CMOS Imager Voltage Pumps for Transistor Leakage and Consistency
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Solution Overview
Problem
CMOS imagers face issues such as leakage in the transfer gate and a need for improved voltage management to account for pixel-to-pixel fabrication differences, which affect the performance and efficiency of the imaging process.
Innovation Solution
The use of voltage pumps to overdrive the reset, transfer, and row select transistors, as well as the incorporation of negative pumps and substrate pumps, to enhance the operational voltage and reduce inconsistencies in transistor performance, allowing for improved image acquisition and processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If voltage pumps are used to overdrive transistors, then transistor operation consistency and image quality improve, but device complexity and fabrication cost increase
Solution Approach 1:
The patent divides the voltage pumping function into separate dedicated pumps for different transistor groups: first voltage pump for reset transistors, second voltage pump for transfer transistors, third voltage pump for row select transistors. This segmentation allows each pump to be optimized for its specific function while maintaining overall system consistency, resolving the contradiction by making complexity manageable through functional division.
Solution Approach 2:
The patent applies different voltage pumping strategies to different parts of the imager: overdrive voltage for reset gates, different overdrive voltages for transfer gates, and specific voltage levels for row select gates. This local quality approach ensures each transistor group receives precisely tailored voltage characteristics, improving overall consistency without uniformly increasing complexity across the entire device.
2Productivity
If overdrive voltage is applied to transfer gate, then charge transfer efficiency improves, but power consumption increases
Solution Approach 1:
The patent employs dynamic voltage control for the transfer gate, where the second voltage pump provides overdrive voltage only during the charge transfer phase when needed, rather than maintaining constant overdrive voltage. This dynamic approach maximizes charge transfer efficiency during critical transfer operations while minimizing power consumption during other operational phases.
Solution Approach 2:
The transfer gate voltage is applied periodically in synchronization with the charge transfer timing, using the second voltage pump to provide overdrive voltage during specific transfer intervals. This periodic action ensures efficient charge transfer occurs when required while allowing power consumption to be reduced during non-transfer periods.
3Manufacturing precision
If multiple voltage pumps are implemented, then pixel-to-pixel fabrication differences are compensated, but manufacturing complexity increases
Solution Approach 1:
The patent uses multiple voltage pumps to dynamically adjust voltage parameters for different transistor groups, compensating for fabrication variations by providing tailored overdrive voltages. The first voltage pump adjusts reset gate voltages, the second adjusts transfer gate voltages, and the third adjusts row select gate voltages, allowing the system to compensate for pixel-to-pixel differences without requiring precise manufacturing control.
4Area of moving object
If transistor size is reduced, then integration density improves, but leakage current increases
Solution Approach 1:
The patent applies preliminary anti-action by using the first voltage pump to provide overdrive voltage to reset gates before charge transfer operations begin. This preliminary voltage application ensures that reset transistors are fully enhanced and properly biased, preventing leakage currents that would otherwise occur in smaller transistors with weaker control over their channel formation.
Solution Approach 2:
The patent changes the voltage parameter for reset gates using the first voltage pump, providing elevated overdrive voltages that compensate for the reduced transistor size. This parameter change ensures that even smaller transistors maintain adequate control over their channels, suppressing leakage current while allowing higher integration density.
Data Source
AI summary
A pixel for an imaging device is described. The pixel includes a photosensitive device provided within a substrate for providing photo-generated charges, a circuit associated with the photosensitive device for providing at least one pixel output signal representative of the photo-generated charges, the circuit includes at least one operative device that is responsive to a first control signal during operation of the associated circuit and a pump circuit. The pump circuit may include substrate pumps, charge pumps and/or voltage pumps. The pixel may also be embedded in an imaging system.