Anomalous drain‐induced barrier lowering effect of thin‐film transistors due to capacitive coupling voltage of light‐shield metal

Abstract

The anomalous drain-induced barrier lowering (DIBL) effect of longchannel thin-film transistors (TFTs) with a light shield (LS) is investigated by two-dimensional (2D) device simulation. In long-channel TFTs with a LS, which is long enough to neglect the DIBL effect and the floating body effect, a decrease of threshold voltage (V th) was observed at high drain voltages. The V th lowering is due to the large potential of the LS induced by the high drain voltage, which lowers the height of the source potential barrier. It is found that the LS-induced DIBL effect can be larger as the length of the LS increases or the thickness of the buried oxide decreases.

The effect of a source-contacted light shield on the electrical characteristics of an LTPS TFT

“…In our previous work [5], the electrical characteristics of a conventional TFT without an LS and a TFT with a floating LS were investigated. For the TFT with a floating LS, an anomalous drain-induced barrier lowering (DIBL) effect was observed, although the TFT was a long-channel device.…”

“…However, for a long-channel TFT with a floating LS, the anomalous DIBL effect occurs because the large potential of the LS lowers the height of the source potential barrier. This large potential is induced by the fringing electric field from the drain because the LS is floating [5]. This anomalous DIBL effect increases the off-region leakage current.…”

Anomalous capacitance characteristics of TFTs with LDD structures in the saturation region

“…Therefore, LTPS TFTs have nonuniform electrical characteristics, even when simultaneously manufactured using the same process. Additionally, LTPS TFTs with a floating substrate suffer from a high-drain electric field that causes drain-induced barrier lowering (DIBL) and a floating body effect (FBE) [4,5]. These deleterious effects alter the V th .…”

“…To prevent variation in the V th of LTPS TFTs, the pixel circuit for V th compensation has been thoroughly researched [1,3,4]. However, despite using this circuit, the V gs −V th of the DTFT can differ from the desired voltage because it is affected by the intrinsic capacitances of the LTPS TFTs used in the V th compensation circuit, such as the gate-to-source capacitance (C gs ) and the gate-to-drain capacitance (C gd ) [9][10][11][12][13].…”