Transparent oxide thin-film transistors (TFTs) have been fabricated by RF magnetron sputtering at room temperature using amorphous indium zinc oxide (IZO) as both active channel and source/drain electrodes and co-sputtered HfO 2 –Al 2 O 3 (HfAlO) as gate dielectric. In spite of its high dielectric constant, HfO 2 has some drawbacks including a high leakage current and rough surface morphologies due to its small band gap (5.31 eV) and microcrystalline structure. In this work, the incorporation of Al 2 O 3 into hafnium (Hf) was achieved by co-sputtering of HfO 2 and Al 2 O 3 without any intentional substrate heating, and its structural and electrical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE) analyses, depending on the Hf:Al compositional ratio. The XRD studies confirmed that the microcrystalline structures of HfO 2 were transformed to amorphous structures of HfAlO at approximately Hf:Al = 3:1 at% ratio. According to the AFM analysis, the HfAlO films (0.54 nm) were considerably smoother than the HfO 2 films (4.27 nm) due to their amorphous structure. The band gap (E g ) deduced by spectroscopic ellipsometry was increased from 5.31 eV (HfO 2 ) to 5.58 eV (HfAlO). The electrical performances of the TFTs, which were made of well-controlled active/electrode IZO materials and co-sputtered HfAlO dielectric material, exhibited a field-effect mobility of more than 10 cm 2 /V s, a threshold voltage of about 1.5 V, a sub-threshold swing of about 0.5 V/decade, and an I on/off ratio of about 10 5 .