Global Shutter Pixel Gate Control for Higher Dynamic Range
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Solution Overview
Problem
The dynamic range of global shutter CMOS image sensors is limited due to the reduced effective area of photodiodes (PD) and memory (MEM) in planar pixels, leading to inferior saturation signal amounts and reduced imaging capabilities compared to CMOS image sensors with only PD.
Innovation Solution
Incorporating a second photoelectric conversion unit on the side of the light incident surface and a voltage application unit that adjusts the gate voltage of the first photoelectric conversion unit based on the charges accumulated by the second unit, allowing for increased saturation signal amounts while minimizing dark current generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a gate electrode and memory unit are added to create a global shutter CMOS image sensor, then the shutter function is improved, but the effective area of photodiodes is reduced and saturation signal amount deteriorates
Solution Approach 1:
The patent applies dynamics by making the gate electrode voltage adjustable and controllable. The gate electrode voltage is not fixed but can be dynamically changed based on accumulated charge amounts, allowing the system to adapt between different operating modes (accumulation mode with high voltage, readout mode with low voltage) to resolve the contradiction between maintaining global shutter function and preserving signal amount
Solution Approach 2:
The patent changes the voltage parameter of the gate electrode based on the accumulated charge amount. When charges are accumulated, a first voltage is applied; when reading out, a second voltage is applied. This parameter change allows the gate electrode to serve dual functions: maintaining charge during accumulation and facilitating charge transfer during readout, thereby resolving the area loss contradiction
2Adaptability or versatility
If photodiodes and charge storage units are divided and formed in planar pixels, then the global shutter function is achieved, but the effective area is reduced and dynamic range deteriorates
Solution Approach 1:
The gate electrode is designed to perform multiple functions: during the accumulation phase, it maintains the charge in the memory unit; during the readout phase, it facilitates charge transfer to the floating diffusion. This multi-functionality allows the structure to achieve global shutter capability while minimizing its impact on the effective photodetector area
Solution Approach 2:
The patent applies preliminary action by pre-charging the gate electrode to a first voltage level before charge accumulation begins. This preliminary preparation ensures that the memory unit is ready to hold charges immediately when photons are converted, enabling efficient use of the pixel area for both photodetection and charge storage functions
3Productivity
If gate voltage is applied to facilitate charge transfer, then charge transfer is improved, but dark current generation increases
Solution Approach 1:
The gate electrode voltage is dynamically adjusted based on the operational phase: a first voltage is applied during charge accumulation to maintain charges, and a second voltage is applied during readout to facilitate transfer. This dynamic control minimizes the time the gate is at high voltage, thereby reducing dark current generation while maintaining charge transfer efficiency
Solution Approach 2:
The control circuit determines the gate electrode voltage based on feedback regarding the accumulated charge amount. This feedback mechanism ensures that high voltage is only applied when necessary for charge transfer, and low voltage is maintained during accumulation to minimize dark current, thus resolving the contradiction between transfer efficiency and dark current suppression
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 expands the dynamic range of the imaging device by enhancing saturation signal amounts under high illuminance while suppressing dark current generation under low illuminance, thereby improving image quality.
Implementation Method 1
a first photoelectric conversion unit having a light incident surface on which light is incident
Implementation Method 2
a second photoelectric conversion unit provided on a side of the light incident surface with respect to the first photoelectric conversion unit
Data Source
AI summary
A solid-state imaging element according to an aspect of the present disclosure includes: a first photoelectric conversion unit (31) having a light incident surface on which light is incident; a first gate electrode (36) provided in the first photoelectric conversion unit (31) via an insulating film (35); a second photoelectric conversion unit (32) provided on a side of the light incident surface with respect to the first photoelectric conversion unit (31); and a voltage application unit that applies a voltage corresponding to the number of charges accumulated by the second photoelectric conversion unit (32) to the first gate electrode (36).