Bottom Gate Thin Film Transistor Substrate for Display Devices
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Solution Overview
Problem
Existing thin film transistor (TFT) substrates in display devices face challenges with low mobility due to amorphous silicon active layers and non-uniform threshold voltage when using polysilicon, leading to complex structures and limited grayscale expression due to top gate transistor designs with large S-factor inclinations and reduced storage capacitor areas.
Innovation Solution
A thin film transistor substrate with a bottom gate structure, utilizing a light shielding layer as the gate electrode and a buffer layer as the gate insulating film, which increases storage capacitor area and improves S-factor properties by allowing a larger usable data voltage range and reducing errors in driving current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a top gate structure is used for the driving transistor, then the transistor can be formed with conventional processes, but the S-factor inclination becomes large resulting in limited grayscale expression
Solution Approach 1:
The patent inverts the conventional top gate structure to a bottom gate structure, where the gate electrode is positioned below the active layer instead of above it. This inversion resolves the S-factor inclination problem by fundamentally changing the electric field distribution and transistor characteristics, enabling superior grayscale expression while remaining compatible with existing manufacturing processes.
2Reliability
If contact holes are prepared in the source/drain metal thin film for the source node, then electrical connection is achieved, but the structure becomes complicated and storage capacitor area is reduced
Solution Approach 1:
The patent merges the source node formation with the light shielding layer by positioning the source node directly on the light shielding layer without requiring separate contact holes in the source/drain metal thin film. This integration eliminates unnecessary structural elements, simplifies the overall device structure, and preserves maximum area for the storage capacitor while maintaining reliable electrical connections.
3Speed
If polysilicon is used for the active layer to achieve high mobility, then driving speed improves, but threshold voltage becomes non-uniform requiring additional compensation circuits
Solution Approach 1:
The patent changes the material parameter of the active layer from polysilicon to oxide semiconductor material. This material substitution fundamentally alters the electrical characteristics, providing both high mobility for fast driving speed and uniform threshold voltage distribution, thereby eliminating the need for additional compensation circuits while maintaining high performance.
Data Source
AI summary
Disclosed is a thin film transistor substrate which facilitates to realize a bottom gate structure where a gate electrode is disposed below an active layer, and to increase an area for a storage capacitor, and a display device including the same, wherein the thin film transistor substrate may include a light shielding layer, a buffer layer for covering the light shielding layer, and a driving transistor prepared on the buffer layer while being overlapped with the light shielding layer, and provided to supply a driving current to an organic light emitting device.