Nanotechnology-Enabled Sensors

116 Chapter 3: Transduction Platforms
The first FET sensors were developed in early 1970 at the Stanford University
by Wise et al. 27 Fig. 3.41 shows one of the original design drawings.
25 They used the device for the measurement of pH. As the device is
very sensitive to the presence of ions in the analyte, for measuring the redox
interactions on the sensitive layer a reference electrode is also needed.
A MOSFET incorporating a reference electrode is called an ion-sensitive
field effect transistor (ISFET). 26 Hence, the incorporation of a reference
electrode is deemed necessary to make an electrochemically sensitive
device.
Fig. 3.41 One of the ISFETs original design drawings. Reprinted with permission
from Elsevier publications. 25,27
For ISFET the equation for the threshold voltage changes to: 25,26
φ Q + Q + Q
= , (3.82)
sol s ox ss b
Vt Eref
−ψ
+ χ + −
+ 2φ
f
q Cox
There are two new parameters that can be seen in this equation: the interfacial
potential, Eref, and the potential at the solution/oxide interface, and
ψ+χ sol . The term ψ+χ sol consists of ψ, which is a function of the analyte’s