Fig. 1. (Color online) (a) Schematic diagram of human neuron network. (b) Structure of synapse, including pre-synapse, post-synapse, and synaptic cleft, which can be illuminated under ultraviolet light as stimuli spikes. (c) Cross-section schematic diagram of the TiO₂-gate/AlGaN/GaN heterostructure synaptic neuromorphic device. (d) Optical image of the complete synaptic device with a chip size of 1 × 1 mm². (e) SEM image of the synaptic device on active area. (f) I−V curve of the synaptic device in the dark state with bias voltage from −4 to 4 V. The inset shows the linear scale of current−voltage and the semi-logarithmic scale of resting power comsuption−voltage with bias voltage from −4 to 0 V.

Fig. 2. (Color online) (a) The spectral response of the TiO₂ gate/AlGaN/GaN heterostructure synaptic device under 3 V bias voltage. (b) Absorption curve of a 10 nm TiO₂ nanolayer. The inset shows the SEM image, including drain electrode, source electrode, and TiO₂ gate area.

Fig. 3. (Color online) (a) EPSC response induced by a stimulating spike (light wavelength: 365 nm; light intensity: 1.35 μW∙cm⁻²; pulse duration: 1 s; V_ds = −2 V). (b) A schematic illustration of the energy of the energy band diagram describes the TiO₂/AlGaN/GaN heterostructure synaptic device under UV light spikes.

Fig. 4. (Color online) (a) A typical PPF behavior induced by two consecutive stimulating spike (light wavelength: 365 nm; intensity: 1.35 μW∙cm⁻²; pulse duration: 100 ms; pulse interval: 300 ms; V_ds = −3 V). (b) The PPF index under ultraviolet light stimuli with different spike interval.

Fig. 5. (Color online) (a) Schematic of the memory model in the human brain. (b) Twenty consecutive stimulating spike (light wavelength: 365 nm; intensity: 1.35 μW∙cm⁻²; spike duration: 50 ms; spike period: 1 s), the inset is the enlarged image of two pulses. (c) The transformation from STP to LTP by 365 nm ultraviolet light spike at different numbers ranging from 1 to 20. V_ds is fixed at −2.5 V.

Fig. 6. (Color online) TiO₂ floating-gate/AlGaN/GaN synaptic device for Pavlov’s dog experiment on associative learning by training. (a) Schematic diagram of the correspondence between bell and voltage, feeding and light stimuli. (b) Five V_ds = −2.8 V voltage did not cause the current to exceed the salivation response threshold of 30 nA, the dog has no response under the neutral stimuli (NS) from bell. (c) Ten ultraviolet light spike induced valid EPSC ( >the threshold of 30 nA), the US (unconditional stimuli) from food can trigger UR (unconditional response, salivation of dog). (d) "Training" process including ten ultraviolet spike and V_ds = −2.8 V bias voltage induced higher EPSC ( >the threshold of 30 nA), salivation of dog. (e) Achieved the threshold of 30 nA through applying V_ds = −2.8 V bias voltage, the CS from bell can trigger CR (conditional response, salivation of dog).