Image Sensor Pixel Bias Timing for Dark Current Suppression
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Solution Overview
Problem
CMOS image sensors generate noise due to dark current and leakage current, which affect image quality and signal-to-noise ratio, especially in low-illumination environments.
Innovation Solution
An image sensor with a pixel array and row driver that adjusts the timing and size of negative voltage applied to the bulk area and device isolation structure to control dark current and leakage current, using a first bulk control voltage during initialization and a second bulk control voltage during charge accumulation, and reverting to the first voltage for signal output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a low voltage is applied to the pixel to reduce power consumption, then power consumption is reduced, but the dark current increases
Solution Approach 1:
The patent applies a first bulk control voltage (more negative) specifically during the reset phase before photocharge accumulation begins. This preliminary action of applying stronger negative voltage during reset effectively suppresses dark current generation and clears residual charges, preparing the pixel for accurate measurement. After reset, the voltage is reduced to a second level during accumulation to maintain low power consumption while accepting minimal dark current since the pixel is already initialized.
Solution Approach 2:
The patent applies a stronger negative bulk control voltage during the reset phase to preemptively counteract dark current generation before it can accumulate and affect the measurement. This preliminary anti-action clears dark current and residual charges from the photodiode and transistor, ensuring that subsequent measurements are not contaminated. After this preliminary cleanup, the voltage is reduced to maintain low power consumption during the actual photocharge accumulation phase.
2Device complexity
If the timing of bulk control voltage is not adjusted, then the circuit operation is simple, but the signal-to-noise ratio deteriorates
Solution Approach 1:
The patent implements dynamic timing adjustment of the bulk control voltage that is synchronized with the pixel operation phase. The control circuit switches between a first bulk control voltage during reset and a second bulk control voltage during photocharge accumulation and readout. This dynamic timing optimization reduces dark current during reset and minimizes transistor leakage current during readout, thereby improving the signal-to-noise ratio without requiring complex additional hardware beyond standard voltage switching capability.
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 leakage current, improving the high-illumination signal-to-noise ratio and dynamic range while preventing charge accumulation and leakage, thereby enhancing image sensor performance.
Implementation Method 1
The image sensor generates images of the object by using a photoelectric transformation element that reacts to the intensity of light reflected from the object
Data Source
AI summary
An image sensor including: a pixel array including a plurality of pixels and a row driver configured to drive the pixel array, wherein each of the plurality of pixels includes at least one photodiode, a transmission transistor, a selection transistor, a device isolation structure, and a bulk area, and the row driver is configured to adjust, for each of preset periods, sizes and application timings of a negative voltage applied to the device isolation structure and a bulk control voltage applied to the bulk area while a first pixel is driven.