Floating Base Bipolar Image Pixel With Reduced Signal Lines
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Solution Overview
Problem
Conventional CMOS image sensors with high performance require multiple signal lines for operation, consuming valuable pixel area and generating noise due to the presence of reset and address transistors, which limits their resolution and sensitivity.
Innovation Solution
The implementation of a CMOS image sensor with small pixels using a floating base bipolar transistor and a pinned photodiode, eliminating reset and address transistors, and incorporating a current sensing column correlated double sampling (CDS) circuit to minimize noise and column-to-column response nonuniformity, while achieving vertical blooming control without additional control lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional CMOS image sensors use multiple signal lines for operation, then device functionality is maintained, but pixel area is consumed and noise is generated
Solution Approach 1:
The patent extracts and eliminates reset and address transistors from the pixel circuit, removing the associated signal lines (reset line, address line, charge transfer line) that generate noise and consume pixel area. Only the essential column output line remains, significantly reducing noise sources while maintaining core functionality.
Solution Approach 2:
The floating base bipolar transistor serves multiple functions simultaneously: it acts as the charge sensing element, the amplifier, and the output driver. This multi-functionality eliminates the need for separate reset and address transistors, reducing the number of signal lines and associated noise while maintaining device operation.
2Area of stationary object
If reset and address transistors are present in the pixel circuit, then pixel operation is enabled, but pixel area is consumed
Solution Approach 1:
The floating base bipolar transistor is designed to perform multiple operations: charge collection from the pinned photodiode, charge amplification through its high gain, and signal output driving. This consolidation of functions into a single transistor dramatically reduces the pixel area required while maintaining full operational capability.
Solution Approach 2:
The patent removes reset and address transistors from the pixel circuit, extracting the essential operational functions and implementing them through the floating base bipolar transistor's inherent characteristics, thereby freeing up significant pixel area.
3Measurement precision
If floating base bipolar transistor is used, then charge gain is achieved and sensitivity is increased, but device complexity increases
Solution Approach 1:
The floating base bipolar transistor provides charge gain through its high current gain (β) characteristic, achieving sensitivity enhancement without requiring additional transistors or complex circuitry. The same transistor that senses charge also amplifies it, eliminating the need for separate buffer amplifiers and reducing overall circuit complexity.
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 design allows for increased pixel sensitivity, reduced noise, and improved charge storage capacity, enabling higher resolution with fewer signal lines and minimizing noise and blooming issues, thus enhancing the overall performance of the image sensor.
Implementation Method 1
Typical image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels
Implementation Method 2
a pixel having an integrated bipolar transistor gain stage where charge received from a pinned diode is multiplied several times before a signal is transferred
Data Source
AI summary
A pixel of an image sensor includes only two signal lines per pixel, a pinned photodiode for sensing light, a floating base bipolar transistor, and no reset and address transistors. The floating base bipolar transistor provides the pixel with a gain, which can increase pixel sensitivity and reduce noise. The pixel also incorporates a vertical blooming control structure for an efficient blooming suppression. The output terminals of the pixel are coupled to a common column output line terminated by a special current sensing correlated double sampling circuit, which is used for subtraction of emitter leakage current. Based on this structure, the pixel has high sensitivity, high response uniformity, low noise, reduced size, and efficient layout.