CNT Network Impacts on Electrolyte-Gated Carbon Nanotube Field-Effect Transistors pH Sensors

Carbon nanotube field-effect transistors (CNT-FETs) are promising platforms for electrolyte-gated sensing, although the influence of CNT network density on device performance for pH measurements remains unknown. In this work, CNT-FETs with controlled network densities were fabricated from aqueous CNT solutions by varying the CNT concentration and the deposition time. Increasing CNT density improved conductivity, reducing channel resistance from the GΩ range to ~100 kΩ and increasing the on-current of electrolyte-gated devices from ~10 nA to ~1 µA. The fabricated CNT-FETs operating in a liquid-gated configuration exhibited on/off ratios ranging between 10³ and 10⁵. The minimum subthreshold swing achieved by the CNT-FETs was 77.5 mV/dec for devices employing a low-density CNT network, compared with 105 mV/dec for those incorporating a high-density CNT network. The high-density CNT networks also exhibited reduced electrostatic gate coupling due to charge screening effects. pH measurements in 1XPBS showed that low-density networks achieved the highest sensitivity of 8.9%/pH, whereas high-density networks showed lower sensitivity of 2.1%/pH, despite producing the largest absolute current response of ~70 nA. Medium-density networks provided the best balance between sensitivity, noise, and signal stability. These findings demonstrate that CNT network density is a critical parameter for optimizing electrolyte-gated CNT-FET pH sensors.