Neurocomputing and Associative Memories Based on Emerging Technologies: Co-optimization of Technology and Architecture

2014-09-01T00:00:00Z (GMT) by Vehbi Calayir
<p>Neurocomputers offer a massively parallel computing paradigm by mimicking the human brain. Their efficient use in statistical information processing has been proposed to overcome critical bottlenecks with traditional computing schemes for applications such as image and speech processing, and associative memory. In neural networks information is generally represented by phase (e.g., oscillatory neural networks) or amplitude (e.g., cellular neural networks). Phase-based neurocomputing is constructed as a network of coupled oscillatory neurons that are connected via programmable phase elements. Representing each neuron circuit with one oscillatory device and implementing programmable phases among neighboring neurons, however, are not clearly feasible from circuits perspective if not impossible. In contrast to nascent oscillatory neurocomputing circuits, mature amplitude-based neural networks offer more efficient circuit solutions using simpler resistive networks where information is carried via voltage- and current-mode signals. Yet, such circuits have not been efficiently realized by CMOS alone due to the needs for an efficient summing mechanism for weighted neural signals and a digitally-controlled weighting element for representing couplings among artificial neurons. Large power consumption and high circuit complexity of such CMOS-based implementations have precluded adoption of amplitude-based neurocomputing circuits as well, and have led researchers to explore the use of emerging technologies for such circuits. Although they provide intriguing properties, previously proposed neurocomputing components based on emerging technologies have not offered a complete and practical solution to efficiently construct an entire system. In this thesis we explore the generalized problem of co-optimization of technology and architecture for such systems, and develop a recipe for device requirements and target capabilities. We describe four plausible technologies, each of which could potentially enable the implementation of an efficient and fully-functional neurocomputing system. We first investigate fully-digital neural network architectures that have been tried before using CMOS technology in which many large-size logic gates such as D flip-flops and look-up tables are required. Using a newly-proposed all-magnetic non-volatile logic family, mLogic, we demonstrate the efficacy of digitizing the oscillators and phase relationships for an oscillatory neural network by exploiting the inherent storage as well as enabling an all-digital cellular neural network hardware with simplified programmability. We perform system-level comparisons of mLogic and 32nm CMOS for both networks consisting of 60 neurons. Although digital implementations based on mLogic offer improvements over CMOS in terms of power and area, analog neurocomputing architectures seem to be more compatible with the greatest portion of emerging technologies and devices. For this purpose in this dissertation we explore several emerging technologies with unique device configurations and features such as mCell devices, ovenized aluminum nitride resonators, and tunable multi-gate graphene devices to efficiently enable two key components required for such analog networks – that is, summing function and weighting with compact D/A (digital-to-analog) conversion capability. We demonstrate novel ways to implement these functions and elaborate on our building blocks for artificial neurons and synapses using each technology. We verify the functionality of each proposed implementation using various image processing applications based on compact circuit simulation models for such post-CMOS devices. Finally, we design a proof-of-concept neurocomputing circuitry containing 20 neurons using 65nm CMOS technology that is based on the primitives that we define for our analog neurocomputing scheme. This allows us to fully recognize the inefficiencies of an all-CMOS implementation for such specific applications. We share our experimental results that are in agreement with circuit simulations for the same image processing applications based on proposed architectures using emerging technologies. Power and area comparisons demonstrate significant improvements for analog neurocomputing circuits when implemented using beyond- CMOS technologies, thereby promising huge opportunities for future energy-efficient computing.</p>