A semiconductor device for a TFT includes a first semiconductor layer to be used as a channel, which is formed on a portion of an insulating layer in correspondence with an underlying gate electrode. The semiconductor device further includes a second semiconductor layer, an ohmic contact layer, and a metal layer formed on the insulating layer and the first semiconductor layer and patterned to expose portions of the insulating layer and the first semiconductor layer. The patterned metal layer forms source and drain electrodes. The semiconductor device also includes a passivation layer, which covers the insulating layer, the first semiconductor layer and the source and drain electrodes, and a pixel electrode, which contacts the drain electrode though a contact hole in the passivation layer.