A diffusion doping approach is adopted to fabricate Al-doped HfO2 resistive random access memory (RRAM) devices, using the new Hf/Al/Hf diffusion structure with a two-step furnace annealing process, to avoid the formation of AlOx interfacial layer between HfO2 layer and electrodes. The uniformity of key switching parameters is improved significantly in the Al-doped HfO2 devices. This performance improvement is attributed to the reduced formation energy of oxygen vacancy (VO) induced by trivalent Al-doping effect and so the improved stability of VO conductive filaments (CFs).