Hybrid Thermal Sensor Trimming for MOSFET SF Variation
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Solution Overview
Problem
Conventional thermal sensors for micro-processors face inaccuracies due to wide variation in the subthreshold factor (SF) of metal oxide semiconductor field effect transistors (MOSFETs) during manufacturing, leading to sensing slope dependencies that affect temperature measurement accuracy.
Innovation Solution
A hybrid thermal sensor system that generates proportional-to-absolute temperature (PTAT) and complementary-to-absolute temperature (CTAT) currents, using a nonlinear feedback loop and Built-In-Self-Test (BIST) apparatus to trim the subthreshold factor, reducing dependency on process variations and eliminating the need for temperature control modules during testing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional thermal sensors are used with standard MOSFETs, then the device complexity and manufacturing cost are low, but the temperature sensing accuracy deteriorates due to wide SF variation
Solution Approach 1:
The thermal sensor is segmented into multiple parallel MOSFETs (first MOSFET and second MOSFET) with different channel widths, allowing independent measurement of subthreshold factor effects and enabling compensation through differential measurement
Solution Approach 2:
A current mirror circuit is introduced as an intermediary to replicate and scale the drain current from the first MOSFET to the second MOSFET, enabling precise comparison and extraction of subthreshold factor variations without direct measurement
2Measurement precision
If transistor size is increased to reduce SF variation, then temperature sensing accuracy improves, but the device area increases violating Moore's law
Solution Approach 1:
Instead of changing the physical size parameter of transistors, the invention changes the electrical parameter (channel width ratio) to create deliberate SF variations that are then measured and compensated, achieving accuracy without area increase
3Measurement precision
If two-temperature-point trimming is performed, then SF accuracy improves, but the testing time and productivity are reduced
Solution Approach 1:
The subthreshold factor is measured and compensated at a single temperature point during manufacturing, and this compensation value is stored for use across the entire temperature range, eliminating the need for repeated measurements at multiple temperature points
Solution Approach 2:
The sensor uses itself to measure the subthreshold factor by comparing currents through MOSFETs with different channel widths under the same bias conditions, enabling self-calibration without external temperature control equipment
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 enhances accuracy by minimizing the impact of SF variations, supports Moore's law by avoiding transistor upsizing, and reduces product costs by eliminating the need for temperature control modules in the test floor, achieving improved temperature sensing with reduced errors.
Implementation Method 1
the sensing slope has full dependency on subthreshold factor (SF) of a metal oxide semiconductor field effect transistor (MOSFET)
Implementation Method 2
generates proportional-to-absolute temperature (PTAT) and complementary-to-absolute temperature (CTAT) currents
Data Source
AI summary
An apparatus is provided which comprises: a thermal sensor comprising one or more n-type devices or p-type devices that suffer from subthreshold factor variation, wherein the thermal sensor is to generate an output digital code representing a temperature; and a calibration circuitry coupled to the thermal sensor, wherein the calibration circuitry is to trim the effects of subthreshold factor variation from the output digital code.