Image Sensor Pixel Transfer Gate Voltage Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Image sensors face challenges in reducing hot pixels and dark current, particularly during long integration periods, as existing methods to mitigate these issues either increase blooming or reduce photodiode well capacity.
Innovation Solution
The image sensor operates in two modes based on illumination levels, with the transfer or reset transistor being positively biased during low light conditions to enhance electron flow and reduce dark current, while maintaining normal operation under bright conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a negative potential is applied to the transfer gate to reduce dark current and hot pixels, then dark current and hot pixel effects are substantially reduced, but anti-blooming performance is reduced and blooming occurs
Solution Approach 1:
The transfer gate voltage is made dynamic rather than static, switching between negative potential during integration period to suppress dark current and positive potential during readout period to enable blooming. This temporal variation in voltage allows the system to achieve both dark current suppression and anti-blooming capability without compromise
Solution Approach 2:
The transfer gate voltage is periodically switched between negative and positive potentials synchronized with the integration and readout periods. During integration period, negative potential suppresses dark current; during readout period, positive potential allows blooming. This periodic voltage switching resolves the contradiction between suppressing dark current and preventing blooming
2Object-affected harmful factors
If the transfer gate potential is held positive to improve anti-blooming performance, then blooming is reduced, but dark current increases and hot pixel effects worsen
Solution Approach 1:
The transfer gate voltage is dynamically switched based on operational mode: negative potential during integration period to suppress dark current, positive potential during readout period to prevent blooming. This dynamic switching allows the system to achieve both dark current suppression and anti-blooming capability
Solution Approach 2:
Periodic voltage switching on the transfer gate synchronized with integration and readout periods achieves both dark current suppression (during integration with negative potential) and blooming prevention (during readout with positive potential)
3Object-generated harmful factors
If the transfer transistor is partially on during integration period to reduce dark current, then dark current is reduced, but photodiode well capacity is reduced
Solution Approach 1:
The transfer gate voltage is periodically switched: negative potential during integration period to suppress dark current, positive potential during readout period to restore full well capacity. This periodic switching allows the photodiode to maintain full well capacity while dark current suppression is achieved during integration
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 effectively reduces dark current and hot pixel defects while preventing blooming and maintaining full well capacity, improving image sensor performance under low light conditions without degrading signal-to-noise ratio.
Implementation Method 1
The well capacity of the photodiode is reduced... the generated photocharge on those pixels cannot spill over the transfer gate
Implementation Method 2
the transfer or reset transistor being positively biased during low light conditions to enhance electron flow and reduce dark current
Data Source
AI summary
A pixel and image sensor formed in accordance with the present invention has two modes of operation: a normal mode and a low light mode. The present invention switches from a normal to a low light mode based upon the amount of illumination on the image sensor. Once the level of illumination is determined, a decision is made by comparing the level of illumination to a threshold whether to operate in normal mode or low light mode. In low light mode, the reset transistor (for a 3T pixel) or the transfer transistor (for a 4T pixel) is biased positive.