A thin film transistor can include an active layer; a gate electrode at least partially overlapping with the active layer; and a source electrode and a drain electrode spaced apart from each other and connected to the active layer, respectively. Also, the active layer includes a channel overlapping with the gate electrode; a first connection portion connected to a first side of the channel portion; and a second connection portion connected to a second side of the channel portion. Also, the channel has a crystalline structure, the first connection portion includes a first amorphous portion contacting the channel, and the second connection portion includes a second amorphous portion contacting the channel.