Stochastic sampling via synaptic delay in spiking RBMs using integrated resistive and threshold switching devices

Spiking neural networks (SNNs) have emerged as promising low-power architectures for next-generation neuromorphic hardware because spike-based operation naturally supports spatiotemporal information processing. Among SNN models, spiking restricted Boltzmann machines (spiking RBMs) enable sampling-based learning and inference, but stable operation requires sufficient stochasticity at the neuron and synapse levels. Under temporally uniform input spike trains, such as sensor-driven inputs, limited intrinsic randomness can degrade learning performance. Here, a delay-based hardware strategy is presented in which synaptic propagation delay serves as a source of stochasticity for sampling in spiking RBMs. The corresponding synaptic unit cell consists of a synapse for weight storage and a delay module for temporal stochasticity. The delay module, based on serially integrated resistive random-access memory (RRAM) and threshold-switching (TS) devices, enables tuning of the TS turn-on delay through the RRAM resistance. Higher resistance increases the mean delay, and the measured delays follow a log-normal distribution. Compact modeling and circuit-level simulation confirm compatibility of the delay behavior with CMOS neuron-synapse circuits. Application of delay distributions to MNIST learning in spiking RBMs yields higher accuracy than both a no-delay baseline and conventional stochastic implementations based on random number generators. The RRAM-TS-based synaptic delay circuit therefore offers an efficient hardware primitive for introducing stochasticity into neuromorphic systems without complex and power-consuming additional peripherals.