Understanding negative-bias-stress-induced instability and hump phenomenon in amorphous In–Ga–Zn–O thin-film transistors: Impact of source/drain contacts and carrier diffusion

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Reliability and stability of oxide thin-film transistors (TFT) are a longstanding unsettled research topic and are critical for practical device applications. This work discussed negative gate bias stress (NBS)-induced instability, including the bias-induced hump phenomenon for amorphous In–Ga–Zn–O (a-IGZO)-TFT. To identify the key factors contributing to NBS instability, we quantitatively evaluated lateral electron diffusion behavior in the a-IGZO channel by transmission line method (TLM) analysis and developed a critical channel length scale model. The effective source/drain (S/D) contact diffusion length (2LD) was extracted, where the lateral carrier distribution (ne) profiles were depicted, showing that nLD was ∼6.5 × 1017 cm−3. The LD was 3%–5% proportional to the maximum extra-carrier spreading length scale (LC) from S/D extension region into the channel, affected >20% of each side of the lateral channel area, and made significant impacts on the TFT stability. This model successfully explained the notable threshold (Vth) roll-off characteristics and mobility variations in the shorter-channel devices. Short-term and long-term NBS stability tests found that short-channel devices with a 5 μm length exhibited large negative Vth shifts accompanied by hump behavior and broadened hysteresis window due to LD and LC diffusion influence. Based on the proposed carrier diffusion model, the NBS-induced TFT instability was attributed to the formation of a highly conductive back-channel layer originating from the electron accumulation by lateral carrier diffusion from S/D region extension. This study provided a quantitative insight into NBS-induced microscopic changes in the a-IGZO channel, contributing to the understanding of bias-instability and the development of a highly stable oxide-TFTs.