Compositionally Modulated TFT Channels for On-Current and Leakage Control
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Solution Overview
Problem
Thin film transistors (TFTs) using semiconducting metal oxide materials like indium gallium zinc oxide face challenges in controlling the material composition of surface layers, leading to high leakage current in the back channel and low on-current in the front channel, due to random distribution of oxide materials during physical vapor deposition.
Innovation Solution
The TFTs are designed with a vertical compositional modulation in the active layer, allowing independent control of material composition in the front and back channels through atomic layer deposition, minimizing surface leakage and enhancing on-current while suppressing leakage current, using a bottom gate or top gate configuration with recessed contact regions for direct contact between electrodes and high-conductivity layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If physical vapor deposition is used to deposit oxide materials, then the materials can be deposited onto the substrate, but the material composition in surface layers becomes randomly distributed
Solution Approach 1:
The active layer is segmented into multiple sub-layers with different material compositions (e.g., IGZO layer, IZO layer, ZnO layer) deposited in sequence. Each sub-layer has a specific thickness and composition designed to control carrier concentration and conductivity in different regions of the active layer, thereby achieving precise control over material composition that cannot be obtained through random PVD deposition.
Solution Approach 2:
Different regions of the active layer are given different local qualities through the multi-layer structure. The IGZO layer provides high conductivity regions, while IZO and ZnO layers provide lower conductivity regions, creating spatially varying material properties within the active layer to simultaneously achieve high on-current and low leakage current.
2Device complexity
If uniform material composition is used in the active layer, then the fabrication process is simple, but leakage current in the back channel increases and on-current in the front channel decreases
Solution Approach 1:
The active layer is divided into functionally distinct sub-layers (IGZO, IZO, ZnO) with different thicknesses and compositions. This segmentation enables independent optimization of front channel and back channel properties, achieving low leakage current in the back channel while maintaining high on-current in the front channel, thus improving overall device reliability.
Solution Approach 2:
The active layer exhibits local quality variations where the IGZO layer provides high conductivity for front channel conduction, while IZO and ZnO layers provide lower conductivity to suppress back channel leakage. This spatial variation in material properties resolves the contradiction between structural simplicity and electrical performance reliability.
3Reliability
If the atomic concentration of post-transition metal elements is increased, then electrical conductivity improves, but leakage current may increase if not properly controlled
Solution Approach 1:
The atomic concentration of post-transition metal elements (In, Ga, Zn) is locally optimized in each sub-layer. The IGZO layer has high In concentration for high conductivity in front channel regions, while IZO and ZnO layers have lower In concentration to suppress leakage in back channel regions. This local composition control achieves high electrical conductivity where needed while minimizing leakage current elsewhere.
Solution Approach 2:
The material composition parameters (atomic ratios of In:Ga:Zn:O) are systematically varied across different sub-layers. By changing these compositional parameters from layer to layer, the patent achieves precise control over carrier concentration and conductivity, maximizing on-current while minimizing leakage current through optimized parameter gradients.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in high-performance TFTs with increased on-current and reduced off-current by modulating the atomic concentration of post-transition metal elements within the compound semiconductor material, optimizing the compositional profile for enhanced electrical conductivity.
Implementation Method 1
The TFTs are designed with a vertical compositional modulation in the active layer, allowing independent control of material composition in the front and back channels through atomic layer deposition
Data Source
AI summary
A stack including an active layer, a gate dielectric, and a gate electrode is formed in a forward or in a reverse order, over a substrate. The active layer includes a front channel layer, a bulk semiconductor layer, and a back channel layer. The front channel layer is formed by depositing a layer stack that include at least one post-transition metal oxide layer, a zinc oxide layer, and at least one acceptor-type oxide layer. The zinc oxide layer or at least one post transition metal oxide layer contacts the gate dielectric, and the at least one acceptor-type oxide layer is most distal from the gate dielectric. The front channel layer provides enhanced channel conductivity, while the back channel layer provides suppressed channel conductivity.