For the first time, we demonstrate a novel CuOx/HfOx channel-less synaptic transistor for neuromorphic computing systems. Engineered 5-nm-thick HfOx switching layer allows lateral current from source to drain (ID) to be highly controlled by gate pulses, achieving a wide dynamic range of ID of ~400. Cu mobile ions provided from the CuOx gate electrode are driven by an electric field across the HfOx layer serving as a solid electrolyte during programming. We thus identify that an appropriate amount of vacancy sites in the electrolyte plays an important role in obtaining analogously modulated synaptic behavior in ion-actuated transistors.