Provided is a thin film semiconductor device such as an organic light-emitting display which includes a thin film transistor (TFT) having a lightly doped region. The thin film semiconductor includes a substrate, a first active pattern, a first lower conductive pattern, and a first upper conductive pattern. The first active pattern is disposed on the substrate and includes a channel region, a lightly doped region, and a heavily doped region. The first lower conductive pattern is disposed on the first active pattern and covers the channel region. The first upper conductive pattern is disposed on the first lower conductive pattern and covers the channel region and the lightly doped region.