SPICE design and analysis of composite based ISFET readout circuits with Al2O3 sensing film

Readout integrated circuits (RIOC) play a vital role in interpreting meaningful results with high precision for physico-chemical models like Ion-sensitive FET. This paper presents a design and analysis of three composite based readout circuits for ion-sensitive field effect transistor (ISFET) and Al
2
O
3
sensing film. Two improved readout circuit of
$0.5\ \mu\mathrm{m}$
technology have been simulated in LTspice XVII software. A macro model of ISFET has been used in all three circuits to check and test relationship between I
d
and V
d
, V
ref
and V
pH
. Relationship between I
d
and V
ref
has also been checked at different pH values to observe the behavior of ISFET for the FVFCS MOSFET readout circuit. All the three developed circuits have been briefly compared through transfer and output characteristic graphs. Simulation results infer average sensitivity value achieved up-to 51.5 mV/pH for proposed FVFCS MOSFET readout circuit. All the three circuits exhibit a linear variation in drain current (I
d
) and equivalent voltage pH (V
pH
) values.