A buffer layer (not shown) may be formed on the substrate 110. The buffer layer may be formed on the entire substrate 110. The buffer layer may prevent or substantially prevent the diffusion of metal atoms and/or impurities from the substrate 110. In addition, the buffer layer may control a rate of heat transfer in a crystallization process for forming an active layer, thereby obtaining substantially uniform the active layer. Further, the buffer layer may improve a surface flatness of the substrate 110 when a surface of the substrate 110 is relatively irregular. The buffer layer may be formed using a silicon compound, a metal oxide, etc.

An active layer 130 may be formed on the substrate 110. The active layer 130 may be formed using an oxide semiconductor, an inorganic semiconductor, an organic semiconductor, etc.

A gate insulation layer 150 may be formed on the active layer 130. The gate insulation layer 150 may cover the active layer 130, and may be formed on the substrate 110. The gate insulation layer 150 may be formed using a silicon compound, a metal oxide, etc.

A gate electrode 170 may be formed on a portion of the gate insulation layer 150 under which the active layer 130 is located. The gate electrode 170 may be formed using a metal, a metal alloy, a metal nitride, a conductive metal oxide, transparent conductive materials, etc. These may be used alone or in a suitable combination thereof. In some example embodiments, the gate electrode 170 may have a multi-layered structure.