A thin film transistor (TFT) using an oxide semiconductor layer as an active layer, a method of manufacturing the TFT, and a flat panel display (FPD) including the TFT are taught. The TFT includes a gate electrode formed on a substrate, an oxide semiconductor layer electrically insulated from the gate electrode by a gate insulating layer, and the oxide semiconductor layer including a channel region, a source region, and a drain region, and a source electrode and a drain electrode respectively electrically contacting the source region and the drain region. The oxide semiconductor layer is formed of an InZnO or IZO layer (indium zinc oxide layer) including Zr. The carrier density of the IZO layer is controlled to be 1×1013 to 1×1018 #cm−3 by controlling an amount of Zr.