A thin-film transistor includes a substrate, a gate electrode over the substrate, an insulating layer over the gate electrode, and a semiconductor layer over the insulating layer. The semiconductor layer includes a channel region, a source region, and a drain region. A source electrode is over the source region, and a drain electrode is over the drain region. The source electrode and the drain electrode each comprise Ni and a metal other than Ni. The channel region, the source region, and the drain region comprise at least one of a polycrystalline silicon that is formed by crystallizing an amorphous silicon layer by thermally diffusing the Ni in the source electrode and the drain electrode into the semiconductor layer and a microcrystalline silicon that is formed by crystallizing an amorphous silicon layer by thermally diffusing the Ni in the source electrode and the drain electrode into the semiconductor layer.