Global Shutter Image Sensor Overflow Voltage Control
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Solution Overview
Problem
Conventional global shutter CMOS image sensors experience image quality degradation due to leakage currents caused by hot carrier generation flowing into photoelectric conversion devices through overflow transistors, even when these transistors are turned off during the photoelectric conversion period.
Innovation Solution
The implementation of an image sensor with an overflow power voltage control unit that sets the overflow power voltage to a low level during the photoelectric conversion period and a high level during the readout period, preventing hot carrier-induced leakage currents by controlling the voltage applied to the overflow transistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an overflow transistor is used in a unit pixel to prevent overflow in the photoelectric conversion device, then the reliability of the image sensor is improved, but image quality degradation occurs due to leakage current from hot carrier generation even when the overflow transistor is turned off
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the power supply voltage to the overflow transistor based on the operational phase. During the photoelectric conversion period, a first (lower) power supply voltage is applied to minimize hot carrier generation and leakage current. During the readout period, a second (higher) power supply voltage is applied to ensure proper transistor operation. This voltage parameter modulation resolves the contradiction by reducing harmful leakage effects when the transistor is off while maintaining reliability when needed.
2Object-affected harmful factors
If the overflow transistor is turned off during the photoelectric conversion period, then leakage current is reduced, but the transistor cannot prevent overflow when it should be active
Solution Approach 1:
The patent implements dynamics by making the overflow transistor's power supply voltage variable rather than static. The voltage dynamically switches between two levels based on the operational phase: a lower voltage during photoelectric conversion to reduce leakage, and a higher voltage during readout to ensure the transistor can effectively prevent overflow. This dynamic voltage adjustment allows the system to optimize both leakage reduction and overflow protection capability at different times.
3Reliability
If a high power supply voltage is applied to the overflow transistor to ensure it functions properly during readout, then the transistor can prevent overflow effectively, but hot carrier generation and leakage current increase during photoelectric conversion
Solution Approach 1:
The patent applies periodic action by rhythmically switching the power supply voltage to the overflow transistor according to the operational phases. During the photoelectric conversion period, a lower voltage is applied to minimize hot carrier generation. During the readout period, a higher voltage is applied to ensure proper transistor function for overflow prevention. This periodic voltage modulation aligns with the operational cycles of the image sensor, resolving the contradiction between preventing hot carrier generation and ensuring transistor functionality.
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 image quality degradation by minimizing leakage currents through the overflow transistor, even when it is off during the photoelectric conversion period, thereby enhancing image quality in global shutter image sensors.
Implementation Method 1
a photoelectric conversion device configured to convert the incident light into charges and to generate the accumulated charges by accumulating the charges
Data Source
AI summary
An image sensor is provided including a pixel array, a correlated double sampling (CDS) unit, an analog-digital converting (ADC) unit, a control unit, and an overflow power voltage control unit. The pixel array includes at least one unit pixel that generates accumulated charges corresponding to incident light in a photoelectric conversion period and outputs an analog signal based on the accumulated charges in a readout period. The CDS unit generates an image signal by performing a CDS operation on the analog signal. An ADC unit converts the image signal into a digital signal. A control unit controls the pixel array, the CDS unit, and the ADC unit. An overflow power voltage control unit controls an overflow power voltage to have a low voltage level in the photoelectric conversion period and controls the overflow power voltage to have a high voltage level in the readout period.