Solid-State Imaging Device Comparator Segmentation for Dark Current Suppression
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Solution Overview
Problem
CMOS image sensors face limitations in achieving a broader dynamic range and higher frame rate due to the influence of dark current from floating diffusion nodes, leading to noise, non-linearity, and image deterioration, while also being cost-inefficient and area-intensive.
Innovation Solution
A solid-state imaging device with a pixel structure that includes a photoelectric conversion element, a transfer element, an output buffer, and a comparator performing divided comparison processing to separate overflow charges and accumulated charges, reducing the impact of dark current through sub-period AD conversion and resetting the floating diffusion node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If an ADC including a comparator is arranged in each pixel to enable global shutter function, then global shutter capability is achieved, but charges overflowing from photodiodes cannot be utilized in real time and dynamic range is limited
Solution Approach 1:
The patent segments the comparison operation into two distinct phases: first comparison processing for overflow charges during the integration period, and second comparison processing for accumulated charges after the integration period. This segmentation enables real-time utilization of overflow charges while maintaining global shutter capability, thereby resolving the contradiction between adaptability and productivity.
2Reliability
If transistor size is enlarged to reduce flicker noise, then random noise is reduced, but area increases
Solution Approach 1:
The patent segments the charge population into overflow charges and accumulated charges, processing them separately through first and second comparison operations. This allows effective utilization of overflow charges that would otherwise be wasted, improving signal-to-noise ratio without requiring larger transistor sizes, thus resolving the contradiction between reliability and area.
Solution Approach 2:
The patent converts the previously harmful overflow charges into useful signal components by performing first comparison processing to digitize them. This transforms a waste resource into a beneficial contribution to the overall signal, improving noise performance without increasing area.
3Reliability
If capacity is added to comparator output to obtain noise filter effect, then random noise is reduced, but inversion delay of comparator deteriorates and frame rate cannot be raised
Solution Approach 1:
The patent segments the comparison operations into two fast, sequential stages rather than one slow stage with heavy filtering. The first comparison processes overflow charges and the second comparison processes accumulated charges, both operating at high speed without requiring large capacity additions, thus resolving the contradiction between reliability and speed.
4Adaptability or versatility
If ADC and memory part are arranged in each pixel, then global shutter is enabled, but effective pixel region cannot be expanded to maximum limit and cost efficiency decreases
Solution Approach 1:
The patent makes the single comparator in each pixel perform multiple functions: first comparison for overflow charges and second comparison for accumulated charges. This multi-functionality eliminates the need for separate ADC and memory structures, maximizing the effective pixel region while maintaining global shutter capability, thus resolving the contradiction between adaptability and area.
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 suppresses dark current influence, enabling broader dynamic range, higher frame rates, reduced noise, and expanded effective pixel regions while optimizing cost and area efficiency.
Implementation Method 1
a photoelectric conversion element which accumulates a charge generated by photoelectric conversion in an integration period
Data Source
AI summary
A comparator in an AD conversion part, under the control of a reading part, performs a first comparison processing outputting a digitized first comparison result signal with respect to a voltage signal corresponding to an overflow charge overflowing from a PD1 to an FD1 in an integration period, and a second comparison processing outputting a digitized second comparison result signal with respect to a voltage signal corresponding to a accumulated charge of the PD1 transferred to the FD1 in a transfer period after the integration period and, in the first comparison processing, the period of the first comparison processing is divided into a plurality of sub periods and, in each of the sub periods, the comparator performs an AD conversion processing comparing the voltage signal of the output buffer part and the reference voltage and outputting the digitized comparison result signal. Due to this, it is possible to suppress an influence of a dark current of the FD, and becomes possible to suppress deterioration of the image while substantially realizing a broader dynamic range and a higher frame rate.