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Noise immunity and low-energy computing have become limiting factors in the semiconductor roadmap as transistor feature sizes shrink. The subject of our study is the probabilistic switch, implemented in the complementary metal-oxidesemiconductor (CMOS) domain, referred to as a probabilistic CMOS (PCMOS) switch, whose behavior is rendered probabilistic by noise. In conducting this study, we are motivated by the possibility of using such probabilistic switches to realize ultra-low energy computing. Based on PCMOS switches realized using 0.5 and 0.25 mm processes, we present detailed analytical models, subsequently verified through HSpice simulations. We consider the thermal noise and power supply noise as our sources for probabilistic behavior. Through one interesting aspect of the study, we characterize the effects of the noise sampling frequency and the output sampling frequency on probabilistic behavior. Finally, we briefly outline the opportunity that such probabilistic switches offer to ultra low-energy computing through the concept of a probabilistic system-on-a-chip (PSoC) architecture (that is based on PCMOS switches); such architectures can achieve significant energy savings and performance improvements at the application level.

Parameter variations, noise susceptibility, and increasing energy dissipation of CMOS devices have been recognized as major challenges in circuit and microarchitecture design in the nanometer regime. Among these, parameter variations and noise susceptibility are increasingly causing CMOS devices to behave in an "unreliable" or "probabilistic" manner. To address these challenges, a shift in design paradigm from current-day deterministic designs to "statistical" or "probabilistic" designs is deemed inevitable. To respond to this need, in this article, we introduce and study an entirely novel family of probabilistic architectures: the probabilistic system-on-a-chip (PSOC). PSOC architectures are based on CMOS devices rendered probabilistic due to noise, referred to as probabilistic CMOS or PCMOS devices. We demonstrate that in addition to harnessing the probabilistic behavior of PCMOS devices, PSOC architectures yield significant improvements, both in energy consumed as well as performance in the context of probabilistic or randomized applications with broad utility. All of our application and architectural savings are quantified using the product of the energy and performance, denoted (energy × performance): The PCMOS-based gains are as high as a substantial multiplicative factor of over 560 when compared to a competing energy-efficient CMOS-based realization. Our architectural design is application specific and involves navigating design space spanning the algorithm (application), its architecture (PSOC), and the probabilistic technology (PCMOS).

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