Modelling and development of detachable extended-gate field effect transistor (EGFET) for portable pH sensor

This study focused on the development and characterisation of a detachable extended-gate field effect transistor (EGFET) for portable pH sensor, in which the detachable sensing component allows separation between the sensing membrane and the transistor, thereby improving sensor flexibility and device reliability compared to conventional ion-sensitive field-effect transistor (ISFET) models. The research addressed gaps in FET-based sensors SPICE modelling and proposed the EGFET sensor SPICE model. The overall study included EGFET SPICE model development, fabrication of integrated all-solid electrodes (IASE), and design and construction of a portable EGFET sensing system. The EGFET SPICE model was optimized through simulations in LTSPICE XVII, and IASE fabrication incorporated spin-coating and thermal evaporation techniques for TiO₂ sensing electrodes and silver/silver chloride (Ag/AgCl) reference electrodes. The fabricated IASE demonstrated high pH sensitivity (67.8 mV/pH) and linearity (0.985) surpassing traditional ISFET systems. Hysteresis, drift, and repeatability tests confirmed the sensor's stability, while physical analyses (FESEM, XRD, XPS, EDX) validated the material quality. The fabricated IASE achieved consistent performance, enabling low-volume, on-site pH sensing. A portable device was developed, transitioning from circuit on breadboard to printed circuit board (PCB) setups. The EGFET system integrated Constant Voltage Constant Current (CVCC) circuits and microcontroller interfaces, enhancing EGFET portability. Breadboard testing provided initial validation of the working readout circuit, while the PCB setup demonstrated improved sensitivity and linearity compared to those on breadboard. Comparative analyses highlighted the superior stability and adaptability of the PCB-based design for real-world applications. This study underscored the potential of EGFET pH sensors in environmental monitoring, biomedical diagnostics, and analytical chemistry. Key contributions of this study included a comprehensive EGFET SPICE model, optimized all-solid electrodes fabrication, and a portable functioning system. These findings established a foundation for future advancements in sensor miniaturization and integration into diverse applications, driving innovation in precision measurement technologies.