Sub-threshold MOSFET Temperature Sensor with Feedback Voltage Matching
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Solution Overview
Problem
Conventional sub-threshold MOSFET temperature sensors face issues due to variability caused by unmatched bulk-to-source voltages and large drain-to-source voltages, leading to gate-induced leakage effects that spoil the desired proportional-to-absolute temperature behavior.
Innovation Solution
A sub-threshold MOSFET temperature sensor design that includes a diode-connected transistor with its drain and gate connected to an output node, along with a sub-threshold transistor having its source and gate connected, utilizing a current mirror and feedback circuit to mirror the sub-threshold leakage current and maintain the drain voltage equal to the output voltage, thereby reducing gate-induced leakage and matching source, bulk, and drain voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional sub-threshold MOSFET temperature sensor is used, then the temperature sensing function is achieved, but the bulk-to-source voltages are not matched causing variability in performance
Solution Approach 1:
The patent applies equipotentiality by connecting the bulk terminals of both the sub-threshold transistor and diode-connected transistor to the same potential (ground), ensuring matched bulk-to-source voltages. This eliminates the voltage mismatch that causes performance variability and improves measurement precision in temperature sensing applications.
2Quantity of substance
If the drain-to-source voltage for the sub-threshold transistor is made large, then sufficient current is available, but gate-induced drain leakage effects increase spoiling PTAT behavior
Solution Approach 1:
The patent employs feedback by using the output voltage from the diode-connected transistor to control the drain voltage of the sub-threshold transistor through a voltage follower configuration. This feedback mechanism maintains the sub-threshold transistor's drain-to-source voltage equal to the output voltage, ensuring low Vds operation that eliminates GIDL effects while maintaining sufficient leakage current for accurate PTAT temperature sensing.
3Quantity of substance
If the sub-threshold transistor is made many times larger than the diode-connected transistor, then sufficient sub-threshold leakage current is produced, but the device area increases
Solution Approach 1:
The patent introduces a current mirror as an intermediary device that copies the small leakage current from the diode-connected transistor and amplifies it to produce the required output current. This allows the use of a much smaller sub-threshold transistor than previously required, significantly reducing die area while maintaining sufficient current levels for accurate temperature sensing.
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 reduces gate-induced leakage current effects and improves temperature sensor accuracy by maintaining low drain-to-source voltage and matching voltages, resulting in enhanced performance and reduced variability.
Implementation Method 1
sub-threshold transistor M2 has a gate-to-source voltage of zero volts. Sub-threshold transistor M2 will thus conduct only a sub-threshold leakage current
Implementation Method 2
The diode connection for diode-connected transistor M1 develops the output voltage on the output node in response to the sub-threshold leakage current from sub-threshold transistor M2
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
A sub-threshold MOSFET temperature sensor is provided in which a subthreshold leakage current through a sub-threshold transistor having its source connected to its gate is mirrored through a diode-connected transistor to produce an output voltage. The feedback maintains a drain voltage for the sub-threshold transistor to equal the output voltage.