Range Imaging Pixel Circuit With Floating Drainage Transistors
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Solution Overview
Problem
Conventional time-of-flight range imaging sensors face challenges in accurately measuring distances due to variations in charge transfer and drainage durations caused by parasitic capacitance, leading to inaccuracies in distance calculations.
Innovation Solution
The range imaging element employs a pixel circuit design with symmetrically arranged charge transfer and drainage transistors, including at least one floating transistor, to ensure consistent charge transfer and drainage paths, thereby maintaining equivalent control pulse characteristics and reducing variations in storage duration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional charge transfer and drainage transistors are used without floating transistor, then the device complexity is reduced, but the measurement precision deteriorates due to variations in charge transfer and drainage durations caused by parasitic capacitance
Solution Approach 1:
The charge drainage function is segmented into two separate transistor operations: one transistor handles charge drainage during the integration period, while another transistor (including floating transistors) handles charge drainage during the transfer period. This segmentation allows independent optimization of each drainage phase, compensating for parasitic capacitance effects and improving measurement precision without excessive complexity increase.
Solution Approach 2:
Floating transistors are introduced as intermediary elements that are electrically disconnected during certain periods. These floating transistors act as mediators to balance the load on active transistors, reduce parasitic capacitance effects, and maintain equivalent control pulse characteristics, thereby improving distance measurement accuracy.
2Measurement precision
If multiple charge transfer transistors (2M) and charge drainage transistors (2N) are used to reduce parasitic capacitance effects, then the measurement precision is improved, but the device complexity increases
Solution Approach 1:
The charge transfer and drainage operations are divided into periodic phases: integration period and transfer period. During the integration period, charge transfer transistors are off while drainage transistors operate. During the transfer period, charge transfer transistors are on while drainage transistors switch roles. This periodic action allows multiple transistors to share functionality across different time periods, improving precision while managing complexity through temporal multiplexing.
3Reliability
If charge transfer and drainage operations are performed with asymmetric transistor configurations, then the ease of manufacture is improved, but the reliability deteriorates due to unequal control pulse characteristics and varied storage durations
Solution Approach 1:
While the overall circuit may appear asymmetric, the invention deliberately creates functional symmetry through the floating transistor configuration. The floating transistors are positioned to create balanced electrical characteristics during different periods, ensuring that control pulses for charge transfer and drainage have equivalent characteristics. This controlled asymmetry achieves reliability improvement while maintaining manufacturability.
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 enhances the accuracy of distance measurements by ensuring consistent charge transfer and drainage operations, resulting in precise distance calculations.
Implementation Method 1
a photoelectric conversion element configured to generate charge based on light incident from a space of which measurement is to be performed
Data Source
AI summary
A range imaging element includes a semiconductor substrate, and a pixel circuit formed on the semiconductor substrate. The pixel circuit includes at least a photoelectric conversion element that generates charge based on light incident from a measurement space, charge storages that store the charge, at least one charge transfer transistor on a transfer path through which the charge is transferred from the photoelectric conversion element to one of the charge storages, and at least one charge drainage transistor on a drainage path through which the charge is drained from the photoelectric conversion element. A surface of the photoelectric conversion element has a rectangular shape in a plan view, the at least one charge drainage transistor includes 2N (N is an integer, N≥1) charge drainage transistors, and at least one of the 2N charge drainage transistors is a floating transistor that is not electrically connected to the photoelectric conversion element.