Neuromorphic devices that mimic a human brain have attracted significant attention in the field of next-generation semiconductors. The human brain can efficiently process information with low power consumption. Several energy efficient artificial synapses have been reported; however, the energy consumption of these synapses is significantly higher than that of the human brain (10 fJ). In this study, we propose the use of double oxide semiconductors for obtaining synaptic transistors with ultra-low energy consumption. The synaptic transistor comprising InZnO (IZO) and InGaZnO (IGZO) exhibits a high mobility and positive turn-on voltage, which are required for ultra-low energy consumption. SiO2 deposited at 200 degrees C by plasma enhanced atomic layer deposition is used as an electric double layer gate insulator. The IZO/IGZO synaptic transistor consumed an ultra-low energy of 0.269 fJ (gate voltage: 3.5 V, 1 ms and drain voltage: 3 mV). Furthermore, the synaptic transistor exhibits various synaptic plasticity features under the brain-like energy conditions, including excitatory post-synaptic current, paired-pulse facilitation, potentiation, and depression. All of the operations of the devices were performed under ambient conditions (25 degrees C, humidity 50%), in the dark probe station. The IZO/IGZO synaptic transistor exhibits similar energy efficiency to a human brain, and this strategy is expected to be utilized for the fabrication of various ultra-low energy consuming synaptic transistors.