Imaging Pixel Charge Transfer Potential Well Design
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Solution Overview
Problem
Existing imaging devices with global electronic shutter operations face challenges in suppressing the reverse flow of charges from a charge holding portion back to the photoelectric converter, leading to distorted images, especially when capturing fast-moving objects.
Innovation Solution
The imaging device incorporates a configuration where the first transfer transistor forms a potential well between the photoelectric converter and the first holding portion, with the maximum charge amount generated by the photoelectric converter, the saturation charge amount of the first holding portion, and the charge amount that can be held in the potential well satisfying the relationship QPD < QGS ≤ QMEM_SAT, ensuring effective charge transfer without reverse flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a charge holding portion is provided separately from a photoelectric converter and floating diffusion within a pixel to implement global electronic shutter, then exposure periods can be synchronized among multiple pixels reducing image distortion, but signal charges may reverse flow from the charge holding portion back to the photoelectric converter during transfer transistor switching
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: photoelectric converter, transfer transistor, charge holding portion, and floating diffusion. This segmentation allows independent optimization of each component's function, particularly enabling the charge holding portion to store charges during global shutter operation while the transfer transistor controls charge flow directionality through timed switching sequences.
Solution Approach 2:
The transfer transistor is turned on before the charge holding portion is fully filled, and charges are transferred to the floating diffusion before the transfer transistor is turned off. This preliminary action sequence ensures that charges are moved through the transfer transistor during a controlled time window when the transistor is conductive, preventing reverse flow when the transistor is off.
2Ease of operation
If transfer transistors are used to move charges from photoelectric converter to charge holding portion, then charge transfer is enabled, but during the process of switching the transfer transistor from on-state back to off-state, signal charges may return to the photoelectric converter
Solution Approach 1:
The transfer transistor operates in periodic cycles: turned on to enable charge transfer from photoelectric converter to charge holding portion, then turned off to prevent reverse flow, then turned on again for the next transfer cycle. This periodic switching pattern synchronizes with the global shutter exposure timing to ensure accurate charge transfer without contamination from reverse flow.
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 the reverse flow of charges, maintaining image linearity and quality even when capturing fast-moving objects, enabling high-quality image acquisition.
Implementation Method 1
a photoelectric converter that generates charges by photoelectric conversion
Data Source
AI summary
An imaging device includes pixels each including a photoelectric converter that generates charges by photoelectric conversion, a first transfer transistor that transfers charges of the photoelectric converter to a first holding portion, a second transfer transistor that transfers charges of the first holding portion to a second holding portion, and an amplifier unit that outputs a signal based on charges held by the second holding portion. The first transfer transistor is configured to form a potential well for the charges between the photoelectric converter and the first holding portion when the first transistor is in an on-state. The maximum charge amount QPD generated by the photoelectric converter during one exposure period, a saturation charge amount QMEM_SAT of the first holding portion, and the maximum charge amount QGS that can be held in the potential well are in a relationship of: QPD<QGS≤QMEM_SAT.