Image Sensor Pixel Circuit With Switchable Conversion Gain
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Solution Overview
Problem
Current image sensors, particularly CMOS image sensors, face challenges in optimizing conversion gain for varying operating modes, which affects signal-to-noise ratio and dynamic range in image capturing and recognition modes.
Innovation Solution
The image sensor design includes a pixel structure with multiple floating diffusion regions and transistors that allow for adjustable conversion gain by varying the size of the floating diffusion region and capacitance, enabling optimal performance across different operating modes through the connection of floating diffusion nodes using metal lines and control of conversion gain transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the floating diffusion region size is increased to improve dynamic range, then the conversion gain decreases, reducing signal-to-noise ratio in low-light conditions
Solution Approach 1:
The patent implements a dual-gate transistor structure where the first gate controls the transfer of photocharges from the photodiode to the floating diffusion region, and the second gate controls the connection between the first and second floating diffusion regions. This dynamic control allows the system to adjust the effective capacitance of the floating diffusion region based on operating conditions, thereby optimizing both dynamic range and signal-to-noise ratio as needed.
Solution Approach 2:
The patent changes the electrical parameters of the pixel circuit by introducing a second floating diffusion region that can be electrically connected or disconnected from the first floating diffusion region through the dual-gate transistor. By controlling the connection state and adjusting the capacitance values, the system can switch between high-gain mode (smaller effective capacitance for better signal-to-noise ratio) and high dynamic range mode (larger effective capacitance for greater dynamic range).
2Adaptability or versatility
If multiple floating diffusion regions are introduced to enable conversion gain control, then the pixel circuit complexity increases, but the manufacturing precision and adaptability improve
Solution Approach 1:
The dual-gate transistor structure serves multiple functions: it controls photocharge transfer to the floating diffusion region, it regulates the connection between the first and second floating diffusion regions, and it enables conversion gain adjustment. This multi-functionality reduces the need for additional dedicated control circuits, thereby limiting the increase in overall pixel circuit complexity while achieving adaptability.
Solution Approach 2:
The patent merges the control functions into a unified dual-gate transistor structure where both gates work together to control the pixel circuit operations. The first floating diffusion region and second floating diffusion region are merged through the transistor connection to form an integrated capacitance system that can be dynamically adjusted, reducing the need for separate control mechanisms.
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 design enhances the signal-to-noise ratio and dynamic range by allowing the image sensor to adapt conversion gain based on operating modes, optimizing performance in both image capturing and recognition modes.
Implementation Method 1
each of the first pixel and the second pixel includes, at least one photodiode
Data Source
AI summary
An image sensor includes: a first pixel connected to a column line; and a second pixel connected to the column line. Each of the first pixel and the second pixel includes: one photodiode; a first floating diffusion region; a second floating diffusion region; one first transistor connected between the one photodiode and the first floating diffusion region; a second transistor connected between the first floating diffusion region and the second floating diffusion region; a third transistor connected to the second floating diffusion region; a fourth transistor including a gate connected to the first floating diffusion region; and a fifth transistor including a drain connected to a source of the fourth transistor and a source connected to the column line. The second floating diffusion region of the first pixel and the second floating diffusion region of the second pixel are electrically connected without an intermediate transistor.