Solid State Imaging Element Charge Transfer Optimization
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High-speed solid-state image sensors face challenges in achieving high sensitivity and minimizing afterimages due to the short time for accumulating photocharges, as the large light-receiving surface of photodiodes leads to incomplete charge transfer to the floating diffusion region within the allotted time.
Innovation Solution
The floating diffusion region is positioned at the edge of the light-receiving surface, with radially extending n-type doped regions surrounding the photodiode, creating a steep potential gradient that efficiently directs photocharges to the center, reducing the need for connecting wires and minimizing capacitance, thus enhancing charge transfer and sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the light-receiving surface of the photodiode is enlarged to increase light reception, then the detection sensitivity is improved, but the photocharges cannot reach the floating diffusion region within the short exposure time, causing afterimages and image degradation
Solution Approach 1:
The light-receiving surface is divided into multiple regions with different conductivity types (n-type and p-type regions) arranged in a specific pattern. This segmentation creates multiple charge collection paths and potential wells that guide photocharges efficiently to the floating diffusion region, reducing transfer time while maintaining large light-receiving area for high sensitivity
Solution Approach 2:
Different regions of the photodiode are doped with different conductivity types to create localized potential distributions. The n-type and p-type regions are strategically positioned to form potential gradients that accelerate photocharge movement toward the floating diffusion region, ensuring fast charge collection across the entire large light-receiving surface
2Ease of operation
If a metallic wire is provided to connect the floating diffusion region to the subsequent circuit, then the electric charges can be transmitted, but the capacitive load of the wire lowers the charge-to-voltage conversion gain and decreases sensitivity
Solution Approach 1:
The metallic connection wire is completely removed from the structure. Instead, the floating diffusion region is directly connected to the subsequent circuit through semiconductor regions (n-type or p-type regions) formed within the photodiode structure itself, eliminating the capacitive load that would be introduced by external metallic wires and preserving high charge-to-voltage conversion gain
Solution Approach 2:
The connection path for charge transmission is merged with the photodiode structure itself. The n-type and p-type regions that form part of the photodiode's internal structure also serve as the charge transmission path to the subsequent circuit, combining the functions of charge collection and charge transmission into a single integrated structure without requiring separate metallic interconnects
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 allows for efficient transfer of electric charges to the floating diffusion region within a short period, improving detection sensitivity and signal-to-noise ratio without compromising imaging speed, while minimizing afterimages and maintaining high sensitivity.
Implementation Method 1
an embedded photodiode is normally used for photoelectric conversion
Implementation Method 2
a potential gradient is formed in the direction from the circumference of the light-receiving surface to the center thereof (i.e. to the floating diffusion region) so that the photocharges created in the photodiode can easily gather at the center
Data Source
Figure 1~2
Figure 3A~3D
Figure 4A~4E
AI summary
A floating diffusion region (13) is formed at an edge of a light-receiving surface of an embedded photodiode, with a transfer gate electrode (12) located therebetween. A first region (111), with radially extending portions centered on the FD region (13), and a second region (112), located to the outside of the first region, are created in the substantially sector-shaped light-receiving surface. A dopant whose conductivity type is the same as the signal charges to be collected in the first region (111) are introduced, whereby an electric field for moving the signal charges from the radially extending sections towards the center is created due to a three-dimensional field effect. As a result, the charge-transfer time is reduced. Additionally, since a circuit element in the subsequent stage can be placed adjacent to the floating diffusion region (13), the parasitic capacitance of the floating diffusion region (13) can be reduced and a highly sensitive element can be obtained. Thus, it is possible to improve the detection sensitivity and signal-to-noise ratio of a solid-state image sensor capable of taking images at ultrahigh speeds (at one million frames per second or higher) without losing the speediness of the imaging operation.