Ion-Sensitive Sensor System Based On Organic Field-Effect Transistor

Since the Ion-Sensitive Field-Effect Transistor(ISFET) was introduced by Piet Bergveld in the 1970's,it has gained great attention from domestic and foreign scholars and has been applied in many detecting fields.Evolving from the silicon FET,the ISFET technology is used for measuring ion concentration in solution and has thus been widely used in bio-/chemical sensing.The ISFET based bio-/chemical can be small in size,easy for integration,maintainance and use,and of low requirements for the testing environment.Therefore,this technology is used in many fields,such as food quality inspection,environmental monitoring,medical diagnosis and chemical analysis.Compared with the silicon FET,despite of poorer device mobility and lower integration density,the organic field-effect transistor(OFET) has competitive advantages for sensor applications,including compatibility with solution based or printing processes,convenience for fabrication on various flexible substrates(e.g.plastic,paper and fabric,etc.),low cost and ease of integration with various functional sensing materials being fabricted on.In recent years,great progress has been made on OFET of reducing operation voltage and improving stability,which has paved the way for integration of ion-sensitive organic field-effect transistor(ISOFET) with low-voltage silicon chips.However,the optimization of a single ISOFET material and performance has been widedly reported,there are several challenges to be addressed for developing ISOFET based sensing systems,such as lack of detailed analysis of the sensitivity of the system,the integration processes,and the calibration approach for the systems.To address these challenges,this thesis has carried out the following studies to realize fabrication of the ISOFET on plastic substrate(including the OFET,the working and reference electrode parts),its integration with low voltage silicon chips,and develop the analytical model for the sensitivity of the system and the calibration approach.1)A model was developed for analysis of the sensitivity of the ISFET sensing system.Its accuracy and universality was verified through electrical characterization of the fabricated OFETs of different subthreshold swing values,and a commercial silicon FET.This model and experimental results prove that the reducing the subthrehold swing of the FET device can improve the detection sensitivity of the entire ISFET sensing system.This part of work would provide the theoretical base for designing and implementing high sensivity ISOFET sensing systems.2)An approach of fabricating all-solid-state flexible reference electrode for ISFET system integration was developed.A non-toxic and low cost polymer was deposited on the Ag/AgCl surface to form nanoscale porous structures for improved stability.The ISFET system presented excellent stability and repeatability for pH measuring.Such an all-solid-state referene electrode would provide a technology basis for developing ISOFET sensing systems.3)A portable signal readout and control system with a calibration approach was developed.Such a calibration approach was shown to be able to reduce the measurement errors caused by non-uniformity of the devices in the system.This system and the calibration approach was verified with pH test experiments,and would provide software/hardware basis for developing ISOFET sensing systems.4)The fabrication of ISOFET(including low-voltage OFET,solid-state work and reference electrode) on plastic substrates,and its integration with silicon-based chips was achieved;combining with calibration approach to achieve reliable and accurate pH sensing.This technology would provide an integrated technology foundation for building ISOFET sensing systems.The research in this thesis has established the theoretical and technical basis for the design and implementation of a high sensitive and reliable ISOFET sensing system for bio-/chemical detection.