TFT Semiconductor Layer Segmentation for Leak Current Suppression
Solution Overview
Problem
In liquid crystal display devices, the stacked structure of poly-Si and a-Si layers in TFTs for drive circuits leads to a phenomenon where the TFTs fail to turn off properly due to leak current when the gate electrode is set at zero or negative voltage, causing malfunction and reduced reliability.
Innovation Solution
The TFTs are designed with a semiconductor layer composed only of an a-Si layer in the pixel region and a poly-Si layer in the drive circuit region, with a specific configuration of n+Si layers and electrodes to suppress leak current, including forming the n+Si layer to cover the side wall portions of the semiconductor layer and ensuring the ends of the gate electrode reside outside the semiconductor layer in the second direction, thereby reducing the OFF current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a stacked structure of poly-Si and a-Si layers is used in TFTs for drive circuits, then high electron mobility is achieved enabling the TFT to function as a switching element, but leak current occurs when the gate electrode is set at zero or negative voltage causing the TFT to fail to turn off properly
Solution Approach 1:
The semiconductor layer is segmented into two distinct materials: poly-Si in the channel formation region for high electron mobility and a-Si in the source/drain regions for low leak current. This segmentation allows each material to perform its optimal function, resolving the contradiction between achieving high switching speed and preventing leak current.
Solution Approach 2:
Different semiconductor materials are used in different local regions of the TFT: poly-Si is used specifically in the channel formation region where high electron mobility is needed for switching, while a-Si is used in the source/drain regions where low leak current is critical for maintaining the OFF state. This local quality differentiation resolves the contradiction between switching performance and leak current suppression.
2Speed
If poly-Si is used for the channel part in TFTs for drive circuits, then high electron mobility is achieved, but the manufacturing process becomes more complex compared to using a-Si
Solution Approach 1:
The invention merges the advantages of both poly-Si and a-Si by forming a stacked structure where poly-Si and a-Si layers are combined in a single semiconductor layer. This allows the manufacturing process to be integrated into the existing TFT fabrication flow while achieving the high electron mobility of poly-Si in the channel region.
Solution Approach 2:
The stacked semiconductor layer structure serves multiple functions simultaneously: the poly-Si layer provides high electron mobility for switching operations, while the a-Si layer suppresses leak current in the source/drain regions. This multi-functionality resolves the contradiction between achieving high performance and maintaining manufacturing simplicity.
3Reliability
If the n+Si layer and source/drain electrodes are formed to cover the side wall portions of the semiconductor layer, then leak current is suppressed, but the manufacturing process requires additional steps
Solution Approach 1:
The n+Si layer is formed preliminarily before the source/drain electrodes, creating a leak current suppression structure in advance. This preliminary action of forming the n+Si layer that covers the side wall portions of the semiconductor layer prevents leak current paths from forming, while the process can be integrated into the existing manufacturing sequence.
Data Source
AI summary
A liquid crystal display device includes a semiconductor layer which is formed of a poly-Si layer and an a-Si layer and formed above a gate electrode with a gate insulating film interposed therebetween. A source electrode or a drain electrode is formed above the semiconductor layer. An n+Si layer is formed between the source electrode or the drain electrode and the semiconductor layer. Since ends of the source electrode or the drain electrode are formed inside ends of the semiconductor layer, leak current at the ends of the semiconductor layer can be reduced.