RADFET Dosimeter Structure for Low-Dose Sensitivity
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Solution Overview
Problem
Conventional RADFET dosimeters have poor voltage sensitivity, limiting their use to high radiation dose applications due to the thin sensing dielectric, which restricts their application in medical fields like dental and CT x-ray procedures where lower dose detection is required.
Innovation Solution
A semiconductor structure with a dielectric on the channel region for storing charge due to irradiation, where the gate electrode and source region are electrically connected, and the bulk is reverse biased with respect to the sensor and drain, allowing for a channel current from the sensor to the drain, enhancing sensitivity by minimizing capacitance and thermal drift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If a thin sensing dielectric is used in conventional RADFET dosimeters, then the device can operate in passive mode without power during irradiation, but the voltage sensitivity is poor and limited to high radiation dose applications
Solution Approach 1:
The device is segmented into distinct functional regions: a high-resistivity bulk region for low capacitance, a channel region for charge collection, and a dielectric layer for charge storage. This segmentation allows each region to be optimized independently, enabling the bulk to provide low capacitance for high sensitivity while the dielectric maintains passive operation capability
Solution Approach 2:
The bulk region resistivity is changed to a high value (greater than 1000 ohm-cm) which fundamentally alters the capacitance characteristics of the device. This parameter change reduces the bulk capacitance contribution, thereby increasing voltage sensitivity while maintaining passive operation
2Measurement precision
If the dielectric thickness is increased to improve sensitivity, then more charge can be stored, but the manufacturing complexity and cost increase significantly
Solution Approach 1:
The dielectric layer is positioned locally over the channel region where charge collection is most effective. This localized approach concentrates the sensing function in the most critical area, achieving high sensitivity without requiring a uniformly thick dielectric across the entire device, thereby simplifying manufacturing
Solution Approach 2:
The device uses a composite structure combining high-resistivity semiconductor bulk material with a dielectric layer. This composite approach leverages the low capacitance property of the high-resistivity bulk and the charge storage capability of the dielectric, achieving high sensitivity with moderate dielectric thickness
3Ease of manufacture
If conventional RADFET structure is used, then the device is simple to manufacture, but external grounding resistors and protection devices are required, increasing device complexity
Solution Approach 1:
The grounding function previously requiring external resistors is merged into the device structure itself through the high-resistivity bulk region. The bulk inherently provides the necessary electrical characteristics, eliminating the need for separate external grounding components and simplifying the overall device configuration
Solution Approach 2:
The high-resistivity bulk region serves multiple functions simultaneously: it provides low capacitance for high voltage sensitivity, acts as an inherent grounding path, and offers ESD protection. This self-service capability eliminates the need for separate external protection devices, reducing device complexity
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 achieves a sensitivity of 6 mV/mGy, enabling the detection of 0.3 mGy radiation doses without a power source, making it suitable for medical applications like dental and CT scans, while reducing thermal drift and external component requirements.
Implementation Method 1
A RADFET operates by trapping charge in the gate dielectric during irradiation, and sensing this charge electrically through changes in the operation of the MOSFET transistor
Implementation Method 2
Electron hole pairs are created in the dielectric by the incident radiation
Implementation Method 3
A vertical electric field across the dielectric, where vertical means between gate and semiconductor regions, is required to force generated electrons to leave the dielectric, leaving behind holes
Data Source
AI summary
The present invention provides a radiation dosimeter with improved sensitivity. The sensor apparatus comprises a semiconductor structure, a means for biasing the semiconductor structure to readout stored charge in its dielectric by exhibiting a sensor current-voltage characteristic which is proportional to the dielectric stored charge, and a means for determining a change in current-voltage characteristic of the sensor terminal due to irradiation. The semiconductor substrate comprises a bulk region of a first conductivity type formed in the substrate, a source region and a drain region formed in the bulk region, wherein the source region and the drain region are of a second conductivity type opposite to the first conductivity type, a channel region formed between the source region and the drain region, a dielectric formed on the channel region for storing charge due to irradiation, and a gate electrode formed on the dielectric. The means for biasing configures the semiconductor structure such that the gate electrode and the source region are electrically connected to form a sensor terminal, and the bulk is biased with a fixed voltage such that it is reverse biased with respect to the sensor and the drain, and the drain is biased with a fixed voltage such that it sinks channel current sourced from the sensor terminal, and wherein the channel region is configured to provide a channel current from the sensor to the drain which is larger in magnitude than the reverse biased bulk-sensor and bulk-drain diode currents.