Variable Threshold Reset for CMOS Image Sensor Dynamic Range
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Solution Overview
Problem
Sub-micron CMOS imaging devices face challenges such as reduced dynamic range, high pixel cross talk, low charge collection efficiency, and increased fixed pattern noise due to scaling limitations, which affect image quality and radiation tolerance.
Innovation Solution
The implementation of double-gated transistors with independently biased gates and variable threshold voltage control in a silicon-on-insulator (SOI) substrate, along with improved readout circuitry and reset methodologies, enhances charge collection and reduces noise and image lag.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If standard CMOS scaling is used to reduce pixel size, then manufacturing cost and power consumption are reduced, but charge collection efficiency decreases and cross talk increases
Solution Approach 1:
The patent applies local quality by creating a deeply-depleted charge collection region with high electric field strength in the pixel active volume, while maintaining standard CMOS scaling elsewhere. This localized high-field region prevents thermal diffusion of signal electrons to neighboring pixels and improves charge collection efficiency without requiring non-standard CMOS processes.
Solution Approach 2:
The patent introduces a vertical dimension solution by creating a deeply-depleted region that extends deep into the silicon substrate beneath the pixel surface. This vertical depletion structure matches the photon absorption depth and prevents lateral electron diffusion, solving the cross-talk problem caused by horizontal scaling.
2Object-generated harmful factors
If transistor threshold voltages are increased to reduce leakage current, then dark current is reduced, but dynamic range decreases
Solution Approach 1:
The patent implements dynamic threshold voltage control for the reset transistor, allowing the threshold to be adjusted based on operating conditions. During reset operations, a lower threshold enables complete charge transfer, while during signal integration, a higher threshold reduces leakage current. This dynamic adjustment maintains both low dark current and full dynamic range.
Solution Approach 2:
The patent changes the threshold voltage parameter of the reset transistor dynamically rather than using a fixed high threshold. By controlling the reset transistor threshold voltage to be lower during specific operations, the patent eliminates the trade-off between leakage reduction and dynamic range preservation.
3Ease of manufacture
If shallow junctions and high doping are used in sub-micron CMOS, then manufacturing is simplified, but photoresponsivity decreases
Solution Approach 1:
The patent segments the silicon substrate into distinct functional regions: a standard sub-micron CMOS region for circuit fabrication with shallow junctions, and a deeply-depleted charge collection region beneath it. This segmentation allows standard manufacturing processes to be used while maintaining high photoresponsivity in the dedicated collection region.
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 improves the dynamic range, reduces noise and image lag, and increases radiation tolerance, leading to higher quality images with improved sensitivity and signal-to-noise ratio.
Implementation Method 1
The PD region is responsible for generating and collecting signal charge
Data Source
AI summary
A CMOS image sensor array has rows and columns of active pixels, and column lines in communication with the active pixels in the respective columns. Each active pixel has an output connected to a column line and includes a photodetector that produces a signal proportional to incident light intensity that is coupled to an active pixel output based on column select and row select signals. Each active pixel has a reset transistor for resetting the active pixel, wherein each reset transistor has a first gate terminal and a second gate terminal. The reset transistors have a variable threshold capability that allows increased sensor array dynamic range or mitigation of the effects of temperature or radiation induced transistor threshold voltage shifts. Row select, column select, and sense transistors can also be configured to have variable thresholds.