The metal gate of the MOSFET of figure 2(a) is replaced by the metal of a reference
electrode, whilst the liquid in which this electrode is present makes contact with the
bare gate insulator (figure 2(b)). Both devices have the same electrical equivalent
circuit, which is symbolised in figure 2(c). Mounting of the chips is of course
different: a MOSFET can be completely encapsulated, whereas for an ISFET source
and drain leads as well as chip edges have to be encapsulated carefully, meanwhile
leaving the gate area open for contact with the liquid.
For both devices the following equation is valid for the non-saturated region (below
pinch-off):