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Self-Heating Stress-Induced Severe Humps in Transfer Characteristics of Amorphous InGaZnO Thin-Film Transistors

Under a self-heating stress (SHS), amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) would exhibit a severe hump in the transfer characteristics. A model based on state transformation of oxygen vacancies is proposed to explain this phenomenon. The channel region of TFT is considerably self-heated if a large current flows through due to the poor thermal conductivity of a-IGZO. The temperature in the channel region can be raised with the high-power SHS so high that the oxygen vacancies there perform a state transformation from deep-donors and/or traps to shallow-donors, making the carrier concentration increase remarkably in the a-IGZO channel layer. The temperature is highest in the center of channel region and thus the state transformation takes place there first, leading to the carrier concentration increasing there first. As a result, the TFT has a lower turn-on voltage in the central channel region than in the rest, bringing finally about the hump in the transfer curves. The model is well verified by annealing the a-IGZO, which shows that the carrier concentration surely increases greatly when the annealing temperature is over 300 °C.