Capacitive Image Sensor Two-Transistor Pixel Structure
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Solution Overview
Problem
Capacitive imaging techniques face challenges in achieving an optimal signal-to-noise ratio and are costly for large-area fingerprint scanners, making it difficult to achieve high pixel densities like 1000 ppi without exceeding manufacturing capabilities.
Innovation Solution
A capacitive image sensor with a two-transistor configuration, including a pixel selection transistor and a source follower transistor, uses a variable capacitance and AC current source to determine output impedance, which represents the proximity of an object, allowing for the creation of high-resolution images up to 1000 ppi.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a three-transistor configuration is used in capacitive image pixels, then the signal-to-noise ratio can be maintained, but the manufacturing cost increases and manufacturing precision becomes difficult to achieve at high pixel densities
Solution Approach 1:
The patent removes the reset transistor from the traditional three-transistor pixel configuration, extracting only the essential components needed for capacitive sensing. This reduction to two transistors per pixel simplifies the pixel structure while maintaining the core functionality of capacitance measurement, thereby reducing manufacturing complexity and cost without significantly compromising signal-to-noise ratio
Solution Approach 2:
The patent modifies the electrical parameters and circuit topology of the pixel by changing from a three-transistor to a two-transistor configuration. This parameter change in circuit architecture reduces the number of TFTs required per pixel, enabling higher pixel densities (e.g., 1000 ppi) to be achieved within manufacturing capabilities while maintaining acceptable performance through optimized biasing and readout schemes
2Manufacturing precision
If the pixel density is increased to 1000 ppi, then the image resolution is improved, but the device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent segments the pixel function into two distinct transistor roles: a selection transistor for row/column addressing and a source follower transistor for signal buffering and readout. This functional segmentation allows for a more efficient layout and reduces the total transistor count per pixel, enabling higher pixel densities to be achieved without proportionally increasing overall device complexity
Solution Approach 2:
The two-transistor pixel design employs multi-functional transistors that perform multiple roles. For example, the source follower transistor serves both as a buffer and as part of the readout circuitry, while the selection transistor handles both row and column selection functions. This multi-functionality reduces the total component count and simplifies the overall device structure, making high-density implementations more feasible
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
The two-transistor configuration enables the capacitive image sensor to achieve high resolution and manufacturability, overcoming the limitations of three-transistor designs by simplifying the pixel structure and improving signal-to-noise ratio without exceeding TFT manufacturer capabilities.
Implementation Method 1
The variable capacitance varies in accordance with the nearness of the object which is affective in altering capacitance
Implementation Method 2
An AC current source is used to interrogate the activated source follower to determine an output impedance of the source follower. The output impedance is a function of the input impedance and is representative of the nearness of an object
Data Source
AI summary
A capacitive image sensor includes a sensor array having capacitive image pixels. Each pixel has a two-transistor configuration including a pixel selection transistor and a source follower transistor. The pixel selection transistor activates the source follower transistor. The source follower is coupled to a variable capacitance that affects an input impedance of the source follower. An AC current is source is used to interrogate the activated source follower to determine an output impedance of the source follower. The output impedance is a function of the input impedance and the output impedance is representative of the nearness of an object.