Ion-sensitive FETs

Figure 3.21 — (A) Integrated FET with two hydrogen ion-sensitive FET elements. (B) Structure of enzyme-modified FET sensor S plastic card FET enzyme-modified FET chip lUM, immobilized urease membrane. (C) Flowthrough cell Bl fixed sensor cell block B2 movable sensor cell block SC flowthrough cell EC electrical connector RP silicone rubber sheet AMP amplifier. (Reproduced from [151] with permission of Elsevier Science Publishers).

The basic structure of an ion-sensitive FET is illustrated in Fig. la. The silicon chip has three principle components a drain (D), a source (S), and an ion-... 

Fig. 1. (a) Structure of eui ion-sensitive FET transducer with single FET element, (b) Structure of an ion-sensitive FET with metal oxide semiconductor FET. D, drain S, source IG, ion-sensitive gate CP, contact pad G, gate RE, reference electrode AS, analyte solution MOSFET, metal oxide semiconductor FET ISFET, ion-sensitive FET. 

Silicon nitride is typically used as the material for a hydrogen ion-sensitive gate, but tantalum oxide and aluminum oxide can also be used (4). FETs sensitive to other ionic species use other ion-sensing membranes (5). The reader is directed to reviews (33-35) of the fabrication, function, and operation of ion-sensitive FETs. 

Two major improvements in the fabrication of an ion-sensitive FET that avoid most of the tedious polymer encapsulauon process have been reported. Matsuo and his coworkers (4, 37) fabricated a probe-type FET with a three-dimensional silicon nitride passivation layer around most of its surface, as shown in Fig. 2. The probe-type FET has one disadvantage Its fabrication requires a three-dimensional process that is uncommon for semiconductor construction facilities. An alternative approach utilizes a silicon-on-sapphire (SOS) wafer for FET fabrication (38, 39). The structure of a SOS-FET is depicted in Fig. 3. It has an island-like silicon layer on a sapphire substrate, in which an ion-sensitive FET is fabricated. The bare lateral sides do not need encapsulation because of the high insulation property of sapphire. 

An enzymatically coupled FET requires two independent hydrogen ion-sensitive FET elements One carries an enzyme-immobilized membrane on its surface, ctdled an enzyme FET, and the other, with no enzymatic activity, is a reference FET. Figure 4 is a schematic representation of an enzymatically coupled FET (shown in the area surrounded by the broken circle). The enzyme immobilized on the surface of the enzyme FET (EIM in Fig. 4) causes a change in... 

A monolithic monofunctional enzymatically coupled FET sensitive to urea was first fabricated by Kuriyama and Kimura s group (44). Their integrated FET chip has two hydrogen ion-sensitive FET elements. One FET element has a urease-immobilized membrane, working as an enzyme FET, and the other has a ultraviolet (UV) light-inactivated, urease-immobilized membrane (see Section 3), working as a reference FET. 

Fig. 9. Photograph of a glucose oxidase-immobilized membrane patterned on jui ion-sensitive FET by the direct photolithographic patterning method enzyme-immobilized membrane (1000 X 200 pm) on right ion-sensitive gate. Left ion-sensitive gate is left blank for comparison.

Fig. 11. Schematic diagram of continuous flow apparatus and structure of an enzymatically coupled FET. (a) Schematic diagram of continuous flow apparams S, enzymatically coupled FET sensor SC, sensor cell WB, water bath D, drtdnage P, peristaltic pump TV, three-way joint EV, electrical valve VC, valve controller WS, washing solution AS, analyte solution, (b) Detailed structure of flow-through cell OR, rubber O-ring. (c) Structure of enzymatically coupled FET (electrical insulation with epoxy resin is not shown here for simplicity) ISFET, ion-sensitive FET EM, enzyme membrane G, thin gold film TC, card edge connector. (Reproduced from Shiono et al. (9), with permission.)...

Fig. 24. Experimental setup of a blood glucose monitoring system. ISFET, ion-sensitive FET. (Reproduced from Ito et al. (52), with permission.)...

Nakako M, Hanazato Y, Maeda M and Shiono S 1986 Neutral lipid enzyme sensors based on ion-sensitive FET Ana/. Chim. Acta 185 179-85... 

In field-effect transistors (FET) a potential is applied via metal contacts between two -type semiconductor areas - called source and drain - in a bulk of otherwise /7-type semiconductor material. A metal layer - called the gate - in contact with a thin insulating layer placed on top of the semiconductor (between source and drain) forms a metal/insulator/semiconductor (MIS) capacitor. If the gate is charged, the semiconductor region below the insulator is influenced by the electric field. The electric field thus affects the current flowing between source and drain. In ion-sensitive FETs (iSFETs) the metal layer on top of the insulating layer is replaced by an ion-sensitive material. This ion-sensitive layer is in contact with the analyte solution, and a reference electrode is placed close to it. 

Another approach to eliminate the inner filling solution of conventional ISEs was introduced also in the 1970s and is based on the use of field effect transistors (FETs). These devices are referred to as ISFETs, that is, ion-sensitive FETs, and belong together with enzyme FETs (EnFETs) and gas sensitive FETs to the larger category of ChemFETs (chemically sensitive FETs). In the case of the ISFET, the ISE membrane is applied to the Si3N4... 

GASFET Gas sensitive FET, e.g. with palladium as H2 sensitive receptor layer CHEMFET CHEMically sensitive FET ISFET Ion Sensitive FET, ion sensitive FET IMFET Immunologic FET. Immunologic reaction... 

Second, a diamond surface with NEA exhibits a novel doping mechanism that relies on the low ionization energy of diamond with NEA. This transfer doping mechanism yields a subsurface hole accumulation layer and a concomitant high surface conductivity (SC) that is discussed in volume 6. Field effect transistors (FETs) based on this kind of SC have been built [33-35], and attempts are being made to exploit the sensitivity of SC in diamond for ion-sensitive electronic devices such as ion-sensitive FETs (ISFETs) [36, 37]. 

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