Floating Diffusion Capacity Modulation for Motion Distortion in CMOS Sensors
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Solution Overview
Problem
Existing CMOS image sensors face challenges in achieving high image quality with wide dynamic range due to notion distortion and quality degradation in moving images, particularly when capturing low and high illuminance scenes at different times.
Innovation Solution
A solid-state imaging device with a pixel structure that includes a photo-electric conversion element, a transfer element, a floating diffusion, a source-follower element, and a capacity changing portion, which allows the capacity of the floating diffusion to be adjusted during a readout period, enabling switching between high and low conversion gains to accommodate varying light conditions without motion distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If imaging by low illuminance and high illuminance are carried out at different times using multiple exposure times, then dynamic range is widened, but notion distortion occurs and moving image quality is degraded
Solution Approach 1:
The floating diffusion capacity is dynamically changed during the readout period by the capacity changing portion in response to a capacity changing signal. This allows the conversion gain to be switched between high and low states within a single readout period, enabling the system to adapt to different illuminance conditions without requiring multiple sequential exposures, thereby preventing motion distortion while maintaining wide dynamic range coverage
Solution Approach 2:
The capacity of the floating diffusion is changed as a parameter during the readout period. By varying the floating diffusion capacity in response to illuminance conditions, the conversion gain is adjusted dynamically - high conversion gain for low illuminance and low conversion gain for high illuminance - all within a single exposure period, thus avoiding the temporal separation issue that causes motion distortion
2Adaptability or versatility
If the capacity of the floating diffusion is changed during the readout period, then conversion gain can be switched to accommodate varying light conditions, but device complexity increases
Solution Approach 1:
The floating diffusion node serves multiple functions: it acts as the charge accumulation node during the accumulation period, the charge-to-voltage conversion node during the readout period, and simultaneously serves as the node whose capacity is modulated by the capacity changing portion. This multi-functionality allows the system to achieve variable conversion gain without adding separate dedicated structures for each function, thereby limiting the increase in device complexity
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 effectively suppresses motion distortion while widening the dynamic range, resulting in higher image quality by allowing both bright and dark signals to be output in a single readout period, thereby enhancing the sensor's ability to handle varying light conditions.
Implementation Method 1
each pixel includes a photo-electric conversion element which accumulates a charge generated by photo-electric conversion in an accumulation period
Implementation Method 2
a source-follower element which converts the charge of the floating diffusion to a voltage signal in accordance with the charge quantity
Data Source
AI summary
A solid-state imaging device, a method for driving the solid-state imaging device, and an electronic apparatus capable of suppressing occurrence of motion distortion while realizing widening of dynamic range and in turn realizing a higher image quality are provided. Each pixel includes a photo diode PD which accumulates a charge generated by photo-electric conversion in an accumulation period, a transfer transistor capable of transferring the accumulated charge in a transfer period, a floating diffusion FD to which the charge accumulated in the photo diode PD is transferred, a source-follower transistor which converts the charge of the floating diffusion FD to a voltage signal in accordance with the charge quantity, and a capacity changing portion capable of changing the capacity of the floating diffusion FD in accordance with a capacity changing signal, the capacity of the floating diffusion FD being changed by the capacity changing portion in a predetermined period in one readout period with respect to the accumulation period and a conversion gain being switched in this one readout period.