  The ion-sensitive field-effect transistor (ISFET) is an emerging technology
that has received much attention in numerous research areas, including
biochemistry, medicine, and security applications. However, compared to other
types of sensors, the complexity of ISFETs make it more challenging to achieve
a sensitive, fast and repeatable response. Therefore, various readout circuits
have been developed to improve the performance of ISFETs, especially to
eliminate the temperature effect. This paper presents a new approach for a
temperature-independent readout circuit that uses the threshold voltage
differences of an ISFET-MOSFET pair. The Linear Technology Simulation Program
with Integrated Circuit Emphasis (LTspice) is used to analyze the ISFET
performance based on the proposed readout circuit characteristics. A
macro-model is used to model ISFET behavior, including the first-level Spice
model for the MOSFET part and Verilog-A to model the surface potential,
reference electrode, and electrolyte of the ISFET to determine the
relationships between variables.In this way, the behavior of the ISFET is
monitored by the output voltage of the readout circuit based on a change in the
electrolyte's hydrogen potential (pH), determined by the simulation. The
proposed readout circuit has a temperature coefficient of 11.9 $ppm/{\deg}C$
for a temperature range of 0-100 ${\deg}C$ and pH between 1 and 13. The
proposed ISFET readout circuit outperforms other designs in terms of simplicity
and not requiring an additional sensor.
