Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 162

(di-electrometers) rely on changes in the dielectric properties of a sensing, material
upon analyte exposure.
Two effects change the capacitance of, e.g., a polymeric sensitive layer upon
absorption of an analyte: swelling and change of the dielectric constant due to
incorporation of the analyte molecules into the polymer matrix. Interdigitated
electrode structures are predominantly used for capacitance measurements. The
sensor devices generally are operated at a frequency of a few kHz up to 500 kHz.
Because of the nominal capacitance of microstructured capacitors (of the order of
1 pF) and the expected capacitance changes are in the range of some attoFarads, an
integrated solution with on-chip circuitry is usually required.
Chemocapacitor sensors principle is based on the two steady states for the
sensitive layer during operation. In the first state, no gaseous analyte molecules are
present in the sampling environment and consequently only air is, therefore,
incorporated into the polymer. As a result, a certain capacitance (C) of the sen-
sitive polymer layer is measured and constitutes the baseline. In the second state,
gaseous analyte molecules are present in the sampling environment. When the
polymer absorb the gaseous analyte, the sensitive polymer layer changes its
electrical (e.g., dielectric constant) and physical properties (e.g., volume V) to
produce deviations (De, DV) from the first state (reference state). The changes in
electrical and physical properties of polymers are the result of reversible incor-
poration of gaseous analyte molecules into the polymer matrix.
Although capacitive sensor responds to capacitance changes, the output signal
is converted to a frequency and this change is delivered as a differential signal
generated using a Sigma-Delta-modulator circuitry between a passive reference
and a polymer-coated sensing capacitor.
6.7.3 Chemotransistor
Field-effect-based transistors, which are the most common electronic components
on current IC logic chips, rely on modulation of the charge carrier density in the
semiconductor surface space-charge region through an electric field upright to the
device surface: an isolated gate-electrode controlled the source-drain current. So
briefly, the field effect transistor (FET) sensor devices are based on the most
common electronic component, a transistor. Also other devices related to the
transistor but more simple are used like a capacitor or a Schottky diode. The use of
a catalytic metal as the gate contact is used to transfer a transistor device to a
sensor. The catalytic gate region of a FET transistor device is interacting with the
gases. Thereby the gate region is charged and the IV characteristics of the tran-
sistor change along the voltage axis. The voltage at a constant current is higher in
oxygen as compared to hydrogen containing ambient. The sensor signal is the
voltage at a constant current, which is possible due to the diode configuration of
the transistor (gate and drain or gate and source are short). In this way, the