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The principle of an ion-selective solid-state device based on a Metal-Oxide Field Effect Transistor (MOSFET) was first developed by P. Bergveld (“Development of an Ion-Sensitive Solid-State Device for Neurophysiological Measurements”, Bergveld P., 1970). As was already known from MOSFETs the conductance within the transistor channel is only present when the applied gate voltage is greater than the threshold voltage:

(1) VG > VTh

The threshold voltage however is influenced by a double layer at the oxide-silicon junction caused by the built-in oxid charge QSS. The resulting value of VTh is often negative and therefore a metallic gate with an applied gate potential is not necessarily needed to form a conductive channel. Instead a MOSFET where the function of the gate junction is basically replaced by an aqueous solution is employed. Thereby a second double layer is formed between the solution and the oxide that interacts with the first one. At this second double layer the potential UEG(pH) occurs, which is dependant on the concentration of OH- ions and therewith the pH value of the solution.

To obtain the value of UEG(pH) the voltage UD and the current IDS are kept at a constant value to define a working point, while the voltage US=URS is measured.

As one can easily see Kirchhoff's law suggests the following relation between the voltages shown in Fig.1:

(2) US = URE + UEG(pH) + UGS

As the system rather detects relative values than absolute ones, this leads to:

(3) ΔUS = ΔURE + ΔUEG(pH) + ΔUGS

In equation (3) ΔUGS = 0 as UD and ID are kept at a constant value. Moreover the point of using a reference electrode is that ΔURE = 0.
Thus the following relation remains:

(4) ΔUS = ΔUEG(pH)

This means while ΔUEG(pH) can't be measured directly, it can still easily be obtained by measuring ΔUS and bearing in mind above equations. Accordingly any changes in the pH value are reflected in a change of the measured voltage US.