Active Pixel Sensor Memory Node Charge Transfer for Dynamic Range
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Solution Overview
Problem
Active pixel sensors face limitations in upward dynamic range due to photodiode saturation, especially in strong light conditions, and existing technologies struggle to implement correlated double sampling in 'global shutter' mode.
Innovation Solution
Increasing the charge storage capacity of the memory node to at least N times that of the photodiode and performing N regularly spaced charge transfers from the photodiode to the memory node during integration time, where N is an integer greater than or equal to 2, to enhance charge accumulation and reading precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the photodiode surface area is increased to improve charge storage capacity, then the upward dynamic range is improved, but the pixel area and manufacturing complexity increase
Solution Approach 1:
The patent introduces a memory node as an intermediate storage element between the photodiode and readout circuit, effectively adding a temporal dimension to charge storage. Charges are transferred from the photodiode to the memory node during integration, allowing the photodiode to be smaller while maintaining adequate charge storage capacity through the memory node's larger capacitance.
Solution Approach 2:
The memory node serves as an intermediary element that decouples the photodiode from the readout circuit. This intermediary node with larger storage capacity allows the photodiode to be optimized for light conversion rather than charge storage, resolving the contradiction between small pixel area and adequate charge storage capacity.
2Quantity of substance
If multiple charge transfers are performed during integration time, then the upward dynamic range is enhanced, but the device complexity increases
Solution Approach 1:
The integration period is segmented into multiple phases with N sequential charge transfers from photodiode to memory node. This segmentation allows the system to accumulate charges in increments, effectively multiplying the charge accumulation capacity while using simple, repetitive transfer operations controlled by a phased clock signal rather than complex control logic.
Solution Approach 2:
The patent employs periodic charge transfer operations during the integration time, with N transfers occurring at regular intervals. This periodic action, synchronized by a clock signal, enables enhanced charge accumulation capacity through multiple transfers while maintaining simple control circuitry based on periodic timing rather than complex variable control.
3Measurement precision
If correlated double sampling is implemented in global shutter mode, then measurement precision is improved, but the device complexity increases
Solution Approach 1:
The memory node stores the accumulated charges from the photodiode during the integration period, preserving the signal state before readout. This preliminary storage action enables the readout circuit to perform correlated double sampling by comparing the stored signal level with a reference level, achieving improved measurement precision in global shutter mode without requiring complex real-time sampling circuitry.
4Quantity of substance
If the memory node charge storage capacity is increased to N times the photodiode capacity, then the upward dynamic range is improved, but the device area increases
Solution Approach 1:
The patent achieves increased charge storage capacity in the memory node by adjusting the capacitance parameter of the memory node capacitor rather than increasing physical area proportionally. Through parameter optimization of the capacitor structure and integration with shared readout circuitry, the system achieves N times the charge capacity while minimizing the area overhead of the memory node.
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 enhances the upward dynamic range of active pixel sensors by allowing multiple charge transfers during integration, effectively multiplying charge accumulation capacity without increasing the photodiode's surface area, thus overcoming saturation issues and enabling precise correlated double sampling in 'global shutter' mode.
Implementation Method 1
a photodiode that converts received photons into electrical charges
Data Source
Figure 1
Figure 2
Figure 3a~3d
AI summary
The invention relates to an active pixel sensor including a photodiode PHD, a memory node MN and a reading node SN, the memory node being provided so as to contain the charges generated by the photodiode at the end of an integration period allowing integration in global shutter mode and reading with correlated double sampling, in which the charge storage capacity of the memory node is provided so to be at least N times higher than the charge storage capacity of the photodiode (N being an integer no lower than 2), and in which, in each integration and reading cycle, during the integration time Tint(i), it is provided for N charge transfers Tri1, Tri2, Tri3 from the photodiode towards the memory node to be carried out, the N transfers being evenly distributed over the integration time. The dynamics of the sensor improve in strong-light environments.