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Metal Oxide sensitive layers are typically made of SnO 2 doped with Pt or Pd.
These sensors can operate at high temperatures (300-5,000°C), which makes them
especially suitable for combustion gases.
Conductive Polymer sensitive layers are usually based on pyrrole, aniline, or thio-
phene. These sensors operate best at room temperatures. Compared to Metal Oxide
sensors these sensors have lower power consumption, and faster response and recov-
ery times. However, they are have lower sensitivity and are sensitive to humidity.
Optical Chemical Sensors
In optical sensors, an optical waveguide is used as the sensitive layer. Chemical reac-
tions between the waveguide and the target chemical substance cause a change in the
optical properties of the waveguide (e.g., the index of reflection). As a result the
amount (or the wavelength) of the light striking the sensor on the end of the wave-
guide varies.
These sensors are highly sensitive, can handle small quantities, are inexpensive,
and easy to sterilize.
Majority (about 60%) of chemical sensors are gas sensors. Most commonly used
chemical sensors include O 2 , pH, CO, CO 2 , NO X , Methane, etc. Table 2.1 gives an
overview of the range of characteristics for some of these sensors available on the
Ion-Sensitive FET Sensor
An ion-sensitive field effect transistor (ISFET) is an ion-sensitive field effect tran-
sistor used to measure ion concentrations in solution; when the ion concentration
(such as pH) changes, the current through the transistor will change accordingly.
Here, the solution is used as the gate electrode. A voltage between substrate and
oxide surfaces arises due to an ions sheath.
An ISFET's source and drain are constructed as for a MOSFET. The gate
electrode is separated from the channel by a barrier which is sensitive to hydrogen
ions and a gap to allow the substance under test to come in contact with the sensi-
tive barrier. An ISFET's threshold voltage depends on the pH of the substance in
contact with its ion-sensitive barrier.
Table 2.1 Overview of important characteristics for some chemical sensors
Sensor type
Response time
Temperature range
4 s
0-150 mm-Hg
20 s
0-5,000 ppvmol
NO x
<40 s
0-100 ppm
−20°C to 50°C
H 2 S
<60 s
0-200 ppm
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