Research Into ISFET Microsensor Readout Circuitry For Biochemical SOC Based On Standard CMOS Technology

Biosensor SOC can be widely used in the areas such as life science, biomedical engineering, food industry, health care, environment monitoring, and so on. Because of its importance, the topic is attracting tremendous research effort today. However, the realization of the biosensor SOC still faces with many challenges. This dissertation presents a thorough study on a monolithic integration of ISFET sensors and readout electronics, aiming at obtaining proper signal condition for further A/D conversion.In this dissertation, the operation principle of the ISFET has been studied and the "floating gate" structure for the ISFET is described. The Ta₂O₅ passivation layer deposited on the gate acts as the pH-sensitive material. Together, a SPICE model of the ISFET is built for circuit simulation.Emphasis of this dissertation is placed on the research into the sensor readout system containing ISFET/REFET (reference FET) pair and the buffer amplifiers as well as a reference electrode structure based on CMOS technology. When the readout system is in operation, two amplifiers are employed to each detect ISFET and REFET signals. The difference of the two signals will be processed through a voltage to current converter that is implemented with a novel circuit structure. This converter circuit is suitable for impedance matching between the cascading stages of the CMOS readout system and can deliver large dynamic range. Therefore, the processed signal should be easily taken by the A/D converter.Usually the input offset voltage of a practical operational amplifier is very large (typically in the order of magnitude of 1-10mV). This dissertation presents an auto-zero stabilization technique that is for the first time used to develop the high precision op-amp for ISFET's signal detection. This technique can significantly reduce the offset, temperature drift and low frequency noise of the op-amp. The op-amp supplys constant bias conditions for the ISFET sensor which can work in either linear or saturation region by programmable control. The amplifier maintains a DC open loop gain of more than 140 dB and an offset voltage of less than 10 uV.