Light and temperature effect on investigation of ion-sensitive field effect transistor (ISFET) sensors / Nurul Izzati Mohammad Noh

Ion-Sensitive Field Effect Transistor (ISFET) is a potentiometric device and most wellknown as a pH measurement sensor. Nowadays, ISFET applications are widely used in those applications such as in biomedical, biochemical, agriculture and environmental monitoring. In viewing the capability of ISFET, there are two limitation factors of ISFET due to the open-gate configuration structure which are light exposure and temperature effect The layout design modification and characterization on the ISFET performance under wafer level and existing/commercial sensor are carried out in reducing the light effect. Several of metal shield design, passivation layer and isolation technique were designed using L – Edit Tanner Tools V12.6 software. The ISFET device is fabricated in the Fabrication Clean Room at MIMOS Berhad using standard CMOS 1 μm fabrication of MIMOS technology. In order to accomplish the reliable characterization on wafer level test, the semiconductor characterization system consists of HP 4145B Semiconductor Parameter Analyzer, Macromanipulator prober and source measure unit (SMU) were utilized. In testing for the analysis of the commercial sensor, three types of standard buffer solution: pH 4, pH 7 and pH 10 and reference electrode were used to analyse the ISFET performance during the experiment. In investigating the temperature effect on the ISFET performance, the effect of channel width-to-length (W/L) ratio on MOSFETISFET structures was investigated from simulation and experimental approach. A metaloxide- semiconductor field-effect-transistor (MOSFET) has been adopted to investigate the isothermal point of an ion-sensitive FET (ISFET), which is needed to suit the readout interfacing circuit of an ISFET sensor. The MOSFET structure with different W/L ratio has been characterized in order to see the effect of W/L ratio to the isothermal point. The Keithley 236 Parameter Analyzer and Semi-auto prober micromanipulator system were used to measure the drain-source current (IDS) versus gate to source voltage (VGS) curves at various temperatures from 30 °C to 60 °C. The simulation result showed that the reduction of W/L ratio can decrease the isothermal point and this was proven by the actual measurement. An investigation towards the responding of light and temperature effect was continued on the modification of ISFET to EGFET sensor using TiO2 sensing membrane. From the modifying sensor, EGFET has shown a good performance in reducing the light and temperature effect.