Channel-Shortening Effect Suppression of a High-Mobility Self-Aligned Oxide TFT Using Trench Structure

Channel-shortening effect (CSE) in oxide thin film transistors (TFTs) is a crucial issue that must be resolved for applications in ultra-high-resolution displays. One of the origins of the CSE is the diffusion of a shallow donor such as hydrogen from other layers into the channel. In this study, we investigated for the first time the CSE of self-aligned Al-doped In-Sn-Zn-O (Al-ITZO) TFTs with planar and trench structures. The TFTs with planar structures exhibited severe negative $\text{V}_{ {{ON}}}$ shifts after an annealing process, whereas the TFTs with trench structures were barely affected, thereby exhibiting excellent ON/ OFF characteristics. The vertical channel in the trench TFT had higher resistance than the horizontal channel because of a back-sidewall roughness and thin channel. The high resistance of vertical channels played a significant role in determining the ON/ OFF characteristics of Al-ITZO TFT, where $\text{V}_{ {{ON}}}$ remained constant until the diffused shallow donors made the resistive vertical channels become conductive. Based on these unique operation characteristics, the suppression of CSE in a trench TFT was demonstrated even under a high annealing temperature. Trench TFT exhibited higher mobility, higher drain currents, and higher stability than planar TFT, thus making it suitable for ultra-high-resolution displays.