The Role of Electrode Position in Gas Sensing and Charge Transport in ZnO Thin Films

Abstract

The influence of electrode positioning on the gas-sensing and charge-transport properties of ZnO thin films remains insufficiently explored despite its importance in resistive sensor design. Here, ZnO thin-film NH3 sensors incorporating top-, bottom-, and hybrid-interdigitated electrode configurations (T-IDT, B-IDT, and H-IDT) were systematically compared under identical synthesis, device geometry, and measurement conditions. Sol–gel spin-coated ZnO films exhibited a polycrystalline hexagonal wurtzite structure, homogeneous elemental distribution, and continuous morphology. Electrical measurements revealed approximately linear I–V characteristics, while equivalent-circuit analysis of the impedance spectra demonstrated configuration-dependent resistive and relaxation behavior, with B-IDT exhibiting the most complex electrical response. Gas-sensing measurements toward 50 ppm NH3 showed a strong dependence on electrode position, with responses at 230°C reaching 163%, 147%, and 106% for B-IDT, H-IDT, and T-IDT, respectively. Quantitative t90 analysis further revealed that the response/recovery dynamics depend on both electrode configuration and operating temperature, indicating that no single electrode configuration consistently provides the fastest dynamic response under all conditions. Repeated exposure–recovery measurements demonstrated stabilization of the sensing response after initial cycling, while concentration-dependent measurements over 5–200 ppm NH3 produced systematic response increases for all configurations. These findings demonstrate that electrode positioning governs how gas-induced conductivity changes are electrically sampled within the same ZnO sensing layer, producing a practical trade-off between response magnitude, resistance, and dynamic behavior. Electrode positioning therefore represents a simple device-level design parameter for tailoring the performance of resistive metal-oxide gas sensors.