CMOS Image Sensor High Voltage Transfer Transistor Dynamic Range
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Solution Overview
Problem
CMOS image sensors with NMOS transistors face a dynamic range reduction due to threshold voltage limitations, leading to decreased image quality under low light conditions, as they cannot transfer signals with voltage levels lower than the threshold voltage.
Innovation Solution
A CMOS image sensor with high voltage supply circuits, including a reference high voltage generator and a high voltage output unit, generates and stabilizes a high voltage level for the unit pixel, ensuring it exceeds the power voltage level, thereby eliminating voltage loss and enhancing dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If NMOS transistors are used in unit pixels, then the circuit can be simplified and manufactured using standard CMOS technology, but the dynamic range decreases due to threshold voltage limitations
Solution Approach 1:
The patent applies parameter changes by introducing a high voltage supply (e.g., 5V or higher) to the gate of the NMOS transfer transistor, which changes the operating parameters of the transistor. This high voltage gate signal overcomes the threshold voltage limitation, enabling the transfer transistor to fully deplete the channel and transfer all accumulated photocharges from the photodiode to the floating diffusion node, thereby extending the dynamic range while maintaining compatibility with standard CMOS manufacturing processes
2Manufacturing precision
If threshold voltage is reduced to increase dynamic range, then more signals can be transferred, but the transistor control capability deteriorates
Solution Approach 1:
Instead of changing the transistor's inherent threshold voltage parameter, the patent changes the gate voltage parameter to a high voltage level. This approach maintains the transistor's normal control characteristics and threshold voltage properties while providing sufficient drive strength to transfer all photocharges, thus preserving both dynamic range and transistor control capability
Solution Approach 2:
The patent segments the voltage supply into different levels: a high voltage supply for the transfer transistor gate and a normal voltage supply for other circuit elements. This segmentation allows the transfer transistor to operate with enhanced performance while other transistors maintain their normal control characteristics, preventing widespread deterioration of transistor control capability
3Manufacturing precision
If high voltage is applied to transfer all photocharges, then dynamic range increases, but power consumption increases
Solution Approach 1:
The high voltage is applied periodically only during the transfer phase when the transfer transistor needs to move photocharges from the photodiode to the floating diffusion node. During other phases (integration, reset, readout), the transfer transistor operates at normal voltage levels. This periodic application of high voltage extends dynamic range while minimizing additional power consumption
Solution Approach 2:
The high voltage supply is applied locally only to the gate of the transfer transistor, not to the entire pixel circuit or other transistors. This localized application ensures that only the specific component needing enhanced performance (the transfer transistor) receives the high voltage, thereby extending dynamic range while limiting the increase in overall power consumption to minimal levels
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 maintains a stable high voltage level for the transistors, allowing for increased electron transfer and improved image quality under low light conditions, reducing noise and enhancing overall image quality by compensating for threshold voltage losses.
Implementation Method 1
a high voltage output unit for generating the high voltage by using the reference high voltage as an operating voltage to thereby output the high voltage to the unit pixel, wherein a level of the high voltage is stably maintained regardless of a variations of the reference high voltage level
Implementation Method 2
The photodiode 10 accumulates electrons corresponding to an incident light
Data Source
AI summary
A CMOS image sensor includes a unit pixel including controlled by a high voltage; a reference high voltage generator for generating a reference high voltage; and a high voltage output unit for generating the high voltage by using the reference high voltage as an operating voltage to thereby output the high voltage to the unit pixel, wherein a level of the high voltage is stably maintained regardless of a variations of the reference high voltage level.