Dark Current Calibration for Photosensitive Devices
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Solution Overview
Problem
Solid state image sensors face performance limitations due to dark current, which is temperature-dependent and influenced by impurities and non-uniformities in doping gradients, making it challenging to accurately measure photon flux at low light levels, especially when ambient light sensors are obscured.
Innovation Solution
A compensation arrangement using temperature information and calibration data to modify dark current values, employing shielded photosensitive devices and temperature-dependent coefficients to subtract dark current from output signals, allowing for accurate photon flux measurement even in low light conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If dark current compensation is not applied, then the device structure remains simple, but measurement precision deteriorates due to dark current interference at low light levels
Solution Approach 1:
The patent applies preliminary action by performing dark current calibration at multiple temperatures beforehand to establish calibration curves. These pre-obtained calibration data are stored in memory and automatically retrieved during operation based on the current temperature, eliminating the need for complex real-time dark current modeling while maintaining high measurement precision.
Solution Approach 2:
The patent introduces an intermediary approach by using shielded reference photodiodes that are isolated from light but subjected to the same temperature conditions. These reference photodiodes serve as intermediaries to measure dark current separately, which is then subtracted from the signal of unshielded photodiodes to achieve precise photon flux measurement.
2Reliability
If temperature-dependent calibration is not used, then the device complexity is reduced, but reliability deteriorates due to dark current variation with temperature
Solution Approach 1:
The patent applies parameter changes by conducting calibration at multiple discrete temperature points (e.g., 20°C, 40°C, 60°C, 80°C) and storing the dark current characteristics at each point. During operation, the system determines the current temperature and selects or interpolates between the nearest calibration points to compensate for dark current, ensuring reliable compensation across the full temperature range.
Solution Approach 2:
The patent uses copying by creating duplicate photodiode structures - both shielded reference photodiodes and unshielded sensing photodiodes - that are identical in construction and subjected to the same environmental conditions except for light exposure. This copying ensures that the reference photodiodes accurately represent the dark current behavior of the sensing photodiodes at each temperature.
3Measurement precision
If shielded reference photodiodes are not used, then the device structure is simpler, but measurement precision deteriorates due to inability to separate dark current from photo current
Solution Approach 1:
The patent applies segmentation by dividing the photodiode array into distinct functional groups: shielded reference photodiodes that measure only dark current, and unshielded sensing photodiodes that measure both dark current and photo current. This segmentation allows separate measurement and subsequent subtraction of dark current components, enabling precise low light level detection.
Solution Approach 2:
The patent extracts the dark current measurement function by using shielded reference photodiodes that are physically isolated from light but remain subject to the same temperature and electrical conditions. This extraction separates the dark current component from the total current measurement, allowing it to be independently quantified and removed from the sensing photodiode signals.
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 effectively reduces the impact of dark current, enabling ambient light sensors to operate in conditions with reduced light transmission, improving the minimum detectable light level and maintaining performance across varying temperatures.
Implementation Method 1
The energy of incident photons removes electrons from the outer orbits of atoms within the photosensitive portion thus generating a charge
Implementation Method 2
The photodiode makes use of an electric field at a P-N junction to cause the photo generated electron to move away from the ion and prevent re-combination and loss of the signal
Implementation Method 3
The temperature sensing device may comprise a proportional to absolute temperature sensor
Data Source
AI summary
An imaging circuit includes at least one photosensitive device that provides an output in response to at least one photon and a compensation circuit configured to provide dark current compensation for the output of said photosensitive device. The applied compensation uses temperature information and temperature dependent calibration information.