We fabricated amorphous oxide semiconductor thin-film transistors (TFTs) using Ge-doped InZnO (Ge-IZO) thin films as active-channel layers. The Ge-IZO thin films were deposited at room temperature by radio-frequency (RF) magnetron co-sputtering system, and then annealed in air for 1 h at 300 °C. Some processing parameters such as sputtering oxygen partial pressure [O2/(Ar + O2)] and sputtering power for GeO2 target were changed to investigate what was the optimal amount of Ge in the Ge-IZO active layer. A small concentration of Ge added to IZO by co-sputtering enhanced the carrier concentration, mobility, and conductivity; but further increase in Ge concentration degraded the device performance. In order to optimize the electrical properties of Ge-IZO TFTs, we tried to adjust the processing parameters and the best Ge-IZO TFT was obtained at a co-sputtering oxygen partial pressure of 2% and GeO2 target power of 10 W. The fabricated Ge-IZO TFT exhibited an on/off ratio of 3.0 x 10(7), a saturation mobility of 13.05 cm2/V·s, a subthreshold swing of 0.95 V/dec, and a threshold voltage of 0 V. XPS and XRD analyses of Ge-IZO films were performed to investigate the binding energies of atoms in Ge-IZO films and the crystallinity of the films. 90% transmittance of visible light was achieved, which makes the technology useful for transparent devices.