TFT Substrate Impurity Gradients for Threshold Voltage Uniformity
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Solution Overview
Problem
Thin film transistors (TFTs) in display apparatuses, particularly those using polysilicon active layers, face challenges with non-uniform threshold voltage distribution, leading to inconsistent luminance and reduced image quality, increased power consumption, and decreased resolution due to variations in threshold voltage.
Innovation Solution
A TFT substrate design with specific impurity concentration gradients and structures, including lightly doped drain and source regions, and channel regions, is implemented to improve threshold voltage uniformity, where the second TFT acts as a driving transistor with a longer channel length and lower impurity concentration, and the first TFT has an LDD structure for improved leakage current reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If polysilicon is used as the semiconductor material in the active layer, then mobility is increased, but threshold voltage uniformity is decreased
Solution Approach 1:
The patent applies local quality by creating different impurity concentration zones within the active layer. Specifically, it introduces a first lightly doped region between the source/drain and channel, and a second lightly doped region with different impurity concentration characteristics. This local variation in impurity concentration allows the channel region to maintain uniform threshold voltage while the source/drain regions provide high mobility, thus resolving the contradiction between mobility and threshold voltage uniformity.
Solution Approach 2:
The patent changes the impurity concentration parameter across different regions of the active layer. The channel region maintains low impurity concentration for uniform threshold voltage, while source/drain regions have high impurity concentration for mobility. The lightly doped regions serve as transition zones with intermediate impurity concentrations, creating a gradient that optimizes both mobility and threshold voltage uniformity simultaneously.
2Manufacturing precision
If additional circuit is provided to compensate the threshold voltage, then threshold voltage uniformity is improved, but pixel area increases and resolution decreases
Solution Approach 1:
The patent extracts the threshold voltage compensation function from the circuit domain and implements it directly in the semiconductor structure through controlled impurity concentration distribution. By incorporating lightly doped regions with specific impurity concentrations in the active layer, the patent achieves threshold voltage uniformity through material structure rather than additional compensation circuits, thereby maintaining small pixel area and high resolution.
3Manufacturing precision
If additional circuit is provided to compensate the threshold voltage, then threshold voltage uniformity is improved, but power consumption increases
Solution Approach 1:
The patent removes the need for active threshold voltage compensation circuits by embedding the compensation mechanism directly into the passive semiconductor structure through impurity concentration control. The lightly doped regions with optimized impurity concentrations provide inherent threshold voltage uniformity without requiring additional active circuits, thereby reducing power consumption while maintaining threshold voltage uniformity.
Data Source
AI summary
A thin film transistor “TFT”) substrate includes a substrate, an active layer over the substrate, and first and second TFTs over the substrate. The active layer includes: a first drain region, a first channel region and a first source region, which function as a drain, a channel and a source of the first TFT: a first lightly doped region between the first drain region and the first channel region: a second lightly doped region between the first channel region and the first source region: and a second drain region, a second channel region and a second source region, which function as a drain, a channel and a source of the second TFT. An impurity concentration at the second drain or source region is lower than an impurity concentration at the first drain or source region and higher than an impurity concentration at the first or second channel region.