Field effect devices

There are two basic types of solid-state chemical sensor (i) potentiometric devices, and (ii) field effect devices, e.g. ion-selective field effect transistors (ISFETs or CHEMFETs). Electrodes of the potentiometric type usually have a metal as the back contact and they also have a high output impedance. Field-effect devices are a variant of the metal oxide field-effect transistor (MOSFET) familiar in electronics, and they have a low output impedance. Hybrid devices attempt to combine the advantages of both. 

A potentiometric sensor is one in which the potential difference developed across the ISM and its interfaces is measured with respect to a reference electrode, and this potential represents directly the concentration of the ion of interest. Most ISMs have a high impedance, and the voltmeter used to measure the potential must be at least of two orders of magnitude higher in impedance. 

The so-called coated wire electrodes (1) are typical examples. A recent review (2) and a book by Freiser (3) give very detailed accounts of the fabrication and range of coated wire sensors studied. Coated wire electrodes are most commonly formed by dipping a copper wire into a polymer solution which, once dry, leaves the metal covered with plastic. The composition of the plastic is often tailored to reproduce compositions which have been used in membrane electrode configurations. A well-documented example is polyvinylchloride (PVC) impregnated with valinomycin (1), which is selective to ions in solution. An extensive collection of ISE literature titles by Moody and Thomas (4) can be used to locate, inter alia, most new devices of this type. 

This structure is of particular interest as it employs a Pt metal wire and it can be easily adapted to the thick film methods Pt metal is added to thick film conductor pastes to aid solderability, thus a Pt-based conductor is very desirable. Most of the electrodes of the kind described above are generally reported to perform adequately , but little information is available on their longevity and operating mechanisms. 

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). 

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Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Chemical and biological sensors (qv) are important appHcations of LB films. In field-effect devices, the tunneling current is a function of the dielectric constant of the organic film (85—90). For example, NO2, an electron acceptor, has been detected by a phthalocyanine (or a porphyrin) LB film. The mechanism of the reaction is a partial oxidation that introduces charge carriers into the film, thus changing its band gap and as a result, its dc-conductivity. Field-effect devices are very sensitive, but not selective. 

Detection of charged macromolecules by means of field-effect devices (FEDs) possibilities and limitations... 

Detection of Charged Macromolecules by Means of Field-Effect Devices (FEDs)... 

Generally, there are a number of ways in which the adsorption and binding of charged macromolecules (in particular, DNA immobilization and hybridization) can affect the electrochemical properties of the analyte-FED interface. In the case of field-effect devices, two basic effects are usually considered ... 

G.. Xuan, J. Kolodzey, V. Kapoor, and G.. Gonye, Characteristics of field-effect devices with gate oxide modification by DNA. Appl. Phys. Lett. 87, 103903-1-3 (2005). 

D. Goncalves, D.M.F. Prazeres, V. Chu, and J.P. Conde, Label-free electronic detection of biomolecules using a-Si H field-effect devices. J. Non-Crystalline Solids 352, 2007-2010 (2006). 

T. Sakata and Y. Miyahara, Detection of DNA recognition events using multi-well field effect devices. Biosens. Bioelectron. 21, 827-832 (2005). 

Greve, D. W. 1998. Field Effect Devices and Applications Prentice Hall, Englewood Cliffs, NJ. 

Echtermeyer TJ, Lemme MC, Bolten J et al (2007) Graphene field-effect devices. Eur Phys J Spec Top 148 19-26... 

Lemme MC, Echtermeyer TJ, Bans M et al (2007) A graphene field-effect device. EEE Electron Device Lett 28 282-284... 

Pierret RE (1990) Field effect devices, vol 4, 2nd edn. Prentice Hall, Reading, MA... 

Since the capacitor, Schottky diode, and transistor all contain an insulating layer under the catalytic metal, they are all referred to in this chapter as field-effect devices. In published literature, the capacitor and diode SiC devices are often referred to as MISiC devices, and the transistor as an MISiC-FET device. 

Figure 2.6 Schematic diagrams of the different field-effect devices described in this chapter. (From [19]. 2003 Springer-Verlag. Reprinted with permission.)...

The gas response of the field-effect devices is determined by the catalytic properties of the contact material, which includes both the catalytic layer and the underlying material. The temperature plays a dominant role in the detection process because the origin of the gas response is found in the chemical reactions that take place on the sensor surface, and it is furthermore also influenced by the mass transport properties of the molecules in the gas phase. This permits arrays of sensors of a common design to be tailor-made for detection of a range of gases and for use in a range of applications... 

Thus, the Pd layer serves multiple purposes its surface catalyzes the dissociation of molecular hydrogen, it selectively forms palladium hydride, and it can be used as the metal gate of the field-effect devices. The scheme in Fig. 6.34 also shows the catalytic reaction involving oxygen. If both oxygen and hydrogen are present, the steady-state response of the Pd IGFET includes the surface-catalyzed oxidation. 

Significant advances have occurred in microfabricated ion sensitive and Pd gated field effect devices and fiber optic, chemically rsnsitive elements. These elements are beginning to find their way into commercial development. Recent advances in these devices are discussed and compared. Pyroelectric sensor devices developed here are reviewed. A discussion of the utility of these devices is presented. 

Engstrom and Carlsson already introduced in 1983 an SLPT [119] for the characterisation of MIS structures, which was extended to chemical gas sensors by Lundstrom et al. [26]. Both SLPT and LAPS base upon the same technique and principle. However, due to the different fields of applications in history, one refers to LAPS for chemical sensors in electrolyte solutions and for biosensors, and the SLPT for gas sensors. A description of the development of a hydrogen sensor based on catalytic field-effect devices including the SLP technique can be found, e.g., in Refs. [120,121]. The SPLT consists of a metal surface as sensitive material which is heated by, for instance, underlying resistive heaters to a specific working-point temperature, and a prober tip replaces the reference electrode (see Fig. 5.10). 

I. Lundstrom, C. Svensson, A. Spetz, H. Sundgren and F. Winquist, From hydrogen sensors to olfactory images—twenty years with catalytic field-effect devices, Sens. Actuators B Chem., 13(1-3) (1993) 16-23. 

I. Lundstrom, Why bother about gas-sensitive field-effect devices Sens. Actuators A Phys., 56(1-2) (1996) 75-82. 

Winquist F, Danielsson B (1990) Semiconducting field effect devices. In Cass AEG (ed) Biosensors-A practical approach. IRL Press, p 171... 

On the basis of the field-effect studies the authors and colleagues at the Royal Signals and Radar Establishment (RSRE), -Malvern, proposed in 1976 the use of a-Si H field-effect devices in the addressing of liquid crystal matrix displays, as an alternative to the thin-film CdSe transistors which had... 

Eriksson, M. Klingvall, R. Lundstrom, I., A combinatorial method for optimization of materials for gas sensitive field-effect devices, In Combinatorial and High-Throughput Discovery and Optimization of Catalysts and Materials, Potyrailo, R. A. Maier, W. F. Eds. CRC Boca Raton, FL, 2006 85-95... 

Filippini, D. Fraigi, L. Aragbn, R. Weimar, U., Thick film Au-gate field-effect devices sensitive to N02, Sens. Actuators. B 2002, 81(2-3), 296-300... 

From numerous results achieved using combinatorial and high-throughput methods, the most successful have been in the areas of molecular imprinting, polymeric compositions, catalytic metals for field-effect devices, and metal oxides for conductometric sensors. In those materials, the desired selectivity and sensitivity have been achieved by the exploration of multidimensional chemical composition and process parameters space at a previously unavailable level of detail at a fraction of time required for conventional one-at-a-time experiments. These new tools provided the opportunity for the more challenging, yet more rewarding explorations that previously were too time consuming to pursue. 

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