Novel, parallel and differential synaptic architecture based on NAND flash memory for high-density and highly-reliable binary neural networks

Abstract

A novel synaptic architecture based on a NAND flash memory structure is proposed as a high-density synapse capable of exclusive NOR (XNOR) operation for binary neural networks (BNNs). For the first time, a 4T2S-based synaptic architecture with a complementary input voltage that implements an equivalent bitwise XNOR operation is proposed. Two adjacent NAND flash strings connected with the word lines are used as one synaptic string with four input transistors connected to the bit-line. By changing the threshold voltage of the NAND flash cells and input voltages in a complementary fashion, the XNOR operation is successfully demonstrated. The large on/off current ratio (-7 x 105) of the NAND flash cells and differential sensing scheme can implement high-density and highly reliable binary neural networks without error correcting codes (ECC), which can reduce the burden of the complementary metal-oxide-semicon ductor (CMOS) overhead. Despite the string structure of NAND flash memory, the parallel read scheme significantly reduces the read-out latency when compared to the sequential read scheme. In addition, we show that with only 1 erase/program pulse, sufficiently low bit-error rate (7.6 x 10-9 %) is achieved without the conventional incremental step pulse programming (ISPP) scheme. Finally, the estimated synapse density of V-NAND flash memory with 128 stacks is -103 times that of the 2T2R synapse in resistive random access memory (RRAM).