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The O2-FET is quite similar to a usual ISFET (Ion-selective Field Effect Transistor), which is commonly used to detect the pH value of a solution (pH ISFET). To accomplish the goal of measuring the dissolved oxygen concentration alongside the pH value, the gate of the transistor is surrounded by a noble metal electrode (NME, see Sohn BK and Kim CS, 1996).
A voltage UNME is applied between an additional Ag/AgCl reference electrode and the NME. Thus the sensor is capable of detetecting dissolved oxygen and changes in the pH value simultaneously depending on the applied voltage.

principle O2FET 060827_customized2.png (20.02 Kb)

When no voltage is applied to the NME the O2-FET works like a pH ISFET.
However, if a potential of UNME = -600 mV is used, an electro-chemical reaction is triggered, which allows for the detection of dissolved oxygen:

(1) O2 + 2H2O + 4e- -> 4OH-

The resulting current INME is measured which is directly dependant on the amount of dissolved oxygen in the solution (Eminaga Y., 2009).