Solid-State Image Pickup Charge Drain Gate Control
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Solution Overview
Problem
In CMOS Image Sensors (CIS) with a global shutter function, most charges generated in the photodiode are lost due to a transitional overflow phenomenon, leading to reduced signal-to-noise ratio (S/N) and dynamic range, especially when exposure times are long, causing focal plane distortion and degrading image quality.
Innovation Solution
A solid-state image pickup apparatus is designed with a photoelectric conversion unit, a first holding unit, a first transfer gate unit, a charge drain unit, and a second drain gate unit that connects the charge drain unit with a constant voltage source, allowing for controlled charge transfer and drainage to prevent overflow losses, including a second holding unit and transfer gate unit for enhanced charge management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a global shutter function is implemented to perform pixel exposure and reading simultaneously for all pixels, then focal plane distortion is prevented, but charges are lost due to transitional overflow phenomenon from the photodiode to the overflow drain
Solution Approach 1:
A control gate is introduced as an intermediary element between the photodiode and the overflow drain. This control gate mediates the charge transfer process by controlling the potential barrier height, preventing direct and uncontrolled overflow of charges from the photodiode to the overflow drain during the global shutter operation, thereby reducing charge loss while maintaining image stability.
Solution Approach 2:
The potential barrier height between the photodiode and overflow drain is dynamically changed by controlling the gate voltage of the control gate. By adjusting this parameter (potential barrier height) based on the operational state, the overflow of charges is controlled - preventing loss during exposure while allowing drainage when needed, thus resolving the contradiction between maintaining charge for image quality and preventing overflow.
2Use of energy by moving object
If exposure time is extended to capture low-light scenes, then more light is accumulated, but charge loss due to overflow phenomenon increases, degrading signal-to-noise ratio and dynamic range
Solution Approach 1:
The control gate acts as an intermediary that regulates charge flow during extended exposure periods. By controlling the potential barrier, it prevents excessive charge accumulation that would lead to overflow loss, thereby maintaining signal-to-noise ratio while still allowing sufficient light accumulation for low-light scene capture.
Solution Approach 2:
The potential barrier height is dynamically adjusted based on exposure conditions. During extended exposure for low-light scenes, the control gate maintains an appropriate barrier height to prevent overflow loss while allowing continuous charge accumulation, thus improving reliability (signal-to-noise ratio) without sacrificing light accumulation capability.
3Area of moving object
If pixels are miniaturized to increase resolution, then more pixels fit in the sensor, but the distance between photodiode and overflow drain is reduced, worsening charge overflow loss
Solution Approach 1:
The control gate is introduced as a necessary intermediary element that becomes even more critical in miniaturized pixels. With reduced distance between photodiode and overflow drain, the control gate's role in mediating charge transfer and preventing uncontrolled overflow becomes essential, allowing high pixel density while controlling charge loss through electrostatic control.
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 configuration effectively suppresses charge loss in the holding unit, thereby improving the signal-to-noise ratio and dynamic range, reducing the degradation of image quality over time, especially in conditions like low-light exposure.
Implementation Method 1
a photodiode (hereinafter, abbreviated to PD (the same holds true for the other components)) 11 that performs photoelectric conversion
Data Source
AI summary
The present disclosure relates to a solid-state image pickup apparatus and an electronic apparatus capable of preventing charges accumulated in a PD from being lost and suppressing reductions of an S/N and a dynamic range. The apparatus according to an embodiment of the present disclosure includes: a photoelectric conversion unit; a first holding unit that holds the charge transferred from the photoelectric conversion unit; a first transfer gate unit that controls the transfer of the charge; a charge drain unit that is a drain destination of the charge generated by the photoelectric conversion unit; a first drain gate unit that controls the transfer of the charge from the photoelectric conversion unit to the charge drain unit; and a second drain gate unit that connects the charge drain unit with a constant voltage source. The present disclosure can be applied to a CIS and an electronic apparatus provided with the CIS.