Capacitor-Type Gas Sensors

In general, the semiconductor is silicon and the oxide is silicon dioxide. The potential on the gate, which is the metal, can be varied to control the charge distribution at the oxide-semiconductor interface. The catalytic gate MOSFET device differs from an ordinary MOSFET in that the gate is metalized with palladium, Pd, 

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

FIGURE 1.3 A Pd-gate MOSFET-type (a) Structure (b) Detection principle (c) curve shift. 

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The capacitor-type gas sensors are one type of the field-effect devices. The field-effect devices can be classified into two types metal-oxide-semiconductor (MOS) capacitors and transistors (MOSFETs), as shown in Figures 1.2 and 1.3, respectively... 

Classical work-function-type gas sensors are specific devices based on the Kelvin method. The Kelvin method ( vibrating capacitor/condenser method, capacitive probe method ), using a so-called Kelvin probe (KP) or Kelvin oscillator, is an established tool for measuring the work function of a sample or, more precisely, the contact potential between the sample and a reference. 

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. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

Alberti K etal 1991 Zeolite coaled interdigital capacitors as a new type of gas sensor Catal. Today 8 509-13... 

Ishihara T., Kometani K., Hashida M., and Yakita Y, Mixed oxide capacitor of BaTiOj-PbO as a new type CO2 gas sensor, Chem. Lett., 18, 1163-1166, 1990. 

Ishihara,T., Kometani, K., Mizuhara, Y. and Takita, Y. (1992), Mixed Oxide Capacitor of CuO-BaTiOs as a New Type CO2 Gas Sensor , Journal of the American Ceramic Society, 75,3,613-18. 

Calorimetric-type gas flow sensors analyze the temperature distribution built up in the environment around a central heater element [2]. Further explanation of its functionality is discussed in the next section. Using thermal electric circuits, thermal behavior of the sensor s stmcture is determined. A number of resistors and capacitors are used in the circuit to function as heat transfer agents. Flquation 11 is modeled for convective resistance and Eq. 12 for thermal capacitance where A is the heater area. The flow velocity can be obtained from Eq. 12. 

Ishihara T, Kometani K, Mizuhara Y, Takita Y (1992b) Mixed oxide capacitor of CuO—BaTiO as a new type CO gas sensor. J Am Ceram Soc 75 613-618... 

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. 

Barium titanate (BaTiOj), a perovskite-type electro-ceramic material, has been extensively studied and utilized due to its dielectric and ferroelectric properties. The wide applications of barium titanates include multiplayer capacitors in electronic circuits, nonlinear resistors, thermal switches, passive memory storage devices, and transducers. In addition, barium titanate can be used for chemical sensors due to its surface sensivity to gas adsorption. 

Since the adsorption of a gas is able to modify the dielectric constant of zeolites, chemical sensors based on interdigital capacitors (IDCs) using zeolites layers as sensitive coatings offer a wide field of applications depending on the type, modification, and working temperature of the coated IDC sensor. 

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