A Gate Modulated ISFET Array with Ultra-High Resolution for Long-Term Chemical Monitoring

This paper introduces a novel CMOS ISFET array that leverages intrinsic gate leakage for chemical signal modulation, eliminating the need for external control signals. Time-domain processing reduces readout complexity, allowing inverter-based amplifiers and digital counters for instrumentation and quantisation. The front-end ISFET pixel features a complementary structure, with its floating gate periodically refreshed to an adjustable level via a reset switch. This reset mechanism, combined with gate modulation and differentiation between adjacent readouts, mitigates common non-idealities such as trapped charge and drift, enhancing the sensing performance when used for long-term monitoring of chemical reactions. Fabricated in the TSMC 65 nm technology, the designed chip comprises a $32 \times 32$ ISFET array, occupying a die area of $0.40375 \mathrm{~mm}^{2}$. The on-chip TDC achieves an ultra-high quantisation resolution of $0.02 \mu \mathrm{~V} / \mathrm{LSB}$. The simulation results demonstrate the system’s suitability for long-term measurements, achieving a minimum detectable resolution of 0.008 pH, which is the finest compared to the state-of-the-art.