Image Sensor with Variable Reverse Bias for Sensitivity
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Solution Overview
Problem
Conventional image sensors face challenges with sensitivity and saturation due to the dependence of avalanche multiplication probability on applied voltage, leading to reduced S/N ratio and varying photon detection across pixels with different color filters, resulting in inconsistent image quality.
Innovation Solution
An image sensor system that applies multiple reverse bias voltages equal to or higher than the breakdown voltage to light receiving elements, with counters to count events beyond a threshold, and a corrector to adjust count values based on applied voltages, optimizing sensitivity and saturation across pixels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single reverse bias voltage is applied to all light receiving elements, then the device structure remains simple, but sensitivity and saturation vary across pixels with different color filters
Solution Approach 1:
The patent applies different reverse bias voltages to light receiving elements with different color filters (e.g., higher voltage to green pixels, lower voltage to red/blue pixels) to compensate for spectral transmittance variations. This local differentiation of voltage parameters equalizes sensitivity across all pixels while maintaining overall system functionality.
Solution Approach 2:
The patent changes the reverse bias voltage parameter across different light receiving elements based on their color filter characteristics. By adjusting this electrical parameter locally, the patent optimizes avalanche multiplication probability and sensitivity for each pixel type, resolving the contradiction between sensitivity consistency and device complexity.
2Measurement precision
If higher reverse bias voltage is applied to increase sensitivity, then avalanche multiplication probability increases, but dead time increases and count rate saturation occurs
Solution Approach 1:
The patent optimizes the reverse bias voltage parameter to achieve the optimal balance between sensitivity and count rate. By carefully selecting voltage values above breakdown voltage but below excessive levels, the patent maximizes avalanche multiplication probability while minimizing dead time effects and preventing count rate saturation.
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 approach enhances image quality by improving sensitivity and saturation characteristics, reducing noise, and compensating for spectral transmittance variations across color filters, leading to improved S/N ratio and consistent image capture.
Implementation Method 1
an image sensor that use an avalanche phenomenon (avalanche breakdown) occurring when avalanche photodiodes (APDs) are operated in Geiger mode. In this type of image sensor, an observable current is produced by avalanche multiplication when a single photon enters the APD
Implementation Method 2
The occurrence probability of avalanche multiplication in the above-described APD is dependent on applied voltage. The larger reverse bias voltage is applied across the APD, the higher the probability of occurrence of avalanche multiplication
Data Source
AI summary
An image sensor comprises: a plurality of light receiving elements whose output voltages vary in response to a photon entering; a plurality of counters each counts a number of events in which the output voltage changes beyond a predetermined threshold, and outputs a count value; and a voltage source that applies a plurality of different reverse bias voltages which are equal to or higher than a breakdown voltage to the plurality of light receiving elements.