CMOS Imager Pixel Dark Current Suppression via Transfer Gate Biasing
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Solution Overview
Problem
Conventional CMOS image sensors suffer from dark current issues due to factors like photosensor junction leakage and fabrication defects, which degrade image quality and can lead to blooming effects, where excess electrons are collected in undesirable regions.
Innovation Solution
Applying a small negative voltage followed by a series of positive voltage pulses to the transfer transistor gate during the charge integration period reduces dark current by promoting electron recombination and creating a path for dark electrons to reach the floating diffusion region without increasing blooming.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CMOS image sensor structures are used, then device complexity is reduced and manufacturing is easier, but dark current increases and image quality degrades
Solution Approach 1:
The patent applies preliminary action by pre-biasing the transfer transistor gate to a negative voltage potential during the integration period before charge transfer occurs. This negative bias is established in advance to suppress dark current generation at the photosensor junction and along isolation edges, preventing harmful effects before they can accumulate and degrade image quality.
Solution Approach 2:
The patent changes the electrical parameter of the transfer transistor gate voltage from the conventional zero or positive bias to a negative voltage potential. This parameter change fundamentally alters the electric field distribution in the pixel, suppressing dark current generation mechanisms while maintaining proper charge transfer functionality when needed.
2Reliability
If methods to reduce dark current are applied, then image quality improves, but blooming effects may increase
Solution Approach 1:
The patent employs dynamic voltage control by switching the transfer transistor gate between negative bias during integration (to suppress dark current) and positive bias during transfer (to enable charge transfer). This dynamic adjustment allows the system to suppress dark current without permanently blocking the charge transfer path, preventing blooming while maintaining image quality.
Solution Approach 2:
The patent applies periodic voltage switching to the transfer transistor gate, alternating between negative bias during the integration period and positive bias during the transfer period. This periodic action rhythmically suppresses dark current when not transferring charge while enabling efficient charge transfer when needed, avoiding both dark current accumulation and blooming.
3Object-generated harmful factors
If negative voltage is applied to transfer transistor gate, then dark current is reduced through electron recombination, but charge transfer efficiency may be affected
Solution Approach 1:
The patent uses dynamic voltage switching to adjust the transfer transistor gate bias according to operational phase: negative voltage during integration to suppress dark current, and positive voltage during transfer to maximize charge transfer efficiency. This dynamic adaptation resolves the contradiction by applying the appropriate voltage condition at the appropriate time.
Solution Approach 2:
The patent implements periodic voltage switching synchronized with the pixel operation cycle, applying negative bias during the integration period to reduce dark current and switching to positive bias during the transfer period to ensure efficient charge transfer. This periodic modulation allows both dark current suppression and high transfer efficiency to be achieved in their respective time windows.
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 method effectively decreases dark current accumulation without reducing pixel capacity and minimizes blooming, as demonstrated by experimental histograms showing reduced dark current across the test pixel array.
Implementation Method 1
When a small negative voltage is applied to the transfer gate, electrons that would normally create dark current problems will instead recombine with holes thereby substantially reducing dark current
Data Source
Figure 1A
Figure 1B~1C
Figure 2
AI summary
A method and apparatus for operating an imager pixel that includes the act of applying a relatively small first polarity voltage and a plurality of pulses of a second polarity voltage on the gate of a transfer transistor during a charge integration period.