Ion-sensitive field effect transistor as a pH sensor.

Area of Science:

• Materials Science
• Electrical Engineering
• Chemistry

Background:

• Ion-Sensitive Field-Effect Transistors (ISFETs) are crucial for chemical sensing.
• Understanding ISFET behavior, particularly threshold voltage drift, is essential for reliable sensor performance.
• Existing models may not fully capture the complex interface phenomena influencing ISFET drift.

Purpose of the Study:

• To model and analyze the static and dynamic behaviors of ISFETs.
• To investigate the mechanisms behind threshold voltage drift in ISFETs.
• To design and simulate a low-noise signal read-out circuit for ISFETs.

Main Methods:

• Modeling ISFETs using a standard NMOS structure and an insulator-electrolyte capacitor.
• Incorporating site-binding theory to describe insulator/electrolyte interface chemistry.
• Utilizing LTspice for circuit simulation and analysis.

Main Results:

• Threshold voltage drift simulation requires two exponential terms to capture initial and long-term effects.
• Slow responding sites and hydration effects are proposed as key factors in threshold voltage drift.
• A low-noise differential signal read-out circuit was designed and simulated.

Conclusions:

• The proposed model, incorporating site-binding theory and considering slow sites and hydration, provides a more comprehensive understanding of ISFET threshold voltage drift.
• The designed read-out circuit demonstrates effective signal processing for ISFETs, with a significant output voltage change across a wide pH range (0-12).
• This research contributes to the development of more stable and accurate ISFET-based sensors.