An index-free sparse neural network using two-dimensional semiconductor ferroelectric field-effect transistors
摘要
The fine-grained dynamic sparsity in biological synapses is an important element in the energy efficiency of the human brain. Emulating such sparsity in an artificial system requires off-chip memory indexing, which has a considerable energy and latency overhead. Here, we report an in-memory sparsity architecture in which index memory is moved next to individual synapses, creating a sparse neural network without external memory indexing. We use a compact building block consisting of two non-volatile ferroelectric field-effect transistors acting as a digital sparsity and an analogue weight. The network is formulated as the Hadamard product of the sparsity and weight matrices, and the hardware, which is comprised of 900 ferroelectric field-effect transistors, is based on wafer-scale chemical-vapour-deposited molybdenum disulfide integrated through back-end-of-line processes. With the system, we demonstrate key synaptic processes—including pruning, weight update and regrowth—in an unstructured and fine-grained manner. We also develop a vectorial approximate update algorithm and optimize training scheduling. Through this software–hardware co-optimization, we achieve 98.4% accuracy in an EMNIST letter recognition task under 75% sparsity. Simulations on large neural networks show a tenfold reduction in latency and a ninefold reduction in energy consumption when compared with a dense network of the same performance.