Global Shutter Image Sensor Leakage Reduction via Negative Gate Voltage
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Solution Overview
Problem
Global shutter image sensors experience increased leakage current due to longer storage times, leading to image distortion and reduced fill factor caused by the use of storage transistors, which affects the capture of fast-moving objects and reduces low-light performance.
Innovation Solution
Applying a negative voltage to the gate of the storage transistor to accumulate holes and reduce surface state leakage, combined with a smaller size for transistors other than the transfer transistor to increase the fill factor and reduce the need for a large shutter transistor, allowing for efficient charge transfer and storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a global shutter is used to capture fast-moving objects, then image quality for fast-moving objects is improved, but leakage current increases due to longer storage times
Solution Approach 1:
The patent applies a negative voltage to the storage transistor gate before charge transfer to preemptively reduce leakage current. This preliminary action prepares the storage transistor to minimize leakage during the extended storage period required for global shutter operation, thereby maintaining image quality while reducing the harmful leakage effect.
Solution Approach 2:
The patent changes the voltage parameter of the storage transistor gate from a standard positive or zero voltage to a negative voltage. This parameter change fundamentally alters the electrical characteristics of the storage transistor, creating a potential well that reduces surface state leakage and enables stable charge storage for the extended durations required by global shutter operation.
2Reliability
If large storage transistors are used to store image charge, then charge storage capability is improved, but fill factor decreases due to larger non-light-sensitive area
Solution Approach 1:
The patent changes the voltage parameter of the storage transistor to negative, which fundamentally improves charge storage capability through enhanced potential well formation. This parameter change enables smaller transistor dimensions to achieve the same storage capability, thereby reducing the non-light-sensitive area and increasing fill factor.
Solution Approach 2:
The patent utilizes the voltage dimension (applying negative voltage) to enhance storage capability rather than relying solely on increasing transistor physical dimensions. By operating in the voltage domain rather than the size domain, the invention achieves improved storage capability while maintaining smaller physical footprint and higher fill factor.
3Adaptability or versatility
If storage transistors are used to store image charge, then global shutter functionality is improved, but surface state leakage increases corrupting image charges
Solution Approach 1:
The patent changes the gate voltage parameter to negative, which fundamentally alters the electrical characteristics of the storage transistor. This parameter change creates a deeper potential well that prevents surface states from releasing stored charge, thereby eliminating surface state leakage while maintaining global shutter functionality.
Solution Approach 2:
The patent applies negative voltage to the storage transistor gate before charge transfer to preemptively counteract surface state leakage. This preliminary anti-action creates an electrical environment that prevents the harmful surface state leakage effect from occurring during charge storage, thereby protecting image charges from corruption.
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 minimizes leakage current, enhances image uniformity, and increases the fill factor, improving the capture of fast-moving objects and low-light performance by reducing the size of non-transfer transistors and utilizing a global shutter effectively.
Implementation Method 1
Applying a negative voltage to the gate of the storage transistor to accumulate holes and reduce surface state leakage
Implementation Method 2
capture an image or still picture... photodiode 120... acquire an image signal or charge within photodiode 120
Data Source
Figure 1A~1B
Figure 2
Figure 3
AI summary
An image sensor includes a photodiode to accumulate an image charge and a storage transistor to store the image charge. A transfer transistor is coupled between the photodiode and an input of the storage transistor to selectively transfer the image charge from the photodiode to the storage transistor. An output transistor is coupled to an output of the storage transistor to selectively transfer the image charge to a readout node and a reset transistor is coupled to the readout node. A controller is configured to apply a negative voltage to a gate of the storage transistor before activating the gate of the storage transistor to store the image charge.