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A Threshold Voltage Model for AOS TFTs Considering a Wide Range of Tail-State Density and Degeneration

There have been significant differences in principle electrical parameters between amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) and silicon-based devices for their distinct conduction mechanisms. Additionally, threshold voltage is one of the key parameters in device characterization and modeling. In this work, a threshold voltage model is developed for AOS TFTs considering the various density of exponential tail states below the conduction band, including degenerate conduction. The threshold condition is defined where the density ratio of free carriers to the trapped carriers reaches a critical value depending on the distribution parameters of tail states. The resulting threshold voltage expression is fully analytical and is of clear physical meaning, with simple parameter extraction methods. Numerical and experimental verifications show that this model provides appropriate values of threshold voltage for devices with different sub-gap tail states, which could be a useful method for identifying the threshold voltage of a large variety of AOS TFTs.