handbook of modern sensors

504 17 Chemical Sensors
(A)
(B)
Fig. 17.4. SnO 2 Wheatstone bridge circuit (A) and its response for different gases (B).
Fig. 17.5. Liquid ChemFET construction and electrical connection.
17.4.2 ChemFET
AchemFET is a chemical field-effect transistor that includes a gas-selective coating or
series of coatings between its transistor gate and the analyte (Fig. 17.5). This chemical
element gives the device a control input that modifies the source–drain conduction
in relationship with selected chemical species. Different materials applied to the gate
react with different chemical species (gases or liquids) and provide differentiation of
species. ChemFETs can be used for detecting H 2 in air, O 2 in blood, some military
nerve gases, NH 3 ,CO 2 , and explosive gases [5].
As in a conventional FET, the chemFET is constructed using thin-film techniques
and commonly employs a p-type silicon body with two n-type silicon diffusion regions
(source and drain). This three-part system is covered with a silicon dioxide
insulator layer separating a final top metal gate electrode above and between the
source and drain. Operation involves applying a voltage, positive with respect to the
silicon to the gate electrode. Electrons are attracted to the surface of the semiconductor
forming a conducting channel between the source and the drain n-regions [6].
In fact, a chemFET is a chemically controlled conductor (resistor). Conductance of
a chemFET is measured by a differential amplifier and is represented by the output