Capacitor sensors operation

SiC capacitor sensors have demonstrated gas-sensitivity to gases such as hydrogen and hydrocarbons [21, 46, 68] up to a maximum temperature of 1,000°C [1, 68]. Devices that can be operated both as MOS capacitors (reverse bias) and as Schottky diodes at temperatures greater than 490°C have also been demonstrated (see Section 2.4.2) [10]. These devices showed sensitivity to combustible gases such as propane, propylene, and CO and were tested at temperatures up to 640°C. 

Si capacitor sensor response is enhanced by raising the operating temperature. Here data are shown for room temperature and 160°C. 

The sensor operation is based on convective cooling by fluid flow of the sensing capacitor, which is initially heated by a polysilicon microheater which is in close proximity to it. Two variables have been used to measure the flow rate voltage [9] and oscillation frequency [10]. 

Figure 6.196 is an illustration of a DP cell which operates by varying the distance between the plates of two adjacent capacitors (see also Fig. 6.11)(24). The high and low pressure signals from the sensor are applied to ceramic diaphragms to which one plate of each capacitor is attached. The subsequent change in the separation of the capacitor plates produces a variation in capacitance which is detected by incorporating the cell into a capacitance bridge, as described in Section 6.5.3 (Fig. 6.316).

When a periodically changing excitation signal is chosen for the operation of chemiresistors, they can be used to detect changes of capacitance (Fig. 8.1b). Therefore, the dielectrometric sensors rely on the chemical modulation of one or more equivalent circuit capacitors, either through the change of the dielectric constant of the chemically sensitive layer or through the chemical modulation of the interfacial charge. 

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. 

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