Stability of amorphous zinc tin oxide thin film transistors (TFTs) is investigated under positive bias stressing (PBS) at temperatures between 65 and 105 °C. The time and temperature dependence of the threshold voltage shift is analyzed using a thermalization energy concept. A maximum energy barrier to defect migration of 0.76 eV and the attempt-to-escape frequency of 10$7$ s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ are extracted. These values are compared with those under PBS of amorphous indium gallium zinc oxide and hydrogenated amorphous silicon TFTs. The oxygen vacancy migration model that was proposed for amorphous oxide semiconductors is contrasted with the defect creation model that was proposed for amorphous silicon.