Shared Pixel Circuit Transistors for CMOS Image Sensor Fill Factor
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Solution Overview
Problem
In CMOS image sensors, the increasing number of pixels requires smaller pixel sizes and reduced metal wiring to enhance fill factor, but there are limitations to reducing the number of transistors and metal wiring widths while maintaining optical path efficiency, especially for global shutter mode operations.
Innovation Solution
The pixel circuit design includes shared reset and source follower transistors with common interconnect lines for toggled control signals, and a shared overflow device between neighboring pixels to minimize metal wiring and enhance fill factor, allowing for efficient light detection and signal processing in both rolling and global shutter modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the number of pixels is increased, then pixel density is improved, but pixel size must be reduced which increases manufacturing complexity
Solution Approach 1:
The patent merges the reset transistor and source follower transistor into a single shared transistor structure that serves both pixels simultaneously. This consolidation reduces the total transistor count and metal wiring requirements, enabling higher pixel density without proportionally increasing pixel circuit complexity
Solution Approach 2:
The shared transistor is designed to perform multiple functions: it acts as the reset transistor for one pixel and the source follower transistor for the same or neighboring pixel. This multi-functionality allows reduced per-pixel component count while maintaining full operational capability across the pixel array
2Area of stationary object
If metal wiring width is reduced, then fill factor is improved, but optical path efficiency deteriorates
Solution Approach 1:
The patent extracts and removes unnecessary metal wiring connections by sharing transistors between pixels. By eliminating redundant wiring paths that would otherwise be required to connect separate reset and source follower transistors to each pixel, the design reduces metal layer occupation and increases photodiode fill factor
3Area of stationary object
If the number of transistors per pixel is reduced, then pixel area is improved, but device complexity increases due to shared contacts
Solution Approach 1:
The patent combines the reset transistor and source follower transistor into a shared structure where one transistor serves dual purposes for adjacent pixels. This merging reduces the total transistor count per pixel while the shared contact design simplifies the interconnect structure rather than complicating it
4Area of stationary object
If photodiode size is increased, then fill factor is improved, but pixel density deteriorates
Solution Approach 1:
By merging transistor functions and sharing contacts between neighboring pixels, the patent reduces the area occupied by active circuit elements. This freed space allows photodiodes to be enlarged within each pixel while maintaining or increasing overall pixel density through more efficient space utilization
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 design minimizes metal wiring, increases the fill factor of photodiodes, and enhances pixel density, improving image sensor performance by allowing for more efficient light detection and signal processing, particularly in high-density pixel arrays.
Implementation Method 1
Each photodiode detects light for converting the detected light into an electrical signal which is the image signal
Data Source
AI summary
Contacts and/or a transistor are shared by neighboring pixel circuits in an image sensor. In addition, a common interconnect line provides common control signals for minimizing metal wiring. Such minimization of space for the shared contacts, transistor, and control signals enhances the fill factor of photodiodes in the image sensor.