CMOS Image Sensor Floating Diffusion Boosting via Transfer Gate Control
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Solution Overview
Problem
Conventional CMOS image sensors face challenges with image lag during the transfer of image charge from photodiodes to floating diffusions, which affects the efficiency of charge transfer and image quality.
Innovation Solution
The implementation of shared floating diffusion boosting by internal transfer gates, where idle transfer gates are set to different voltage levels to boost the voltage on the shared floating diffusion during the transfer operation, facilitating charge transfer and reducing image lag through capacitive coupling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If image charge is transferred from photodiodes to floating diffusions in conventional CMOS pixels, then charge conversion to voltage signal is achieved, but image lag occurs during the transfer process
Solution Approach 1:
The floating diffusion voltage is boosted in advance before the charge transfer operation begins. By pre-charging the floating diffusion node to a higher voltage level through dedicated boosting circuitry, the potential well is deepened beforehand, ensuring that image charge transfers more completely and rapidly without leaving residual charge that would cause lag in subsequent frames.
Solution Approach 2:
The voltage level of the floating diffusion is dynamically changed from a standard reset level to an elevated boosted level during the transfer phase. This parameter change creates a deeper potential well that enhances charge collection efficiency and reduces transfer time, thereby minimizing image lag while maintaining high charge transfer efficiency.
2Productivity
If miniaturization and integration are pursued to meet higher resolution and lower power demands, then image sensor performance improves, but charge transfer challenges increase due to smaller node sizes
Solution Approach 1:
Instead of uniformly scaling all components, the invention applies localized quality enhancement by implementing floating diffusion voltage boosting specifically at the charge transfer interface. This local intervention compensates for the reduced charge collection capacity inherent in miniaturized pixels, maintaining high charge transfer efficiency despite smaller node sizes and enabling continued productivity gains from miniaturization.
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 enhances charge transfer efficiency and reduces image lag, improving the overall image quality by effectively managing the voltage levels on the shared floating diffusion nodes.
Implementation Method 1
facilitating charge transfer and reducing image lag through capacitive coupling
Data Source
AI summary
A shared pixel includes a plurality of transfer gates coupled between respective photodiodes and a shared floating diffusion. Each transfer gate is coupled to receive a transfer control signal to independently control a transfer of the image charge from the corresponding photodiodes to the shared floating diffusion. Each transfer control signal is set to one of an ON value, a first OFF value, and a second OFF value. One of the control signals that is coupled to an active transfer gate is set to the ON value during a transfer operation. The control signals coupled to idle transfer gates are set to the first OFF value during a reset period prior to the transfer operation, and are set to the second OFF value during the transfer operation.