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
$\text{L}_{\text {LDD}}$
.