Design of ROIC with Temperature Compensation Circuitry for ISFET based pH Meter

Abstract

This work presents the design of readout circuitry of ISFET (Ion Sensitive Field Effect Transistor) based pH meter with temperature compensation using 0.18 μm UMC technology. The readout circuit is used for reading out the concentration of H+ ions, which denotes the pH value of solution. Any change in pH directly affects the threshold voltage of ISFET. To measure this change in pH, a gate CIMP (complementary ISFET/MOSFET pair) technique based sensing readout circuitry is designed, which is applicable in CMOS based Microsystems and also eliminates body bias effect. The performance of the pH meter is affected by temperature variation. The ISFET is thermally instable due to semiconductor properties and pH dependency on temperature, which in turn affects the pH reading of the solution at a temperature other than room temperature. In this work, VT extractor based temperature compensation circuitry is designed for compensating the temperature variation effect of ISFET. Since the temperature coefficient of NMOS VT extractor circuit is negative, it is used to cancel out the temperature variation of ISFET. A new architecture of PMOS VT extractor is proposed, which has been used to implement a VT extractors based voltage reference circuit. This voltage reference circuit provides temperature independent biasing to both readout circuit and temperature compensation circuitry for reducing effect of temperature on ISFET characteristics.