MOS Capacitor Switching for X-ray Sensor Leakage and Noise
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Solution Overview
Problem
CMOS active pixel image sensors used for X-ray detection face issues such as process spreading in threshold voltage, leakage currents, and noise coupling due to switching FETs, which affect the accuracy and reliability of X-ray radiation measurement.
Innovation Solution
A multi full-well pixel design replaces the switching FET with a MOS capacitor, utilizing a semiconductor-side contact to control charge distribution and operate in low and high capacitance states, thereby reducing leakage currents and noise impact, and using a MOS capacitor with a metal or conductive material on top of an oxide layer to manage charge concentration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a switching FET is used to connect storage capacitors, then the pixel can operate in different saturation levels, but noise on the switching signal is coupled to the voltage at node N when the FET is conductive
Solution Approach 1:
The patent extracts the switching function from the FET and implements it through capacitor coupling mechanisms instead. The switching FET that directly connected node N to capacitors is replaced by capacitive switching that couples signals indirectly, removing the direct noise coupling path while preserving the ability to switch between different saturation levels.
Solution Approach 2:
The patent introduces capacitive coupling as an intermediary between the switching control and the voltage at node N. Instead of direct FET connection, the switching signal is transmitted through capacitors that block DC noise coupling while allowing AC signal transmission, thus mediating the control function without direct noise paths.
2Measurement precision
If X-ray radiation penetrates through the scintillator layer, then the detector can detect high energy X-rays, but leakage currents and traps are generated in the CMOS circuitry
Solution Approach 1:
The patent implements radiation hardening techniques in advance during the CMOS fabrication process. Specific process steps are applied beforehand to create radiation-resistant structures in the circuitry, cushioning against the damaging effects of X-ray induced leakage currents and traps before they can degrade performance.
Solution Approach 2:
The patent converts the harmful effect of high energy X-ray penetration into a benefit by designing the scintillator thickness and material properties to optimize the trade-off. The scintillator is engineered to allow sufficient X-ray transmission for detection while minimizing the generation of harmful leakage currents, turning the potential harm into optimized detection capability.
3Adaptability or versatility
If the total storage capacitor size is increased to detect higher X-ray radiation levels, then the dynamic range is extended, but the voltage at node N drops below the reliable measurement threshold
Solution Approach 1:
The patent implements dynamic switching between different capacitor configurations. The total storage capacitance is not fixed but can be dynamically adjusted by switching between single capacitor mode and dual capacitor parallel mode, allowing the system to adapt the voltage drop characteristics to match the expected X-ray radiation level while maintaining reliable measurement throughout the dynamic range.
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 minimizes the impact of leakage currents and noise on the sensor performance, allowing for more accurate X-ray radiation measurement and extending the dynamic range of detectable X-ray radiation without saturation.
Implementation Method 1
A scintillator layer is typically used to convert the X-ray to light having a wavelength for which the photodiode has a high sensitivity
Implementation Method 2
This light will impinge on photodiode D1, thereby generating a photocurrent
Data Source
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AI summary
The present invention is related to a multi full-well pixel for a metal-oxide-semiconductor (MOS) active pixel image sensor. It is further related to a MOS active pixel image sensor comprising a plurality of such pixels. The invention is particularly related to active pixel image sensors realized in complementary MOS (CMOS) technology. According to the invention, a MOS capacitor is used as a switchable capacitor, wherein the gate electrode is connected to the voltage that is to be read out. Semiconductor-side contacts of the MOS capacitor are used to apply a switching control signal that allows the effective capacitance of the MOS capacitor to be selected and being radiation-hard for damaging X-ray radiation..