Capacitor-type structures

In this review the basis for the chemical sensitivity of these devices will be explored and the various device structures used for these sensors will be discussed. A survey of the performance of the diode-type and capacitor-type structures will be presented and a comparison of characteristics of these two classes of solid state gas sensors will be given. 

General Behavior. For the same reasons discussed in the diode performance section, most of the work on capacitor-type C-I-S sensors has focused on structures for detecting hydrogen or hydrogen-bearing gases. Consequently, this section will mainly examine the performance of C-I-S hydrogen sensors however, detection of CO will be discussed. 

FIGURE 1.2 A Pd-gate MOS capacitor-type (a) structure (b) C-Vcurve shift. 

In addition, special capacitor cell structures are designed to meet the requirements of some instrumental measurements (especially in situ techniques) for electrolytes. Here, we describe three types of cells used for in situ Raman, nuclear magnetic resonance (NMR), and small-angle neutron scattering (SANS) characterizations. 

In addition to their potential use as structural composites, these macroscopic assemblies of nanocarbons have shown promise as mechanical sensors [83], artificial muscles [84], capacitors [85], electrical wires [59], battery elements [85], dye-sensitized solar cells [86], transparent conductors [87], etc. What stands out is not only the wide range of properties of these type of materials but also the possibility of engineering them to produce such diverse structures, ranging from transparent films to woven fibers. This versatility derives from their hierarchical structure consisting of multiple nano building blocks that are assembled from bottom to top. 

Field-effect transistors (Appendix C) are miniature cousins of the Kelvin probe. The most common is the insulated gate field-effect transistor. The heart of the insulated gate field-effect transistor is the Metal-Insulator-Semiconductor (MIS) capacitor. Let us form this capacitor from palladium (to be modulated by hydrogen), silicon dioxide (insulator), and p-type silicon (semiconductor), and examine the energy levels in this structure (Fig. 6.32). 

Pd MOS STRUCTURES The Pd MOS device (capacitor and field effect transistor) has been extensively studied as a model chemical sensor system and as a practical element for the detection of hydrogen molecules in a gas. There have been two outstanding reviews of the status of the Pd MOS sensor with primary emphasis on the reactions at the surface (7,8). In this section, the use of the device as a model chemical sensor will be emphasized. As will be seen, the results are applicable not only to the Pd based devices, they also shed light on the operation of chemfet type systems as well. Because of its simplicity and the control that can be exercised in its fabrication, the discussion will focus on the study of the Pd-MOSCAP structure exclusively. The insights gained from these studies are immediately applicable to the more useful Pd-MOSFET. 

It is desirable to have, as a background to a discussion of ceramic capacitors, some knowledge of alternative dielectrics and the capacitor structures used with them. The principal types are described below and their capacitance ranges indicated in Fig. 5.8. 

There are two types of tantalum electrolytic capacitor wet and solid . Both varieties consist of a porous anode made by sintering tantalum powder at 1800 °C in vacuum. In the wet type the porous structure is impregnated with sulphuric acid, anodized to form a thin layer of Ta205 and encapsulated in a tantalum container that also serves as the cathode. The use of sulphuric acid gives a lower e.s.r. than that of the aluminium electrolytic and increases the temperature range within which the unit can be run. In the solid type the liquid... 

The surface after adsorption will be chained with a potential, as in Figure 9.14, so that primary adsorption can be treated in terms of a capacitor model called the Stem model [43]. The other type of adsorption that can occur involves an exchange of ions in the diffuse layer with those of the surface. In the case of ion exchange, the primary ions are chemically bound to the structure of the solid and exchanged between ions in the diffuse double layer. 

In addition to the multilevel metallization and formation of interconnects, anodic processing of A1 was employed for the fabrication of integrated passive components thin film capacitors and inductors.56,57 For example, localized porous-type anodization of A1 films was used to convert 20- am-thick A1 to the dielectric layer of porous AI2O3 and to define metal-dielectric-metal structures.56 The... 

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