(A) study on suppression of channel-shortening effect of high-mobility thin film transistor using trench structure

Abstract

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 (H) 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 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 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, resulting in a high mobility and high stability.