CMOS Image Sensor Level Shifting for Low-Noise Pixel Scanning
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Solution Overview
Problem
Conventional solid-state imaging devices experience degradation in image quality due to increased potential differences between control pulses, leading to random noise when expanding the operation margin of pixels.
Innovation Solution
A solid-state imaging device with a pixel unit on a semiconductor substrate, including a receiving unit, transfer transistor, charge detection unit, reset transistor, and a vertical scanning unit that utilizes level shift circuits and buffer circuits to convert and buffer power supply voltages, ensuring increased voltage amplitudes for transfer and reset transistors while minimizing noise transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the potential differences between control pulses are increased to expand the operation margin of pixels, then the operation margin is improved, but the through-currents in level shift circuits increase causing random noise and image quality degradation
Solution Approach 1:
The level shift circuit is divided into multiple independent circuits (first level shift circuit for transfer transistor, second level shift circuit for reset transistor, third level shift circuit for selection transistor). Each circuit operates independently with its own power supply voltage, preventing noise coupling and allowing optimized voltage levels for each transistor type.
Solution Approach 2:
Different power supply voltages are applied to different level shift circuits based on their specific requirements. The first level shift circuit uses a higher power supply voltage to drive the transfer transistor effectively, while other circuits use appropriate voltages for their functions. This localized voltage optimization expands operation margins without causing excessive currents in all circuits.
2Strength
If the power supply voltage is increased to provide sufficient voltage amplitude for transfer and reset transistors, then the transistor operation is improved, but the currents through level shift circuits increase causing noise
Solution Approach 1:
The power supply system is segmented into multiple independent power supply voltage sources (first power supply voltage, second power supply voltage, third power supply voltage). Each level shift circuit uses the appropriate voltage source for its specific transistor type, providing sufficient voltage amplitude where needed while avoiding excessive currents in other circuits.
Solution Approach 2:
The patent changes the voltage parameter by introducing multiple power supply voltages with different amplitude levels. The first power supply voltage has a larger amplitude for transfer transistor operation, while second and third power supply voltages have appropriate amplitudes for reset and selection transistors. This parameter differentiation allows adequate voltage drive without proportional current increase across all circuits.
3Reliability
If the level shift circuits are designed to provide large voltage differences for pixel operation, then the pixel operation margin is improved, but the noise transmission to pixel unit increases
Solution Approach 1:
The level shift circuits are segmented into separate independent circuits, each handling specific transistor control. This segmentation isolates the high-voltage-difference first level shift circuit from other circuits, preventing noise generated by large voltage swings from coupling into circuits that directly control pixel operations.
Solution Approach 2:
The buffer circuit acts as an intermediary between the level shift circuits and the pixel unit. It isolates the pixel unit from direct connection to level shift circuits with large voltage differences, filtering and conditioning the signals to prevent noise transmission while maintaining the beneficial voltage amplitudes for proper transistor operation.
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 prevents image quality degradation from random noise while expanding the operation margin of pixels, reducing through-currents and noise transmission to the pixel unit.
Implementation Method 1
a receiving unit which stores signal charges obtained by photoelectrically converting incident light
Data Source
AI summary
A solid-state imaging device according to the present disclosure includes: a pixel unit in which unit pixels are arranged two-dimensionally, each of the unit pixels including: a photodiode which stores signal charges; a transfer transistor for transferring the signal charges stored in the photodiode; a charge detection unit which temporarily stores the transferred signal charges; and a reset transistor for resetting the signal charges stored in the charge detection unit; and a vertical scanning unit which drives the pixel unit, the vertical scanning unit including: a row selection unit; a level shift circuit for converting a level of an externally inputted power supply voltage; and a buffer circuit for buffering a voltage whose level has been converted by the level shift circuit, the level shift circuit including: a step-down level shift circuit; and a step-up level shift circuit isolated from the step-down level shift circuit by a well.