Active CMOS Pixel Structure for Wide Dynamic Range Imaging
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Solution Overview
Problem
Conventional CMOS cameras face challenges with dynamic range and illumination variations, leading to image saturation and loss of information, especially in low-light conditions, due to limitations in photodiode operation and transistor configurations.
Innovation Solution
An active pixel structure with a photodiode that allows reading of voltage polarity variations during exposure, utilizing a depleted NMOS transistor with negative threshold voltage and capacitive coupling, enabling operation across linear, mixed, and logarithmic areas to improve signal-to-noise ratio and compactness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If both NMOS and PMOS transistors are present within a same pixel, then the pixel can achieve proper transistor functionality, but it requires an insulation case causing loss of useful surface area
Solution Approach 1:
The patent merges both NMOS and PMOS transistors within a single insulation case structure, allowing them to share the same isolation region. This combining approach enables full transistor functionality while minimizing the total area occupied by the insulation case, thereby resolving the contradiction between functional versatility and surface area utilization.
2Adaptability or versatility
If the photodiode operates in photovoltaic mode, then it generates voltage proportional to logarithm of illumination level, but the output signal has very low amplitude limiting signal-to-noise ratio
Solution Approach 1:
The patent implements dynamic operation of the photodiode, allowing it to switch between photovoltaic mode (for high dynamic range and logarithmic response) and photodetector mode (for high signal amplitude and good signal-to-noise ratio). This dynamic adaptability enables the system to optimize between functional dynamics and measurement precision depending on illumination conditions.
3Reliability
If the short-circuit switching transistor has residual conductivity during disabling, then it affects submicron CMOS technology performance, but eliminating it requires additional design complexity
Solution Approach 1:
The patent addresses the residual conductivity issue by carefully controlling the transistor design parameters, specifically the width-to-length ratio (W/L) of the switching transistor. By optimizing these parameters, the patent reduces residual conductivity to acceptable levels for submicron CMOS technology without introducing additional complex circuit structures, thereby resolving the contradiction between reliability and device complexity.
4Measurement precision
If only linear area of photodiode voltage variation is utilized, then good image quality is obtained, but the dynamic range remains small
Solution Approach 1:
The patent utilizes the complete dynamic range of the photodiode by allowing operation across linear, mixed, and logarithmic areas. The system dynamically adapts to different illumination levels, operating in linear mode for low light (maintaining good image quality) and transitioning to logarithmic mode for high light (extending dynamic range), thereby resolving the contradiction between image quality and dynamic range.
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 provides high functional dynamics and improved signal-to-noise ratio, allowing for better image quality and extended dynamic range without the need for insulation cases, thus enhancing the performance of CMOS cameras in varying illumination conditions.
Implementation Method 1
a photodiode generates a negative voltage in an open circuit, the absolute value of which is proportional to the logarithm of the illumination level of the photodiode
Implementation Method 2
utilizing a depleted NMOS transistor with negative threshold voltage and capacitive coupling
Data Source
AI summary
The invention relates to a structure of an active pixel of the CMOS type (1) that comprises: at least one photodiode (10), characterized in that it comprises means for reading any bias voltage in the evolution phase of the photodiode (10) upon exposure.