Nanotechnology-Enabled Sensors

3.5 Solid State Transducers 115
and source and drain and source, respectively. Furthermore, the threshold
voltage, Vt, is defined as: 25,26
φ −φ
Q + Q + Q
= , (3.81)
m s ox ss b
Vt −
+ 2φ
f
q Cox
where φm and φs are the metal and semiconductor workfunctions respectively.
Qox Qss and Qb are the accumulated charges in the oxide, oxide/
semiconductor interface, and the depletion charge in the semiconductor,
respectively. Finally, the last term, φf, depends on the doping level of the
semiconducting material. As a consequence Vt being dependent on charge
and capacitance, a FETs I-V characteristics are strongly affected by
changes in humidity, ions, chemical species and the presence of dielectric
materials in the environment.
FETs may be implemented for applications in liquid media, as shown in
Fig. 3.40, in which the metal gate is replaced with a reference electrode in
a liquid touching the gate metal oxide layer. Insulating layers are added to
isolate the connections to the drain and source from the liquid. A sensitive
layer is deposited over the oxide layer, forming the gate, and as a result of
chemical reactions occurring on this surface when free electrons or holes
in the semiconductor become attracted or repelled to the gate surface.
Fig. 3.40 A schematic representation of an ISFET.