We report the effect of germanium doping on the active layer of amorphous Zinc -Tin-Oxide (a-ZTO) thin film transistor (TFT). Amorphous thin film samples were prepared by RF magnetron sputtering using single targets composed of Zn2Ge0.05Sn0.95O4 and Zn2SnO4 with variable oxygen contents in the sputtering gases. In comparison with undoped, Ge-doped a-ZTO films exhibited five order of magnitude lower carrier density with a significantly higher Hall-mobility, which might be due to suppressed oxygen vacancies in the a-ZTO lattice since the Ge substituent for the Sn site has relatively higher oxygen affinity. Thus, the bulk and interface trap densities of Ge-doped a-ZTO film were decreased one order of magnitude to 7.047 x 10(18) eV(-1)cm(3) and 3.52 x 10(11) eV(-1)cm(2), respectively. A bottom-gate TFT with the Ge-doped a-ZTO active layer showed considerably improved performance with a reduced SS, positively shifted Vth, and two orders of magnitude increased Ion/Ioff ratio, attributable to the doped Ge ions.