BCMD Transistor Global Shutter Pixel Design
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Solution Overview
Problem
CMOS image sensors face challenges in achieving global shuttering without increasing pixel size, as existing methods require additional charge storage sites that consume valuable area and generate noise, while also suffering from exposure time skew in rolling shutter mode.
Innovation Solution
The use of BCMD transistors with lateral or vertical reset capability, allowing for charge storage and sensing in the same structure, reducing noise and pixel area, and enabling operation in both rolling and global shutter modes with multiple charge storage sites per pixel and variable conversion gain through concentric BCMD transistor gates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If additional charge storage sites are added to achieve global shuttering, then global shutter capability is improved, but pixel area increases
Solution Approach 1:
The patent merges the charge storage function and charge sensing function into a single BCMD transistor structure. The bulk charge storage region is formed within the transistor body, allowing simultaneous charge accumulation and threshold voltage modulation without requiring separate storage sites, thereby achieving global shutter capability without increasing pixel area
Solution Approach 2:
The BCMD transistor serves multiple functions: it acts as both the charge storage site (through its bulk region) and the charge sensing element (through threshold voltage modulation). This multi-functional design eliminates the need for dedicated separate storage structures, resolving the area constraint while maintaining global shutter capability
2Adaptability or versatility
If additional charge storage sites are added to achieve global shuttering, then global shutter capability is improved, but noise increases
Solution Approach 1:
The patent converts the typically harmful kTC reset noise into a beneficial signal by using the same BCMD transistor for both charge storage and sensing. The threshold voltage modulation directly reflects the stored charge amount, transforming what would be noise in traditional architectures into the useful sensing mechanism, thereby achieving global shutter without additional noise
Solution Approach 2:
By merging storage and sensing in the same BCMD transistor, the patent eliminates the need for separate readout circuitry that would introduce additional noise sources. The direct threshold voltage modulation provides a clean signal path from charge storage to measurement
3Area of stationary object
If rolling shutter mode is used to reduce pixel area, then pixel area is reduced, but exposure time skew occurs causing distortion
Solution Approach 1:
The patent implements preliminary charge transfer to the BCMD transistor before the readout process begins. All pixels complete their integration and transfer charges simultaneously to their respective BCMD transistors, establishing a frozen charge state that represents the entire scene at one moment in time. This preliminary action enables global shutter functionality without requiring increased pixel area
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 solution enables high-performance CMOS image sensors to operate in both shutter modes without increasing pixel size, reducing noise, and providing variable conversion gain, thus improving image quality and reducing distortion from moving objects.
Implementation Method 1
Typical image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels
Implementation Method 2
charge is stored in a bulk charge storage region of the BCMD transistor and the stored charge modulates a threshold voltage of the BCMD transistor
Data Source
AI summary
The invention describes image sensor array pixels with global and rolling shutter capabilities that utilize multiple BCMD transistors for storing and sensing charge for a single photodiode. This configuration improves the Dynamic Range (DR) of the sensor, by allowing sensing different image signals from a single pixel without saturation, a low level signal with long integration time followed by a high level signal with short integration time. Signal processing circuits can process these signals into a single Wide Dynamic Range (WDR) output. Further disclosed are pixels that use multiple-gate BCMD transistors for charge storage and sensing having multiple concentric gates, which allows changing the conversion gain of the BCMD transistors. Variable conversion gain is a useful feature when building WDR sensors since low conversion gain and high well capacity allows detection of high level signals and, at the same time, low level signals with high conversion gain and low noise.