SWIR Image Sensor Threshold Voltage Compensation
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Solution Overview
Problem
The integration of photodiodes and amplifiers on a silicon substrate for SWIR image sensors leads to performance degradation due to threshold voltage shifts in the MOS transistors, causing reliability issues and limiting resolution.
Innovation Solution
An image sensor with a compensation unit that applies specific compensation voltages to transistors forming the amplifier unit to counteract threshold voltage shifts, thereby maintaining performance and improving reliability and resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If photodiodes and amplifiers are monolithically integrated on a silicon substrate, then manufacturing cost is reduced and resolution is improved, but threshold voltage shift occurs in MOS transistors causing performance degradation
Solution Approach 1:
The patent applies preliminary action by forming a compensation voltage line and applying compensation voltage before the photodiode formation process. This pre-compensation approach anticipates the threshold voltage shift that will occur during subsequent high-temperature photodiode formation, allowing the MOS transistor to maintain stable operation despite the upcoming thermal stress.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the compensation voltage applied to the MOS transistor gate. The compensation voltage is specifically tuned to counteract the threshold voltage shift caused by photodiode formation temperature. By changing the voltage parameter in response to the thermal process conditions, the transistor operates point remains stable throughout the integration process.
2Ease of manufacture
If photodiodes are formed after amplifiers on the same substrate, then monolithic integration is achieved, but threshold voltage shift degrades amplifier performance
Solution Approach 1:
The patent introduces a compensation voltage line as an intermediary element between the MOS transistor and the external control circuitry. This compensation voltage line serves as a mediator that transmits the corrective voltage signal to the transistor gate, enabling precise control of the transistor operating point without requiring physical modification of the transistor structure itself.
Solution Approach 2:
The patent implements feedback by monitoring or calculating the threshold voltage shift that occurs during photodiode formation and applying a compensating voltage in response. The compensation voltage is determined based on the expected or measured threshold shift, creating a closed-loop control system that maintains amplifier performance despite process variations.
3Reliability
If separate substrates are used for photodiodes and amplifiers, then coupling reliability is maintained, but manufacturing cost increases and resolution is limited
Solution Approach 1:
The patent applies merging by integrating both the photodiode and amplifier circuits onto a single silicon substrate, eliminating the need for separate substrates and complex coupling interfaces. This monolithic integration reduces manufacturing steps, lowers costs, and improves spatial resolution by eliminating coupling limitations between separate components.
Solution Approach 2:
The patent converts the harmful effect of threshold voltage shift during photodiode formation into a beneficial control mechanism. By intentionally designing the compensation voltage line and applying corrective voltage, the previously problematic thermal interaction between photodiode and amplifier formation processes becomes a controllable parameter that can be optimized for performance.
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 compensation unit effectively prevents performance degradation of the MOS transistors, enhancing the image sensor's reliability and resolution by stabilizing the threshold voltage shifts during the integration process.
Implementation Method 1
a photoelectric transformation unit for generating charges according to photoelectric transformation in each of pixels
Data Source
AI summary
Provided are an image sensor compensating for property degradation of a metal-oxide-semiconductor (MOS) resulting from a threshold voltage shift that may occur when photodiodes and a MOS circuit of configuring an amplifier are integrated on the same substrate, and a method for driving the image sensor.