Field-effect transistor sensor configuration

There are three major classes of palladium-based hydrogen sensors [4], The most popular class of palladium-based sensors is based on palladium resistors. A thin film of palladium deposited between two metal contacts shows a change in conductivity on exposure to hydrogen due to the phase transition in palladium. The palladium field-effect transistors (FETs) or capacitors constitute the second class, wherein the sensor architecture is in a transistor mode or capacitor configuration. The third class of palladium sensors includes optical sensors consisting of a layer of palladium coated on an optically active material that transforms the hydrogen concentration to an optical signal. 

Volume has been the most significant limitation on the size and construction of microreference electrodes, a limitation that complements the small size of the microfabricated ion sensors (Section 6.23.2). There have been many attempts to prepare a liquid junction free microreference electrode that would be comparable in size with the integrated ion sensors, such as ion-sensitive field-effect transistors (Section 6.23.2). These attempts have followed broadly three tines of reasoning scaling down of a macroscopic reference electrode (Comte and Janata, 1978 Smith and Scott, 1986), elimination of the reference solution compartment while preserving the internal element structure (e.g., Ag/AgCl), and utilization of inert materials such as polyfluorinated hydrocarbons and the tike, particularly in the so-called reference FET configuration. 

Carbon nanotubes, especially SWNTs, with their fascinating electrical properties, dimensional proximity to biomacromolecules (e.g., DNA of 1 nm in size), and high sensitivity to surrounding environments, are ideal components in biosensors not only as electrodes for signal transmission but also as detectors for sensing biomolecules and biospecies. In terms of configuration and detection mechanism, biosensors based on carbon nanotubes may be divided into two categories electrochemical sensors and field effect transistor (FET) sensors. Since a number of recent reviews on the former have been published,6,62,63 our focus here is mostly on FET sensors. 

From the above discussion, it is evident that BR is a bifunctional electronic material [64] it is sensitive to light as well as to ions such as H+, Cl, and Ca. In the motion detector developed by Miyasaka etal, BR is configured as a photon sensor. In the cyclic-GMP cascade, a photon, via its action on rhodopsin, triggers the hydrolysis of cyclic-GMP, and thus in turn regulates the release of energy stored as a Na+ gradient. The hypothetical trigger mechanism based on the surface potential thus works like a field effect transistor (FET), or more precisely, a phototransistor. 

Figure 14.7 Four representative configurations of carbon nanotube sensors (a) field-effect transistor with a single single-walled carbon nanotube conduction channel (b) carbon nanotube film resistive sensor (c) carbon nanotube network field-effect transistor (d) entangled vertical carbon nanotube film as a resistive sensor.

CNT-based gas sensors have shown good electrical response as well. It was established that chemiresistors and chemical field-effect transistors are probably the most promising types of gas sensors based on CNTs (Kong et al. 2000 Bondavalli et al. 2009 Wang and Yeow 2009 Zhang and Zhang 2009 Hu, et al. 2010). Typical configuration of such sensors is shown in Fig. 1.10. It has to be pointed out that for this kind of sensor the research has essentially focused on SWCNTs, because MWCNTs are only metallic and therefore unsuitable to fabricate chemiresistors and transistors. 

With some reservations, CNTs-based FET sensors with bottom-gate configuration can also be attributed to TFT sensors. Carbon nanotubes (CNTs) are one-dimensional molecular structures, which demonstrate very high carrier mobility in field-effect transistors (Avouris et al. 2007). Carbon-based gas sensors will be discussed in detail in Chap. 1 (Vol. 2). In this chapter we will briefly analyze the prospects for CNTs in TFT-based gas sensor design. 

Figure 2 Different configurations for electrical sensors (a) electrochemical sensor (b) chemiresistor (c) organic field-effect transistor and (d) organic electrochemical transistor.

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