A TFT includes a gate electrode, an active layer, a source electrode, a drain electrode, and a buffer layer. The gate electrode is formed on the substrate; the active layer is formed on the gate electrode. The source and drain electrodes, formed on the active layer, are separated by a predetermined distance. The buffer layer is formed between the active layer and the source and drain electrodes. The buffer layer has a substantially continuously varying content ratio corresponding to a buffer layer thickness. The buffer layer is formed to suppress oxidation of the active layer, and reduce contact resistance.