CMOS Image Sensor Gate Potential Control for Charge Management
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Solution Overview
Problem
In CMOS image sensors, the flow of excessive charges from the photoelectric conversion unit to the charge holding unit can lead to deteriorated image quality, increased dark current, and reduced saturation capacity, especially when exposure periods differ between the two units.
Innovation Solution
A photoelectric conversion apparatus with multiple charge holding units and strategically controlled gate potentials, where the potential of the first gate is set to a first potential and the second gate to a second potential, with the first potential being higher or lower than the second potential, depending on the operational phase, to manage charge transfer and prevent false signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the gate of the charge holding unit side remains constantly ON while the photoelectric conversion unit is storing charges, then excessive charges flowing into the charge holding unit is suppressed, but dark current increases
Solution Approach 1:
The gate potential of the charge holding unit is dynamically adjusted between a first potential during charge storage and a second potential during charge transfer, rather than remaining constantly ON. This dynamic control suppresses excessive charge flow during storage while preventing dark current accumulation by switching to the second potential during transfer periods.
Solution Approach 2:
The gate potential is switched periodically between the first potential (during charge storage) and the second potential (during charge transfer). This periodic action allows the system to suppress excessive charges during storage phases while clearing accumulated dark current during transfer phases, resolving the contradiction between suppressing excessive charges and preventing dark current increase.
2Object-affected harmful factors
If an overflow gate is disposed in the photoelectric conversion unit and the potential difference between the overflow gate and the gate of the charge holding unit is increased, then excessive charges flowing in is suppressed, but the saturation capacity of the photoelectric conversion unit decreases
Solution Approach 1:
The invention changes the potential parameter of the charge holding unit gate between two distinct values (first potential during storage, second potential during transfer) rather than using a fixed potential difference. This allows suppression of excessive charges during storage while maintaining saturation capacity by switching to the second potential for charge transfer, avoiding the need to permanently increase the potential difference which would reduce saturation capacity.
3Reliability
If multiple charge holding units are used to alternately store charges, then signal quality deterioration is reduced, but device complexity increases
Solution Approach 1:
The charge holding function is segmented into multiple charge holding units that can be alternately used. One unit stores charges while another transfers charges, and they switch roles periodically. This segmentation allows simultaneous charge storage and transfer operations, preventing signal quality deterioration while managing complexity through systematic alternation of unit functions.
Solution Approach 2:
The multiple charge holding units operate in periodic alternation, with each unit switching between charge storage and charge transfer modes. This periodic action enables continuous operation without signal quality deterioration while distributing the complexity across multiple units that follow a regular switching pattern, making the system manageable despite the increased number of components.
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 reduces signal quality deterioration, suppresses dark current increase, and maintains saturation capacity, ensuring improved image quality by alternately using charge holding units and controlling gate potentials.
Implementation Method 1
a photoelectric conversion unit that stores charges generated by photoelectric conversion
Data Source
AI summary
A photoelectric conversion apparatus including a plurality of pixels each having: a photoelectric conversion unit; a first charge holding unit; a second charge holding unit; a first gate; and a second gate. In a period when the photoelectric conversion unit is storing charges to be transferred to the first charge holding unit, the potential of the first gate is set to a first potential, and the potential of the second gate is set to a second potential. Both the first potential and the second potential are higher than the potential of the first gate in a period when the charges of the photoelectric conversion unit are transferred to the first charge holding unit and the potential of the second gate in a period when the charges of the photoelectric conversion unit are transferred to the second charge holding unit. The first potential is lower than the second potential.