Sensors and Actuators - Fachbereich Physik der Universität ...

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 picture 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.
Ersatzschaltkreis und Impedanzspektrum einer Interdigitalsschaltung mit Imaginär- (y) und
Realteil (x) des Wechselstromwiderstands (=Impedanz). Die einzelnen Beiträge Re und Ce
ändern sich als Funktion der Umgebungsvariablen. Die Frequenz f nimmt nach rechts hin
ab.
Biosensoren
Der Begriff Biosensor wird für einen Aufbau verwendet, in dem eine biologische
Komponente (z.B. ein Enzym, ein Antikörper oder ein Mikroorganismus) unmittelbar mit
einem Signalwandler (z.B. einer Elektrode, einer optischen Faser, einem Piezokristall oder
einem Transistor) verbunden ist.
227