Bandgap Reference Layout for Mechanical Stress Voltage Shift Mitigation
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Solution Overview
Problem
Bandgap voltage reference circuits are susceptible to mechanical stress-induced voltage shifts due to the solder-down process, leading to temperature coefficient shifts and performance degradation, which existing solutions often require additional circuitry and complex characterization efforts to mitigate.
Innovation Solution
A spatially distributed transistor array is employed, where bipolar junction transistors are intentionally positioned at different die locations to experience varying mechanical stress, allowing the absolute temperature coefficient shift to be canceled by a relative temperature coefficient shift, without the need for extra sensors or circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If transistors are positioned close together in a standard integrated circuit layout, then manufacturing precision and device matching are improved, but mechanical stress-induced voltage shifts increase due to uniform stress exposure
Solution Approach 1:
The transistor array is segmented into multiple groups positioned at different locations on the die, specifically placing some transistors near the center and others near the edges. This segmentation allows different portions of the array to experience different mechanical stress levels, thereby reducing the overall stress-induced voltage shift while maintaining device matching through careful design of the segmentation pattern.
Solution Approach 2:
Different transistors within the array are assigned different local positions with respect to stress exposure. Transistors near the die center experience higher stress while those near the edges experience lower stress. This local quality variation in stress exposure creates a differential effect that compensates for stress-induced voltage shifts, improving voltage stability without sacrificing manufacturing precision.
2Reliability
If additional sensors and circuitry are added to compensate for mechanical stress, then voltage stability is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
The transistor array itself serves the dual function of both generating the voltage reference and compensating for mechanical stress effects. By strategically positioning transistors within the array to experience different stress levels, the circuit uses its own internal structure to self-compensate for stress-induced voltage shifts, eliminating the need for external sensors or additional compensation circuitry.
Solution Approach 2:
The transistor array is designed to perform multiple functions simultaneously: it generates the voltage reference signal and also provides stress compensation. The same transistors that are used for voltage generation are also used for stress sensing and compensation through their differential positioning, thereby reducing device complexity while improving voltage stability.
3Reliability
If transistors are positioned at different die locations to experience varying stress, then stress-induced voltage shifts are reduced, but manufacturing precision requirements increase
Solution Approach 1:
The transistor array is divided into segments positioned at predetermined locations that are relatively insensitive to small positioning variations. By using segmentation with adequate spacing between groups and employing layout techniques that average out local variations, the design reduces the impact of positioning precision errors while maintaining the stress compensation benefit.
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 approach effectively reduces mechanical stress-induced voltage shifts, maintaining temperature stability and reducing the complexity and cost of the circuit design, while being insensitive to process variations and applicable across various semiconductor materials and technologies.
Implementation Method 1
mechanical stress-induced voltage shifts
Implementation Method 2
voltage shift induced by mechanical stress in bandgap voltage reference circuits
Data Source
AI summary
A bandgap voltage reference circuit includes first and second transistors (e.g., 3-terminal BJTs or diode-connected BJTs), and a PTAT element (e.g., resistance or capacitance). The first transistor is at a first die location, and operates with a first base-emitter voltage. The second transistor is at a second die location, and operates with a second base-emitter voltage. Each of the first and second transistors may include multiple individual parallel-connected transistors. The PTAT element is operatively coupled to the first and second transistors such that a voltage difference between the first and second base-emitter voltages drops across the PTAT element. The first and second locations are separated by a distance (e.g., 1.5% or more of die length, or such that the respective centroids of the first and second transistor are spaced from one another). Such spatial distribution helps mitigate voltage shift induced by mechanical stress, and is insensitive to process variation.