Anomalous Kink Effect Induced by Bottom-Shield-Metal in LTPS TFTs on Plastic Substrates

Abstract

The anomalous kink effect was investigated in n-type low-temperature polycrystalline silicon thin-film transistors with the source-contacted bottom-shield-metal (SBSM) layer on a polyimide substrate. Using experiments and technology computer-aided design simulation, it was found that the SBSM layer plays a critical role in the anomalous kink effect. The SBSM layer modulated the carrier concentration within the lightly doped drain (LDD) region according to the applied drain voltage ( $\text{V} _{\text{DS}}$ ). In addition, this carrier modulation caused changes in the lateral electric field at simultaneously the channel/LDD junction and the LDD/n+ junction. Thus, multiple kink effects, different from the conventional kink effect, occurred in two different $\text{V} _{\text{DS}}$ regions with different slopes depending on the LDD length ( $\text{L}_{\text{LDD}}$ ) and the overlap length ( $\text{L}_{\text{BSM}}$ ) between the LDD region and the SBSM layer in the drain region. We propose a novel asymmetric design to suppress the SBSM-induced anomalous kink effect in all $\text{V}_{\text{DS}}$ regimes. This design has the following requirements with respect to $\text{L}_{\text {LDD}}$ and $\text{L}_{\text {BSM}}$ at the drain region: $\text{L}_{\text {BSM}}$ greater than $1.0~\mu \text{m}$ , $\text{L}_{\text {LDD}} - \text{L}_{\text {BSM}}$ (LLDD) greater than $1.0~\mu \text{m}$ , and $\text{L}_{\text {BSM}}$ equal to half of $\text{L}_{\text {LDD}}$ .