CMOS-based area-and-power-efficient neuron and synapse circuits for time-domain analog spiking neural networks

Conventional neural structures tend to communicate through analog quantities such as currents or voltages, however, as CMOS devices shrink and supply voltages decrease, the dynamic range of voltage/current-domain analog circuits becomes narrower, the available margin becomes smaller, and noise immunity decreases. More than that, the use of operational amplifiers (op-amps) and clocked or asynchronous comparators in conventional designs leads to high energy consumption and large chip area, which would be detrimental to building spiking neural networks. In view of this, we propose a neural structure for generating and transmitting time-domain signals, including a neuron module, a synapse module, and two weight modules. The proposed neural structure is driven by leakage currents in the transistor triode region and does not use op-amps and comparators, thus providing higher energy and area efficiency compared to conventional designs. In addition, the structure provides greater noise immunity due to internal communication via time-domain signals, which simplifies the wiring between the modules. The proposed neural structure is fabricated using TSMC 65 nm CMOS technology. The proposed neuron and synapse occupy an area of 127 um2 and 231 um2, respectively, while achieving millisecond time constants. Actual chip measurements show that the proposed structure successfully implements the temporal signal communication function with millisecond time constants, which is a critical step toward hardware reservoir computing for human-computer interaction.