Stacked Photodiode Structure for Ambient Light Sensing
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Solution Overview
Problem
Existing ambient light sensors face challenges in manufacturing cost due to additional coating processes and part-to-part variation in dual diode configurations, making it difficult to detect sub-lux lighting conditions effectively.
Innovation Solution
A stacked photodiode structure comprising a first-conductivity-type substrate and second-conductivity-type well regions with PN junctions responsive to visible and IR light spectra, eliminating the need for additional coatings and reducing silicon area consumption, integrated with charge balancing ADCs and a control circuit for accurate light sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single photodiode with coating is used to emulate human eye response, then the manufacturing cost increases due to additional coating process, but the device complexity is reduced
Solution Approach 1:
The patent combines multiple photodiode functions into a single integrated structure with stacked photodiodes of different conductivities. This merging eliminates the need for separate coating processes on individual photodiodes while maintaining the spectral response characteristics, thereby reducing manufacturing cost without significantly increasing device complexity
Solution Approach 2:
The patent uses photodiodes with different conductivity types (n-type and p-type) stacked together to create a composite structure. Each layer has different spectral sensitivity characteristics, and their combination provides the desired human eye response emulation without requiring additional coating materials or processes
2Measurement precision
If dual diodes are used to detect visible and IR light spectrum, then the silicon area occupied doubles, but the measurement precision is improved
Solution Approach 1:
The patent stacks photodiodes vertically in a nested configuration where one photodiode is positioned above another. This vertical nesting allows multiple sensing functions (visible light detection and IR light detection) to be integrated within a single silicon footprint, effectively halving the area requirement while maintaining the dual-spectrum measurement capability
Solution Approach 2:
The patent transitions from a planar arrangement of dual diodes to a vertical stacked configuration. By utilizing the vertical dimension (z-axis) instead of expanding in the horizontal plane, the patent achieves three-dimensional integration that reduces the silicon area occupation while preserving the measurement precision of both visible and IR light spectra
3Adaptability or versatility
If dual diodes are used to emulate human eye response, then part to part variation increases due to matching requirements, but the adaptability is improved
Solution Approach 1:
The patent merges the functionality of multiple independent diodes into a single integrated stacked structure. This combination ensures that all photodiodes are fabricated together in the same process batch, automatically matching their electrical and optical characteristics. The merged structure eliminates the need for post-fabrication matching, thereby reducing part-to-part variation while maintaining the adaptability to detect multiple light spectra
4Area of stationary object
If a single photodiode structure is used, then the photodiode area is reduced, but the measurement precision for sub-lux lighting conditions deteriorates
Solution Approach 1:
The patent implements a nested stacked photodiode structure where multiple sensing elements are vertically integrated. This nesting allows the effective sensing area to be maintained across multiple layers, increasing the total light collection capability within a compact footprint. The nested configuration enables sub-lux lighting detection by accumulating photons across stacked photodiodes, thereby improving measurement precision without requiring large individual photodiode areas
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 solution provides lower manufacturing costs, improved performance stability, and reduced photodiode area consumption while maintaining identical performance, effectively sensing ambient light without the need for complex matching of independent diodes.
Implementation Method 1
A first PN junction between the first-conductivity-type well region and the second-conductivity-type well region generates a first photocurrent that is responsive mainly to visible light spectrum incident from the first surface into the stacked photodiode structure
Implementation Method 2
A second PN junction between the second-conductivity-type well region and the first-conductivity-type substrate generates a second photocurrent that is responsive mainly to IR light spectrum incident from the first surface into the stacked photodiode structure
Data Source
AI summary
A stacked photodiode structure comprises a first-conductivity-type substrate, a second-conductivity-type well region and a first-conductivity-type well region. The first-conductivity-type substrate has a first surface for light incidence and a grounding terminal. The second-conductivity-type well region is formed in the first-conductivity-type substrate and adjacent to the first surface. The first-conductivity-type well region is formed in the second-conductivity-type well region and adjacent to the first surface. A PN junction between the first-conductivity-type well region and the second-conductivity-type well region generates free electrons responsive to visible light spectrum. A PN junction between the second-conductivity-type well region and the first-conductivity-type substrate generates free holes and free electrons responsive to mainly IR light. The difference between a first photocurrent generated from an anode terminal of the first-conductivity-type well region and a second photocurrent generated from a cathode terminal of the second-conductivity-type well region represents the intensity of incident IR light.