The second active layer 131 may be formed of an oxide semiconductor. Since an oxide semiconductor material has a larger bandgap compared to a silicon material, electrons do not cross the bandgap in an off state, and thus, an off-current is low. Accordingly, a thin film transistor including an active layer formed of an oxide semiconductor may be suitable for a switching thin film transistor having a short on time and a long off time, but is not limited thereto. Depending on characteristics of the display device, it may be applied as a driving thin film transistor. And, since the off-current is small, a size of an auxiliary capacitor may be reduced, which is suitable for a high-resolution display device. For example, the second active layer 131 may be formed of a metal oxide, for example, may be formed of various metal oxides such as indium-gallium-zinc-oxide (IGZO) and the like. A description is made assuming that the second active layer 131 of the second thin film transistor 130 is formed of IGZO among various metal oxides, but the present disclosure is not limited thereto. The second active layer 131 of the second thin film transistor 130 may be formed of another metal oxide such as IZO(indium-zinc-oxide), IGTO(indium-gallium-tin-oxide), or IGO(indium-gallium-oxide), rather than IGZO.

The second active layer 131 may be formed by depositing a metal oxide on the second buffer layer 114, performing a heat treatment process for stabilization, and then, patterning the metal oxide.

An insulating material and a metallic material may be sequentially formed on the entire surface of the substrate including the second active layer 131, and a photoresist pattern may be formed on the metallic material.