FeFET Multi-Bit Content-Addressable Memories for In-Memory Nearest Neighbor Search
Nearest neighbor (NN) search computations are at the core of many applications such as few-shot learning, classification, and hyperdimensional computing. As such, efficient hardware support for NN search is highly desired. In-memory computing using emerging devices offers attractive solutions for NN search. Solutions based on ternary content-addressable memories (TCAMs) offer high energy and latency improvements for NN search at the expense of accuracy. In this work, we propose a novel distance function that can be natively evaluated with multi-bit content-addressable memories (MCAMs) based on ferroelectric FETs (FeFETs) to perform a single-step, in-memory NN search. We evaluate the efficacy of FeFET MCAMs in the context of few-shot learning applications with different datasets. As an example, we achieve a 78.54% accuracy for a 5-way, 5-shot classification task for the mini-ImageNet dataset (only 1.5% lower than software-based implementations) when using a 3-bit MCAM for NN search. We consider the effects of FeFET threshold voltage variations on the application accuracy and analyze the area and search energy requirements of FeFET MCAMs for accurate operations. Our results indicate that MCAMs require 2x lower area and search energy than TCAMs to achieve the same accuracy. Furthermore, we experimentally demonstrate a 2-bit implementation of FeFET MCAM using AND arrays from GLOBALFOUNDRIES to further validate the design concept.