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

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}} (L LDD ) greater than 1.0~\mu \text{m} , and \text{L}_{\text {BSM}} equal to half of \text{L}_{\text {LDD}} .