Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 156

6.7 Chemo Sensor (Electro-Chemical Sensors)
• Chemoresistor
• Chemocapacitor
• Chemotransistors
A chemical state is determined by the different concentrations, partial pressures,
or activities of particles such as atoms, molecules, ions, or biologically relevant
compounds to be detected in the gas, liquid, or solid phase. The chemical gas
sensor basic principle is based on that analyte molecules come into contact with a
chemically sensitive material which cause a change in the properties of the
material, follow-on in a change in the electrical signal. The most common material
used for gas sensors is the tin-dioxide semiconductor, which is doped in order to
provide selectivity.
Typically, the sensors are contained in an instrument that regulates the flow of
air. Sampling is done in three phases such as baseline, sampling, and recovery
phases. The gas to be analyzed is exposed to the sensor array in the sampling phase
while a reference gas is used during baseline and recovery phases in order to return
the sensor values to the initial state. Valuable information is not only obtained in
the transient and static sensor values in the sampling phase but also obtained in the
dynamic appearance in the recovery phase.
6.7.1 Chemoresistor Sensors
The typical structure of a planar chemoresistor is shown in Fig. 6.11 that employs
constant voltage circuit. The resistance of the chemically sensitive layer changes
with the amount of adsorbed chemical. The resistivity measurement is done either
via a Wheatstone bridge arrangement or by recording the current at an applied
voltage in a direct current (DC) mode or in a low-amplitude, low-frequency
alternating current (AC) mode to avoid electrode polarization.
1. Metal Oxide Semiconductor (MOS)
MOS sensors are used for observing the electrical-resistance changes that occur
when vapors are adsorbed onto a semiconductor surface [ 17 ]. MOS sensor con-
tains a ceramic structure heated by wire and coated with a metal oxide semicon-
ducting film. So usually it is called metal oxide or ceramic gas sensors [ 18 ]. MOS
sensor also label as Taguchi and Figaro because of the inventor and the first
company to manufacture likewise.
In general, sensors are prepared by depositing a thin porous film of a metal-
oxide material (usually tin oxide) onto an electrically heated ceramic pellet and
annealing at high temperatures. The oxygen available in the air adsorbed onto the
sensor surface removes electrons from the conduction band of the semiconductor