Monolithic pH Sensing via Frequency Modulation in 2D NC-ISFET-Based Ring Oscillators

This work presents a frequency-based transduction technique to evaluate the sensing attributes of a two-dimensional (2D) material-based Ion-sensitive field effect transistor (ISFET) for pH sensing applications. The proposed transduction technique utilizes a 2D-ISFET-based ring oscillator and includes the development of a Verilog-A compatible compact model of 2D-ISFET, including the negative capacitance (NC) concept. To highlight the correctness and accuracy of the model and designed oscillator responses, the work presents a rigorous validation against experimental/TCAD data. A three-stage ring oscillator designed using 2D-NCISFET exhibits a frequency sensitivity of 225.9 kHz/pH, a resolution up to 0.013 pH, and sub-1V V_DD operation ability. The performed analysis shows superior sensing attributes of frequency-based transduction using 2D-NCISFET compared to 2D-ISFET-based design, a similar trend observed in conventional voltage-based sensitivity. Presented work also highlights the impact of ferroelectric thickness, oxide thickness and power supply voltage scaling on the sensitivity of the frequency-based transduction technique.