Gate Voltage Regulation of Surface Properties in Polyethylenimine-Doped Indium Oxide Transistors for Enhanced Detection of Low-Concentration NO₂ at Room Temperature

Nitrogen dioxide (NO₂), a toxic environmental pollutant, requires high-performance sensors for ppb-level detection. While indium oxide thin-film transistors (In₂O₃ TFTs) show promise, conventional devices require costly vacuum equipment, unlike solution-processed spin-coating, which is suitable for scalable fabrication. Channel doping enhances gas sensing performance but degrades transistor output current. This work introduces polyethylenimine (PEI) as the electron dopant for solution-processed In₂O₃ TFTs. PEI provides abundant electrons for NO₂ interaction, but the limited current-driving capability of resistive sensors fundamentally restricts their detection sensitivity to such low-amplitude signals. Gate voltage-regulated surface electronic states in thin films significantly enhance the current-driving capability, enabling ultrasensitive NO₂ detection down to subppb concentrations. The 1% PEI-doped In₂O₃ TFT demonstrates a saturation drain current of 0.065 mA, representing a 1.91-fold enhancement over the undoped counterpart (0.034 mA). Furthermore, the optimized 1% PEI: In₂O₃ TFT achieves a 23% response toward 10 ppb NO₂.