Image Sensor Transfer Transistor Pseudo Pinch-Off Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional CMOS image sensors face issues with unstable reset and transfer operations due to decreased operating voltage, leading to increased dark current and fixed pattern noise, especially when the photodiode is not fully depleted, which affects image quality and noise characteristics.
Innovation Solution
The implementation of a pseudo pinch-off mode for the transfer transistor, achieved through an intermediary circuit that adjusts the driving switching signals, including a turn-on level down shift circuit and rising relief circuit, to maintain the transfer transistor in a stable pseudo pinch-off state, reducing the impact of electron overflow and improving reset and transfer conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If operating voltage is decreased to reduce power consumption, then power efficiency is improved, but reset and transfer operations become unstable leading to increased dark current and noise
Solution Approach 1:
The patent changes the voltage parameter by introducing a boosted voltage (VDD_BOOST) that is higher than the normal operating voltage (VDD). This boosted voltage is applied specifically to the gate of the transfer transistor during reset and transfer operations, allowing these critical operations to occur at a higher voltage level even when the overall system operates at low voltage for power efficiency. The intermediary circuit dynamically adjusts the voltage level based on the operational phase.
Solution Approach 2:
The patent introduces an intermediary circuit that acts as a mediator between the low-voltage power supply and the transfer transistor gate. This intermediary circuit includes voltage boosting functionality that temporarily raises the gate voltage to VDD_BOOST during critical operations, then returns it to normal levels. This intermediary mechanism enables the transfer transistor to achieve proper depletion and transfer at low operating voltages without compromising reset and transfer stability.
2Device complexity
If conventional voltage levels are used for transfer transistor operation, then circuit simplicity is maintained, but electron overflow occurs and dark current increases
Solution Approach 1:
The patent modifies the gate voltage parameter of the transfer transistor by introducing a boosted voltage level (VDD_BOOST) that exceeds the normal operating voltage. This parameter change ensures the transfer transistor achieves sufficient depletion depth during reset operations, preventing electron overflow into the photodiode and thereby reducing dark current generation. The boosted voltage creates a higher potential barrier that confines electrons properly.
Solution Approach 2:
The patent applies preliminary anti-action by using the boosted gate voltage to preemptively prevent electron overflow before it can occur. During the reset phase, the elevated voltage establishes a strong depletion region in the transfer transistor channel that acts as a barrier, preventing electrons from the photodiode from leaking into the transfer transistor. This preliminary establishment of proper depletion conditions eliminates the harmful electron overflow effect.
3Use of energy by moving object
If photodiode is not fully depleted during reset, then power consumption is reduced, but fixed pattern noise increases due to inconsistent charge transfer
Solution Approach 1:
The patent changes the voltage parameter applied to the transfer transistor gate during reset operations by introducing a boosted voltage (VDD_BOOST) that is higher than the normal operating voltage. This parameter change ensures consistent and complete depletion of the transfer transistor channel, enabling reliable charge transfer from the photodiode to the floating diffusion node. The higher voltage guarantees that the transfer transistor acts as an effective barrier, preventing residual charge from remaining in the photodiode and causing fixed pattern noise.
Solution Approach 2:
The patent introduces an intermediary voltage boosting circuit that mediates between the low-power operating state and the high-voltage reset state. This intermediary circuit provides the necessary voltage elevation only during critical reset and transfer operations, ensuring complete photodiode depletion and consistent charge transfer. After these operations, the voltage returns to normal levels, maintaining low power consumption during other operational phases. This intermediary mechanism reconciles the conflicting requirements of complete depletion and power efficiency.
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 suppresses dark current and noise, even in low operating voltage environments, while maintaining image quality by ensuring consistent reset and transfer operations, and can be implemented with minimal modification to existing image sensor structures.
Implementation Method 1
a photodiode for generating a photoelectron
Data Source
AI summary
An image sensor operated in a pseudo pinch-off condition capable of reducing a reset voltage of a photodiode and reducing a dark current and fixed pattern noise generated due to discordance of characteristics between pixels is presented. The image sensor has a photosensitive pixel, a driving circuit and an intermediary circuit. The photosensitive pixel can have a photodiode generating a photoelectrons, a transfer transistor transferring the photoelectrons to a diffusion node, and a reset transistor resetting the diffusion node. The driving circuit generates a driving switching signal with respect to the transfer and resist transistors. The intermediary circuit changes characteristics of the signal to drive the photosensitive pixel in a pseudo pinch-off mode.