We prepared Cu/amorphous silicon (a-Si)/Si structures with active device diameters of 5, 10, and 20 μm. The Cu/a-Si/Si devices showed bipolar resistive switching (RS) behaviors and the ON/OFF ratio was larger than 10 6. In the low-resistance state (LRS), the current-voltage (I-V) curves of the 10- and 20-μm-sized devices were linear. In contrast, the LRS I-V curves for the 5-μm-sized devices were symmetric and nonlinear, which could be explained by tunnelling conduction. The Cu/a-Si/Si devices consist of materials compatible with conventional CMOS processes and can be candidates for nonvolatile memory devices.