Improved AC Drain Bias Stability in In–Zn–O TFTs with an Hf-Doped Heterobilayer Structure

Metal oxide semiconductors are widely used in display technologies due to their high electron mobility, low leakage current, and robust switching characteristics. However, ensuring stability under AC bias stress, which is an inherent condition for practical device operation, remains a critical challenge. In particular, hot carrier effects (HCE) have been identified as a key mechanism for device instability under AC bias stress, as they induce oxygen vacancies (V_O) and acceptor-like defect states. In this study, we selected IZO, a material with excellent properties but weak M–O bonds, to improve AC stability. To address this challenge, we propose a heterobilayer channel thin-film transistor (TFT) consisting of an indium–zinc oxide (IZO) bottom layer and a Hf-doped IZO top layer as a solution to enhance AC bias stability as well as electron mobility. The Hf-doped IZO top layer forms strong bonds with oxygen, effectively reducing oxygen vacancies and V_O-related defect states, while inhibiting excessive hot carrier accumulation near the drain electrode. Meanwhile, the IZO bottom layer provides an abundance of oxygen vacancies, contributing to enhanced mobility. The fabricated TFT with the IZO:Hf/IZO bilayer channel exhibits a mobility of 7.3 cm²/(V s) and an I_on degradation rate of only 4% after 1000 s, demonstrating excellent device stability under AC drain bias stress. In addition, negligible hysteresis and excellent reproducibility were also achieved even under AC bias conditions. After stress, the threshold voltage shift was only 0.11 V, with a current on/off ratio of 1.8 × 10⁶ and a subthreshold swing of 512 mV/dec. TCAD simulations further validated the heterobilayer structure in improving stability under AC drain bias stress by demonstrating its effectiveness in suppressing defect generation.