A semiconductor device is provided, including a lower conducting layer formed above a substrate, an upper conducting layer, and a memory cell structure formed on the lower conducting layer (such as formed between the lower and upper conducting layers). The memory cell structure includes a bottom electrode formed on the lower conducting layer and electrically connected to the lower conducting layer, a transitional metal oxide (TMO) layer formed on the bottom electrode, a TMO sidewall oxides formed at sidewalls of the TMO layer, a top electrode formed on the TMO layer, and spacers formed on the bottom electrode. The upper conducting layer is formed on the top electrode and electrically connected to the top electrode.