A thin film transistor is provided, which includes a gate electrode layer over a substrate, a gate insulating layer over the gate electrode layer, a layer including an amorphous semiconductor over the gate insulating layer, a pair of crystal regions over the layer including the amorphous semiconductor, and source and drain regions over and in contact with the pair of crystal regions. The source and drain regions include a microcrystalline semiconductor layer to which an impurity imparting one conductivity type is added.