Charge Injection Suppression in Active Pixel Sensors
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Solution Overview
Problem
CMOS imager circuits face errors due to unwanted charge injection, which cannot be fully subtracted using correlated double sampling (CDS), leading to noise in output signals and reduced dynamic range, especially as the floating diffusion region area scales for higher conversion gain.
Innovation Solution
Maintaining the RST signal at a positive voltage greater than ground but less than the threshold voltage of the reset transistor during the readout period to minimize charge injection, thereby increasing the available voltage swing and reducing lag without contributing to noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the floating diffusion region area is scaled down to increase conversion gain, then the conversion gain is improved, but charge injection errors increase and cannot be fully subtracted using CDS
Solution Approach 1:
The patent changes the voltage parameter of the RST signal from a conventional ground level to a positive voltage level (e.g., +0.3V to +1.0V) during the readout period. This parameter change suppresses charge injection errors by preventing charge redistribution from the reset transistor gate-source overlap capacitor into the floating diffusion region, thereby resolving the contradiction between high conversion gain and charge injection errors.
2Device complexity
If the RST signal is maintained at ground level during readout, then the circuit operation is simple, but charge injection occurs reducing dynamic range
Solution Approach 1:
The patent modifies the RST signal voltage parameter from ground level to a controlled positive voltage level during the readout period. This change suppresses charge injection into the floating diffusion region, thereby increasing the available voltage swing and dynamic range while maintaining manageable circuit complexity through controlled voltage level management.
3Measurement precision
If correlated double sampling is used to subtract charge injection errors, then some noise is reduced, but residual noise remains and dynamic range is reduced
Solution Approach 1:
The patent applies preliminary anti-action by maintaining the RST signal at a positive voltage level during readout to prevent charge injection errors from occurring in the first place. This proactive approach eliminates the need for CDS subtraction and preserves the full dynamic range while achieving superior noise performance compared to reactive CDS methods.
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 suppresses charge injection, enhances the dynamic range, and improves the peak conversion gain by maintaining a positive voltage level for the RST signal during readout, reducing unwanted charge leakage and increasing the linear full well capacity.
Implementation Method 1
each pixel cell receives photons of light and converts those photons into charge carried by electrons
Data Source
AI summary
A method and apparatus are provided for operation of an image sensor during signal readout. During a reset operation the gate of a reset transistor coupled to the storage node receives a voltage greater than a threshold voltage to produce a reset of the storage node. During a period where photogenerated charges stored at the storage node are read out the gate of the reset transistor receives a voltage VRST<sub2>—</sub2>LOW greater than ground, but less than a maximum voltage which can be stored at the storage node.