CMOS Image Sensor Pixel Circuit with Charge Pump and Drive Transistor
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Solution Overview
Problem
Existing CMOS image sensors require a select transistor for each pixel, which complicates the manufacturing process and increases costs, while also limiting the fill factor and simplifying the layout of the unit pixel.
Innovation Solution
The image sensor design excludes the select transistor and expands the function of the reset transistor by using a charge pump circuit to control the drive transistor, allowing for three-transistor operations with additional gate signals from an external device to manage the floating diffusion region and output signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a select transistor is included in each pixel of the CMOS image sensor, then the pixel can be controlled to output signals, but the manufacturing process becomes complicated and costs increase
Solution Approach 1:
The patent removes the select transistor from the pixel structure, extracting this component from the system. The functions previously performed by the select transistor are redistributed to the reset transistor and drive transistor, simplifying the pixel structure while maintaining signal output control capability
Solution Approach 2:
The reset transistor is assigned multiple functions: it serves as both the reset transistor and the select transistor. By controlling the reset transistor with multiple control signals (first control signal for reset function, second control signal for select function), a single transistor performs the work of what would traditionally require two transistors
2Reliability
If a select transistor is included in each pixel, then signal output can be controlled, but the fill factor is reduced and layout is simplified
Solution Approach 1:
By removing the select transistor from the pixel structure, the physical area occupied by this component is eliminated, directly increasing the fill factor (the ratio of photodetector area to total pixel area). The extraction of this unnecessary component allows more space for light-sensitive elements
Solution Approach 2:
The reset transistor performs dual functions as both reset and select transistor, reducing the total transistor count from four to three per pixel. This reduction in component count directly increases the available area for the photodetector, improving the fill factor
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 approach simplifies the manufacturing process, reduces costs, and enhances the fill factor by allowing the drive transistor to perform the function of the select transistor, while maintaining effective control over the operation of the image sensor.
Implementation Method 1
a charge pump circuit which supplies control signals having various levels and acting as a terminal to drain charges of a floating diffusion region
Implementation Method 2
a photodiode (PD) serving as a photoelectric conversion section
Data Source
AI summary
An image sensor includes a charge pump circuit supplying first to third signals having sequentially decreasing voltage levels, a reset transistor having a drain and a gate connected with the charge pump circuit to form a diode connection and receiving the first to third signals, a photodiode generating photocharges, a transfer transistor forming a series connection between the photodiode and the reset transistor, a floating diffusion region forming a parallel connection between the transfer transistor and the reset transistor and storing the photocharges, and a drive transistor connected with the floating diffusion region, the reset transistor, a power supply voltage terminal, and a unit pixel output terminal. A gate of the transfer transistor receives a turn-off voltage if the first or second signal is supplied, and receives a turn-off voltage or a turn-on voltage if the third signal is supplied.