CMOS Image Sensor Boosted Voltage Charge Transfer
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Solution Overview
Problem
Conventional CMOS image sensors suffer from inefficient charge transfer, leading to afterimage effects and reduced charge integration capacity due to residual charge in the photodiode, which complicates image read operations and reduces image quality.
Innovation Solution
A CMOS image sensor design that employs a boosted voltage signal only during the charge transfer period, using a boosting capacitor and loading capacitance to generate a voltage higher than the power voltage, which is selectively applied to the charge transfer element to enhance charge transfer efficiency without increasing the overall size or complexity of the sensor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional charge transfer element is used in a CMOS image sensor, then the device complexity is low and manufacturing is simple, but charge transfer efficiency is poor resulting in afterimage effects and residual charge
Solution Approach 1:
The charge transfer element uses a dynamic voltage boosting mechanism where a boosting capacitor is charged during the integration period and then discharged during the transfer period to provide a temporary voltage boost. This dynamic approach enhances charge transfer efficiency without requiring a permanently complex structure, as the boosting circuitry is activated only when needed.
Solution Approach 2:
A boosting capacitor is introduced as an intermediary element between the power supply and the charge transfer element. This capacitor stores energy and releases it during the charge transfer period to enhance the transfer voltage, thereby improving charge transfer efficiency without directly modifying the fundamental pixel structure.
2Reliability
If a boosted voltage signal is applied continuously to the charge transfer element, then charge transfer efficiency is improved, but power consumption increases and the sensor size increases
Solution Approach 1:
The boosting capacitor is charged and discharged periodically in synchronization with the image capture and transfer cycles. During the integration period, the capacitor is charged; during the transfer period, it discharges to boost the transfer voltage. This periodic operation provides enhanced charge transfer efficiency only when needed, avoiding continuous power consumption.
Solution Approach 2:
The boosting capacitor recovers and stores energy during the integration period when charge accumulation occurs, then discards this stored energy in the form of a voltage boost during the transfer period. This energy recycling approach minimizes overall power consumption by utilizing the existing power supply voltage during charging and only consuming additional energy when the boost is actually needed for transfer.
3Reliability
If a boosted voltage signal is applied continuously to the charge transfer element, then charge transfer efficiency is improved, but the sensor size increases
Solution Approach 1:
The boosting circuitry operates dynamically only during the charge transfer period rather than being permanently active. The boosting capacitor is charged during integration and discharged during transfer, providing the necessary voltage enhancement temporarily. This dynamic operation allows the use of smaller capacitor values compared to a continuously operating boost circuit, thereby reducing the overall sensor area.
4Device complexity
If conventional charge transfer is used, then the sensor design is compact and simple, but afterimage effects occur due to residual charge in the photodiode
Solution Approach 1:
The boosting capacitor serves as an intermediary that temporarily enhances the transfer voltage during the charge transfer period. This voltage enhancement ensures complete evacuation of charge from the photodiode to the floating diffusion, eliminating residual charge that causes afterimage effects, while the capacitor itself occupies minimal space compared to other possible solutions.
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 effectively reduces afterimage effects, enhances charge integration capacity, and improves image quality by ensuring complete charge transfer from the photoelectric conversion element to the charge detection element, while maintaining a compact sensor design.
Implementation Method 1
using a boosting capacitor and loading capacitance to generate a voltage higher than the power voltage
Implementation Method 2
Each pixel within the plurality of pixels forming the pixel array typically includes a photoelectric conversion element capable of accumulating a quantity of electrical charge in relation to an amount of detected energy (e.g., visible light, etc.). When photons impact the surface of a photoelectric conversion element, free charge carriers are produced.
Data Source
AI summary
Provided is a complementary metal oxide semiconductor (CMOS) image sensor. The CMOS image sensor includes a pixel array unit having a matrix-type array of unit pixels, each unit pixel including a charge transfer element transferring charge collected in a photoelectric conversion element to a charge detection element. The charge transfer element also receives a boosted voltage signal higher than an external power voltage.