Effects of active layer thickness on performance and stability of dual-active-layer amorphous InGaZnO thin-film transistors

Dual-active-layer (DAL) amorphous InGaZnO (IGZO) thin-film transistors (TFTs) are fabricated at low temperature without post-annealing. A bottom low-resistance (low-R) IGZO layer and a top high-resistance (high-R) IGZO layer constitute the DAL homojunction with smooth and high-quality interface by in situ modulation of oxygen composition. The performance of the DAL TFT is significantly improved when compared to that of a single-active-layer TFT. A detailed investigation was carried out regarding the effects of the thickness of both layers on the electrical properties and gate bias stress stabilities. It is found that the low-R layer improves the mobility, ON/OFF ratio, threshold voltage and hysteresis voltage by passivating the defects and providing a smooth interface. The high-R IGZO layer has a great impact on the hysteresis, which changes from clockwise to counterclockwise. The best TFT shows a mobility of 5.41 cm²/V.s, a subthreshold swing of 95.0 mV/dec, an ON/OFF ratio of 6.70×10⁷, a threshold voltage of 0.24 V, and a hysteresis voltage of 0.13 V. The value of threshold voltage shifts under positive gate bias stress decreases when increasing the thickness of both layers.