Matched-Transistor PUF Cells for Persistent Random Number Generation
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Solution Overview
Problem
Implementing a Physical Unclonable Function (PUF) apparatus that generates a persistent, random number with equal probability for each output, while being low-cost in terms of power consumption and area, and suitable for integration in devices like IoT devices, is challenging due to the need for random manufacturing variations and reliable on-state characteristic comparisons.
Innovation Solution
A PUF apparatus comprising a plurality of PUF cells with matched pairs of transistors of the same type, where the on-state characteristics are compared to determine a transistor difference value, generating a persistent random number based on random manufacturing differences, and using these cells for both selection and determination mechanisms to reduce size and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If matched pairs of transistors are used for PUF cell comparison, then manufacturing precision is improved, but device complexity increases
Solution Approach 1:
The selection mechanism and determination mechanism are merged into a single unified structure using matched pairs of transistors. The first transistor in each matched pair serves dual purposes: it acts as part of the determination mechanism for generating the PUF output and simultaneously functions as the selection mechanism for selecting among multiple PUF cells. This merging eliminates the need for separate selection transistors, thereby reducing device complexity while maintaining manufacturing precision through the matched pair configuration.
Solution Approach 2:
Each transistor in the matched pair is designed to perform multiple functions. The first transistor serves as both a determination element (for comparing on-state characteristics) and a selection element (for selecting the active PUF cell). This multi-functionality reduces the total number of components required in each PUF cell, simplifying the overall device structure while maintaining the precision needed for reliable PUF operation.
2Reliability
If separate selection and determination mechanisms are used, then reliability is improved, but area increases
Solution Approach 1:
The patent combines the selection mechanism and determination mechanism into a single integrated structure within each PUF cell. By using the first transistor of each matched pair to serve both as a selection element and a determination element, the patent reduces the total area required per PUF cell while maintaining the functional separation needed for reliable operation. The matched pair configuration ensures that even with shared components, the determination reliability is preserved through comparison of on-state characteristics.
3Loss of information
If multiple transistor types are used in PUF cells, then output randomness is improved, but manufacturing precision deteriorates
Solution Approach 1:
The patent uses matched pairs of transistors where each pair consists of transistors with identical or very similar characteristics (same type, size, and configuration). This local uniformity within each matched pair ensures high manufacturing precision and reliable matching. The randomness of the PUF output is achieved not through varying transistor types within pairs, but through the aggregate effect of many such matched pairs, each contributing a binary decision based on their unique manufacturing variations.
Solution Approach 2:
Within each matched pair, the transistors are designed to be homogeneous in terms of type, size, and configuration. This homogeneity ensures that manufacturing variations affect both transistors in a pair similarly, allowing for reliable comparison of their on-state characteristics. The overall PUF output randomness emerges from the collective behavior of many such homogeneous pairs, each contributing independent binary decisions based on their unique manufacturing fingerprints.
Data Source
AI summary
The present disclosure relates to a PUF apparatus and method for generating a persistent, random number. The generated number is random in that each particular instance of PUF apparatus should generate a randomly different number to all other instances of PUF apparatus, and is persistent in that each particular instance of the PUF apparatus should repeatedly generate the same number, within acceptable error correction tolerances. The persistent, random number is determined by selecting one or more PUF cells, each comprising a matched pair of transistors that are of identical design, and comparing an on-state characteristic of the pair (e.g., turn-on threshold voltage or gate-source voltage). The difference in on-state characteristic of each selected pair of transistors is caused by random manufacturing differences between the transistors. This causes the randomness between each different instance of PUF apparatus, and should be relatively stable over time to provide persistence of the generated number.