CMOS Image Sensor Pixel With Thin Oxide Read Transistor
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Solution Overview
Problem
CMOS image sensors face significant noise issues, particularly in low light conditions, due to sources like read noise and 1/f noise from the source-follower read transistor, which are not adequately addressed by existing noise reduction methods like CDS and CMS.
Innovation Solution
The design incorporates a CMOS image sensor pixel with a thin oxide read transistor, which increases gate-substrate surface capacitance and reduces 1/f noise, along with a pinched diode photodiode and specific transistor configurations to isolate and bias the read transistor within voltage constraints suitable for thin oxide operation, while maintaining dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional source-follower read transistor with standard oxide thickness is used, then the transistor can operate at higher voltages and maintain dynamic range, but it generates significant read noise including 1/f noise that degrades measurement precision
Solution Approach 1:
The transistor system is segmented into two distinct transistors: a thick-oxide transfer transistor for voltage control and a thin-oxide read transistor for low-noise signal reading. This segmentation allows each transistor to be optimized for its specific function, with the thin-oxide transistor providing reduced 1/f noise while the thick-oxide transistor handles the dynamic range requirements.
Solution Approach 2:
A thick-oxide transfer transistor acts as an intermediary between the photodiode and the thin-oxide read transistor. It transfers the photogenerated charge to the read node while isolating the thin-oxide transistor from high voltage stress, enabling the read transistor to operate at lower voltages with reduced noise without compromising the dynamic range of the overall pixel.
2Measurement precision
If a thin oxide read transistor is used to reduce 1/f noise, then read noise is significantly reduced, but the transistor cannot withstand the full voltage range required for dynamic range operation
Solution Approach 1:
The voltage handling function is segmented from the noise-critical read operation. The thick-oxide transfer transistor handles the full voltage range for dynamic range adaptation, while the thin-oxide read transistor operates only in the lower voltage regime necessary for low-noise signal reading, thus resolving the conflict between noise performance and voltage adaptability.
Solution Approach 2:
The thick-oxide transfer transistor serves as a voltage intermediary that buffers the thin-oxide read transistor from high voltage conditions. It transfers charge at the required voltage levels while presenting a lower voltage environment to the read transistor, enabling the thin-oxide device to achieve its low-noise potential without being constrained by voltage breakdown limitations.
3Adaptability or versatility
If the body region of the read transistor is isolated and biased at a different potential, then the thin oxide transistor can operate within voltage constraints, but the transistor configuration becomes more complex
Solution Approach 1:
The body region of the read transistor is given special local treatment through isolation and separate biasing. This local quality modification allows the read transistor to operate at optimized voltage levels independent of the rest of the pixel circuitry, enabling thin-oxide operation while maintaining the ability to handle full dynamic range through the transfer transistor.
Solution Approach 2:
The biasing parameters of the read transistor are changed by isolating its body region and applying a dedicated bias voltage. This parameter change enables the thin-oxide transistor to operate within its voltage constraints while the transfer transistor maintains the overall dynamic range, effectively decoupling the voltage requirements of the two transistors.
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 configuration significantly reduces read noise, particularly the 1/f component, enhancing the sensor's performance in low light conditions by minimizing noise variance and maintaining measurement range.
Implementation Method 1
The thin oxide read transistor, whose gate insulator has a thickness less than the gate insulator thickness of the transfer and reset transistors, has a gate-substrate surface capacitance that is greater than the gate-substrate surface capacitance of a read transistor having a gate insulator of the same type as the transfer and reset transistors. The read noise, and more particularly the 1/f component, of a pixel is significantly reduced
Implementation Method 2
Each pixel comprises a photodiode used in reverse, the junction capacitance of which is discharged by a photocurrent according to a light intensity received
Data Source
Figure 1~4
AI summary
The invention relates to a CMOS image sensor comprising a pixel (300) including: a photodiode (101) in series with a MOS transistor (103) between a first reference potential (GND) and a readout node (SN); a MOS transistor (105) connecting the readout node (SN) to a second reference potential (VDDH); and a third MOS transistor (307) mounted as a source follower between the readout node (SN) and a readout circuit, wherein the oxide thickness of the third transistor is less than that of the first and second transistors, the voltage difference between the first and second reference potentials is greater than the maximum voltage that can be applied between two terminals of the third transistor, and the body or drain region of the third transistor is connected to a third reference potential (VL) between the first and second potentials.