3 - Structures

The structure and operation principles of impedance-type sensors are the most simple. Animations show some possible device structures: the sheet capacitor and the film resistor, respectively. Measurand changes alter the permittivity and/or resistivity of the sensitive layer, which, in turn, produces the variation of capacitance and/or resistance values of the sensor. There are also generator type effects that can be applied to produce voltage output at sheet capacitors; the basis is the polarization or surface charge density variations within the dielectrics (e.g., as a result of the piezoelectric or pyroelectric effect.) The interdigital device structure is rarely used in integrated devices since only small capacitance and resistance values can be realized. It is, however, a very popular structure in the sensorics because of its large free surface, which can interact with the environment. The mathematical expressions for the design are here much more complicated. The capacitance value is determined not only by the geometry and the permittivity of the dielectric layer, but also by the permittivity of the substrate and the gas or liquid environment. This interdigital structure can also be used as a resistor. At AC investigations of the interdigital structure, both capacitive and resistive behaviors must be taken into account. The equivalent circuit model is shown, including interface impedance as well. In the model, Rb is the bulk resistance of the film; Re and Ce are the dielectrics/electrode interface resistance and capacitance values, respectively; and Cg is the geometric capacitance, which consists of the bulk dielectrics capacitance and the stray capacitance. The parameters can be measured experimentally by the complex impedance spectra method using the so-called Cole-Cole plots. The animation also shows a typical complex impedance diagram for interdigital structures using layers with relatively low permittivity and large resistivity. If Cg-Rb and Ce-Re pairs are dominating different frequency ranges, Rb and Cg can be determined simply from the complex impedance spectra, as it is shown.