Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 164

voltage is not applied, the back-to-back p-n junction between the drain and the
source prevents current flowing. When a positive voltage applied on the gate in
respect to the source, negative carriers provide a conduction channel between the
source and the drain. Vertical as well as the lateral field controlled the current so it
is also known as a field effect transistor.
The microchemo sensor uses the structure of a MOSFET in which the gate is
made of a gas sensitive metal such as Pd as it was first proposed by Lundstrom
et al. in 1975 [ 20 ]. The metals that compose the gates of a transistor are replaced
with catalytic metals or metal alloys (e.g., platinum (Pt), palladium (Pd), iridium
(Ir), alloys, etc.) and then left exposed to air. The interaction of adsorbed gases
alters the surface-charge density and thus changes the potential of the device. The
choice of the operational temperature, the metal used on the gate and varying the
microstructure of the metal help in achieving the MOSFET selectivity.
The general structure of the MOSFET is shown in Fig. 6.19 . Three components
contained in a sensor are: a catalytic metal, an insulator, and a transistor (semi-
conductor). The device basic principle is to allow gaseous compounds to react with
the catalytic metal and produce species that are able to diffuse through the metal
film and absorb onto a metal insulator. Due to the absorption voltage will change
and it will change the current voltage characteristics of the sensor (as shown in
Fig. 6.19 ). Different catalytic metals may be used to vary the sensitivities of the
sensor. When VOCs come in contact with a catalytic metal, it produces a reaction,
the products which diffuse through the gate changing its surface potential. The
voltage shift depends on the gas concentration. The sensitivity and selectivity can
be enhanced by varying the thickness/type of metal catalyst and changing the
operating temperature, usually 100-200 C. They too are susceptible to drift
similar to conductivity sensors.
Advantage:

• MOSFET are robust sensors.
• Low sensitivity to humidity.
• Sensor reproducibility is good.
• Low-cost sensor.

Disadvantage:

• Running temperature can affect the selectivity and sensitivity of sensor.
• To achieve good quality and reproducibility, high-quality manufacturing

expertise is needed.
• Low sensitivity to defined gases especially ammonia and carbon dioxide.
• Baseline drift.
• Low sensitivity to moisture.

Application:
• Food cooking, fermenting, and wine-making processes.
• Ethylene measurement during fruit ripening process.