CMOS Pixel Reset Gate Voltage Control for Dark Current Reduction
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Solution Overview
Problem
Conventional CMOS Image Sensors experience degradation in low-light characteristics due to dark current generated from adjacent photo detecting devices caused by high gate voltage of the reset transistor, which affects image quality and blooming control.
Innovation Solution
A unit pixel apparatus and operation method that control the gate voltage of the reset transistor by applying a lower voltage than the first supply voltage during exposure and readout periods, and using a floating diffusion node reset voltage supply unit to manage the reset voltage, thereby reducing dark current and improving blooming control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high gate voltage is applied to the reset transistor to improve blooming control, then blooming effect is reduced, but dark current increases causing degradation in low-light characteristics
Solution Approach 1:
The patent applies dynamic voltage control to the reset transistor gate, switching between a first voltage (during exposure) and a second voltage (during readout). This dynamic adjustment allows the system to optimize both blooming control and dark current suppression at different operational stages, resolving the contradiction between these two performance aspects.
Solution Approach 2:
The patent implements periodic voltage switching to the reset transistor gate, alternating between a first voltage level during the exposure period and a second voltage level during the readout period. This periodic action enables the system to achieve effective blooming control during exposure while minimizing dark current during readout, thus resolving the technical contradiction.
2Reliability
If a high gate voltage is applied to the reset transistor, then reset function is improved, but noise in dark or low light images increases
Solution Approach 1:
The patent dynamically adjusts the reset transistor gate voltage based on the operational phase: applying a first voltage during exposure to ensure proper reset function, and switching to a second voltage during readout to minimize noise generation. This dynamic control resolves the contradiction between reset effectiveness and noise suppression.
Solution Approach 2:
The patent applies the first voltage to the reset transistor gate in advance during the exposure period, preparing the floating diffusion node for proper reset operation before readout begins. This preliminary action ensures the reset function is established while preventing noise generation during the critical readout phase.
3Reliability
If the reset transistor gate voltage is increased, then floating diffusion node reset is improved, but image quality in low light deteriorates
Solution Approach 1:
The patent implements dynamic voltage control, switching between a first voltage level (during exposure) that ensures proper floating diffusion node reset, and a second voltage level (during readout) that preserves image quality in low-light conditions. This temporal separation of voltage levels resolves the contradiction between reset effectiveness and image quality.
Solution Approach 2:
The patent applies periodic voltage switching to the reset transistor gate, using a first voltage during the exposure period to ensure complete floating diffusion node reset, and switching to a second voltage during the readout period to maintain image quality. This periodic action resolves the contradiction by applying different voltage levels at appropriate operational stages.
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 solution effectively reduces low-light characteristic degradation and enhances blooming effect by minimizing dark current and optimizing the reset voltage, leading to improved image quality in CMOS Image Sensors.
Implementation Method 1
The photo sensitive device PD performs a photoelectric conversion function. That is, the photo sensitive device PD receives light from outside, and generates photo charges based on the received light.
Data Source
AI summary
A unit pixel apparatus may include a plurality of unit pixels including a reset transistor, each unit pixel being suitable for outputting a pixel signal corresponding to incident light; a reset transistor gate voltage transmission unit suitable for transmitting a plurality of reset transistor gate voltages; and a first voltage switching unit suitable for transmitting a first supply voltage among the plurality of reset transistor gate voltages to a gate terminal of the reset transistor in each of the unit pixels during a period from an exposure start time to just before a readout time.