shown in Fig. I. A MOSFET consists of a p-type silicon substrate in which two n-type diffusions are realized, which are called the source and the drain. The structure is covered with an insulating layer (usually Si02), and a metal gate electrode is deposited over the area between the source and the drain. 

Enzyme sensors can measure analytes that are the substrates of enzymatic reactions. Thermometric sensors can measure the heat produced by the enzyme reaction [31], while optical or electrochemical transducers measure a product produced or cofactor consumed in the reaction. For example, several urea sensors are based on the hydrolysis of urea by urease producing ammonia, which can be detected by an ammonium ion-selective ISE or ISFET [48] or a conductometric device [49]. Amperometric enzyme sensors are based on the measurement of an electroactive product or cofactor [50] an example is the glucose oxidase-based sensor for glucose, the most commercially successful biosensor. Enzymes are incorporated in amperometric sensors in functionalised monolayers [51], entrapped in polymers [52], carbon pastes [53] or zeolites [54]. Other catalytic biological systems such as micro-organisms, abzymes, organelles and tissue slices have also been combined with electrochemical transducers. 

The ion-sensitive surface of the ISFET is naturally sensitive to pH changes, but the device may be modified so that it becomes sensitive to other species by coating the silicon nitride gate insulator with a polymer containing molecules that tend to form complexes with species other than hydronium ion. Furthermore, several ISFETs may be fabricated on the same substrate so that multiple measurements may be made at the same time. All the ISFETs may detect the same species to enhance accuracy and reliability, or each ISFET may be coated with a different polymer so that measurements of several different species may be made. Their small size (about 1 to 2 mm ), rapid response time relative to glass electrodes, and ruggedness suggest that ISFETs may be the ion detectors of the future for many applications. 

For ISFET devices in particular, because of their poor isolation and the impurities penetration in the substrate in chemical environment, device encapsulation... 

The electric field applied on the gate can change the charge-carrier distribution in the surface layer. Positive field repulsing the positive holes can make inversion. This means that in the surface layer of the substrate, an n-type conductive channel is formed. In ion-selective field effect transistors [24, 25] (ISFET), the gate is coated with an ion-sensitive layer and an appropriate electrode body is applied (Figure 7.1). 

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