Fig. 2. Three biological functions and corresponding synaptic characteristics.

a The device simulates three typical functions of synapses: forming memory, synaptic plasticity, and stimulating membrane potential. b The PPF index is a measure of synaptic facilitation defined as the ratio of the amplitudes of the first (A₁) and second (A₂) EPSCs plotted against the pulse interval ($\mathrm{\Delta }\mathrm{t}$). c EPSC behaviors activated and modified by electric pulses with 9 different widths (50 ms, 100 ms, 150 ms, 200 ms, 250 ms, 300 ms, 350 ms, 400 ms, and 450 ms) at V_DS = 0.5 V. d EPSC triggered by 5 single electric pulses with different amplitudes (3 V, 4 V, 5 V, 6 V, and 7 V). e Low-pass filtering characteristics are shown by 10 continuous pulses of different frequencies (10 Hz, 12.5 Hz, 20 Hz, 30 Hz, and 40 Hz) applied to the presynaptic terminal. f The LTP/LTD characteristics demonstrate the controllable range and level of conductance. g Threshold effect of the synaptic device as biological neurons. h Trend of the STDP curve as the weight update rule transforms the temporal information