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

Abstract

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sensing film. Two improved readout circuit of 0.5 μm 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> , V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ref</sub> and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pH</sub> . Relationship between I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ref</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> ) and equivalent voltage pH (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pH</sub> ) values.