Image Sensor Pixel Layout With Parallel Source Follower Noise Reduction
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Solution Overview
Problem
Image sensors face challenges in reducing signal noise, particularly thermal noise and flicker noise, which affect image quality due to the inherent characteristics of source follower transistors.
Innovation Solution
The design incorporates a source follower transistor with one source region and two drain regions adjacent to the source follower gate electrode, operating in a parallel structure to reduce noise and increase current, while also utilizing pixel separation parts and impurity regions to improve dark current characteristics and ground connections for reduced noise transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If source follower transistors are used in conventional image sensors, then pixel signal amplification is achieved, but thermal noise and flicker noise increase
Solution Approach 1:
The source follower transistor is segmented into multiple parallel transistors (first source follower transistor and second source follower transistor) sharing a common gate electrode. This segmentation distributes the signal amplification across multiple devices, reducing the noise contribution from each individual transistor while maintaining the required gain.
Solution Approach 2:
Multiple source follower transistors are merged in parallel configuration with a shared gate electrode and combined output. The parallel merging of transistor channels increases the total current capacity and averages out the thermal and flicker noise, improving the signal-to-noise ratio.
2Measurement precision
If pixel density is increased, then image sensor resolution improves, but noise transmission between adjacent pixels increases
Solution Approach 1:
Ground electrodes are extracted and positioned between adjacent pixel regions, physically separating the electrical fields of neighboring pixels. This extraction of ground references into the inter-pixel spaces creates electrical isolation barriers that prevent noise transmission while allowing tighter pixel packing for higher resolution.
Solution Approach 2:
Ground electrodes serve as intermediary elements positioned between adjacent pixels. These intermediary ground structures act as electrical shields and reference potential barriers, mediating the interaction between neighboring pixels and preventing noise coupling while enabling higher pixel density.
3Object-generated harmful factors
If transistor size is increased to reduce noise, then noise performance improves, but device area increases
Solution Approach 1:
Multiple smaller transistors are merged in parallel to achieve the equivalent noise performance of a single large transistor, but with reduced total area. The combined current from multiple devices provides the necessary signal strength and noise averaging without requiring each individual transistor to be large.
Solution Approach 2:
The transistor layout transitions from a single large planar device to multiple smaller devices arranged in a two-dimensional parallel configuration. This dimensional reorganization allows the system to achieve the same electrical performance (noise reduction) while optimizing the spatial footprint through parallel stacking rather than series expansion.
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 configuration enhances image sensor performance by reducing noise, improving linearity of voltage-current graphs, and increasing current, resulting in better image quality and higher integration density.
Implementation Method 1
The photodiode serves to convert incident light into an electrical signal
Data Source
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Figure 2B
AI summary
An image sensor includes a pixel separation part in a substrate and configured to separate pixels, the pixels including a first pixel, the pixel separation part including first to fourth sidewalls that at least partially define the first pixel, a first source follower gate electrode on the first pixel and adjacent to the first sidewall and the second sidewall, a first impurity region adjacent to a first corner where the first sidewall and the second sidewall meet, a second impurity region adjacent to a second corner where the second sidewall and the third sidewall meet, and a third impurity region adjacent to a third corner where the first sidewall and the fourth sidewall meet. The first to third impurity regions are adjacent to the first source follower gate electrode. The second impurity region and the third impurity region are electrically connected to each other.