Detection capacitor sensors

Schalwig et al. have tested the feasibility of using a SiC MOS capacitor sensor containing a contact metal of 40-nm TaSi plus 45-nm Pt to detect NO and HC after the catalytic converter. This was carried out by simulating lean burn engine exhausts [116]. It was observed that the sensor signal increased for NO detection and decreased for HC detection. This could permit this sensor to be used in a sensor array to differentiate these two gases. 

The effect of the presence of water vapor on hydrogen detection at room temperature for C-I-S capacitor sensors is seen in Figure 16. The data show that water vapor tends to reduce the sensitivity to... 

Thus, zeolite-coated IDCs have been tested for sensing n-butane [317] and also, NH3, NO, and CO [318,319] on Na-Y and NaPtY zeolite-based sensors at temperatures high enough to where chemical reactions may also occur (above 200°C). The response time is of the order of seconds and the cross-sensitivity to water is small at high temperatures, at which no water condensation occurs in the zeolite-pore system. Under certain conditions, selectivity of these reactive chemical sensors is remarkable. Thus, the detection of 10 ppm of n-butane with a NaPtY interdigitated capacitor with no response to CO and H2 has been reported [318]. Similarly, Moos et al. [320] described a ZSM-5 based capacitor sensor with on-chip heating for temperatures up to 450°C capable of detecting NH3 with no cross-sensitivity to CO, hydrocarbons, and O2. 

Starting from the analytical description, we can create a first rough design of the sensor system, including the resonant frequencies for drive and detection and the sizes of the sensing element, the electrostatic comb structures, and the detecting capacitors. While this work is done, the physical properties of the technology already have to be considered. 

FIGURE 7 Contactless coupled conductivity detection (C D) is based on two cylindrical metal electrodes, actuator, and pickup electrode, which are placed on the separation capillary. Schematically, it represents a series of a capacitor, an ohmic resistor, and a second capacitor (from Innovative Sensor Technologies GmbH). 

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. 

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. 

Mode 2 devices which rely on a different detection principle are the Kelvin probe sensor and the CHEMFET. In the first case, a vibrating capacitor measures the change of the work function (see Figure 2), while in the second case the interaction is detected in the field-effect transistor mode.29 31... 

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