Pixel Circuit Signal Storage Node Leakage Reduction
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Solution Overview
Problem
Global shutter image sensors face issues with in-pixel leakage current and read-out errors due to varying time intervals for holding image data, leading to inaccurate data from pixel units read out later, such as those in the last row.
Innovation Solution
A pixel circuit architecture comprising a photodiode, transmission circuit, reset circuit, signal storage circuit, and buffer circuit, where the signal storage circuit stores charges on a node with a reduced leakage path, and the buffer circuit generates output signals when selected for read-out, reducing leakage current effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If image data is held in pixel units for varying time intervals until read-out, then global shutter functionality is achieved, but leakage current causes signal inaccuracy
Solution Approach 1:
The pixel circuit is segmented into distinct functional blocks: photodiode for charge accumulation, transmission circuit for charge transfer, signal storage circuit for holding charges, and buffer circuit for read-out. This segmentation allows each component to be optimized for its specific function, with the storage circuit designed to minimize leakage while maintaining global shutter capability.
Solution Approach 2:
A signal storage circuit acts as an intermediary between the photodiode and the read-out circuitry. This intermediate storage node holds the accumulated charges during varying time intervals without being directly connected to the read-out path, thereby isolating the signal from leakage currents that would otherwise affect accuracy during extended holding periods.
2Productivity
If pixel units in later rows hold image data for longer periods, then sequential read-out is enabled, but leakage current increases causing read-out errors
Solution Approach 1:
The signal storage circuit is prepared in advance to receive and hold charges from the photodiode during the entire exposure and transfer period. By having the storage circuit ready and isolated before read-out, the system can accommodate varying hold times for different row groups without degrading signal quality, enabling flexible sequential read-out patterns.
Solution Approach 2:
The transmission circuit serves as an intermediary that transfers charges from the photodiode to the storage circuit before read-out. This intermediate transfer allows the photodiode to be reset early while the storage circuit maintains the signal, decoupling the exposure timing from the read-out timing and enabling later rows to be read out after longer hold periods without accuracy loss.
3Ease of manufacture
If a simple pixel circuit structure is used, then manufacturing is easier, but in-pixel leakage paths cause signal loss
Solution Approach 1:
The storage circuit node is specifically designed with different electrical characteristics than ordinary floating diffusion nodes. By creating a localized region with optimized properties (such as adjusted doping or geometry) at the storage node, the circuit achieves reduced leakage without requiring complete redesign of the entire pixel structure, maintaining manufacturability while improving signal retention.
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
The proposed architecture reduces signal errors caused by leakage currents, ensuring more accurate image data by minimizing leakage paths during the holding phase and effectively buffering signals for accurate read-out operations.
Implementation Method 1
The PD is arranged to accumulate charges in response to incident radiation, to generate a PD signal
Data Source
AI summary
A pixel circuit is disclosed. The pixel circuit includes a photodiode (PD), a transmission circuit, a reset circuit, a signal storage circuit and a buffer circuit. The transmission circuit is coupled between the PD and an ordinary floating diffusion (FD) node. The reset circuit is coupled to the ordinary FD node. The signal storage circuit is coupled to the ordinary FD node. The buffer circuit is coupled to the ordinary FD node. The signal storage circuit may store a PD signal on a specific node having a reduced leakage path in comparison with the ordinary FD node during a holding phase of the pixel circuit, wherein the holding phase is a time interval starting from a first time point at which the PD signal is stored on the specific node and ending at a second time point at which the pixel circuit is selected for performing a read-out operation.