CMOS Image Sensor Pixel Charge Transfer Noise Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Solid state imaging devices, such as CMOS image sensors, face challenges in achieving global exposure with all pixels at the same exposure period without distortion, while also reducing noise due to threshold irregularities in pixel transistors, which affects image quality and dynamic range.
Innovation Solution
The implementation of a unit pixel structure with a charge holding region (memory unit) separate from the floating diffusion, where photo-generated charges are divided and accumulated as first and second signal charges, allowing the first transfer gate's threshold irregularities to be cancelled out, thereby reducing noise and maintaining image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If photo-generated charges are accumulated in the p-n junction capacitance of photodiodes and read out by MOS transistors, then the device can perform image sensing, but the exposure period does not match for all pixels causing image distortion when the subject is moving
Solution Approach 1:
The pixel structure is segmented into multiple photodiodes per pixel unit, with each photodiode having its own transfer gate. This segmentation allows independent control of charge transfer timing for each photodiode, enabling precise exposure period matching across all pixels while maintaining image quality
Solution Approach 2:
Transfer gates are introduced as intermediary elements between the photodiodes and the readout circuitry. These transfer gates act as mediators that can precisely control the timing of charge transfer, ensuring that all pixels complete their exposure period simultaneously even though they are read out sequentially, thereby preventing image distortion
2Reliability
If a mechanical shutter is used to control exposure time for all pixels, then global exposure can be realized, but the device size increases and mechanical driving speed limits simultaneity
Solution Approach 1:
The mechanical shutter system is completely replaced with an electrical control system using transfer gates. Each transfer gate is electrically controlled to transfer charges from photodiodes to floating diffusions simultaneously across all pixels, achieving global exposure without any mechanical moving parts, thereby reducing device complexity and improving simultaneity
Solution Approach 2:
Transfer gates serve as electrical intermediaries that replace the mechanical shutter's light-blocking function. By controlling the timing of charge transfer through these electrical intermediaries, the system achieves global exposure with precise timing control, eliminating the need for mechanical shielding mechanisms
3Productivity
If the transfer gate is turned on to transfer accumulated charges to the floating diffusion, then charges are transferred to the floating diffusion, but threshold irregularities in the transfer gate cause noise in the image
Solution Approach 1:
Multiple photodiodes are merged into a single pixel unit, sharing common readout circuitry. This merging allows the system to average out the threshold irregularities of individual transfer gates, reducing noise while maintaining efficient charge transfer to the floating diffusion
Solution Approach 2:
The patent uses multiple identical photodiode-transfer gate structures within each pixel unit. By creating copies of the transfer gate mechanism, the system can statistically average out threshold irregularities across multiple copies, thereby reducing noise while maintaining high charge transfer efficiency
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 enables global exposure with reduced noise and improved image quality by cancelling out charge deviations due to threshold irregularities, while maintaining a wide dynamic range and high signal-to-noise ratio.
Implementation Method 1
a CMOS image sensor which reads out photo-generated charges accumulated in the p-n junction capacitance of photodiodes, which are photoelectric conversion devices
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
A solid state imaging device includes: multiple unit pixels including a photoelectric converter (PD) generating electrical charge in accordance with incident light quantity and accumulating the charge, a first transfer gate (TRX) transferring the accumulated charge, a charge holding region (MEM) holding the transferred charge, a second transfer gate (TRG) transferring the held charge, and a floating diffusion region (FD) converting the transferred charge into voltage; an intermediate charge transfer unit (TRX) transferring, to the charge holding region (MEM), a charge exceeding a predetermined charge amount as a first signal charge; and a pixel driving unit setting the first transfer gate (TRX) to a non-conducting state, set the second transfer gate (TRG) to a conducting state, transfer the first signal charge to the floating diffusion region (FD), set the second transfer gate (TRG) to a non-conducting state, set the first transfer gate (TRX) to a conducting state, and transfer the accumulated charge to the charge holding region (MEM) as a second signal charge.