In this paper, zirconium–aluminum–oxide (ZAO) dielectric layers were prepared by a solution method with intent to combine the high dielectric constant with a low leakage current density. As a result, dielectric layers with improved electrical properties as expected can be obtained by spin-coating the mixed precursor. The chemical and physical properties of the films were measured by thermogravimetric differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and a UV spectrometer. It is observed that the oxygen defects and the hydroxide in the films are reduced with the addition of high-bond-energy zirconia, while the films can remain large optical bond gaps thanks to the presence of alumina. The metal-insulator-metal (MIM) devices were fabricated, and it was seen that with a molar ratio of Zr:Al = 3:1 and an annealing temperature of 500 °C, the dielectric layer afforded the highest dielectric constant of 21.1, as well as a relatively low leakage current of 2.5 10−6 A/cm2@1MV/cm. Furthermore, the indium–gallium–zinc oxide thin-film transistors (IGZO-TFTs) with an optimal ZAO dielectric layer were prepared by the solution method and a mobility of 14.89 cm2/Vs, and a threshold voltage swing of 0.11 V/dec and a 6.1 106 on/off ratio were achieved at an annealing temperature of 500 °C.