By providing appropriate TFT structures arranged in various circuits of the semiconductor device in response to the functions required by the circuits, it is made possible to improve the operating performances and the reliability of a semiconductor device, reduce power consumption as well as realizing reduced manufacturing cost and increase in yield by lessening the number of processing steps. An LDD region of a TFT is formed to have a concentration gradient of an impurity element for controlling conductivity which becomes higher as the distance from a drain region decreases. In order to form such an LDD region having a concentration gradient of an impurity element, the present invention uses a method in which a gate electrode having a taper portion is provided to thereby dope an ionized impurity element for controlling conductivity accelerated in the electric field so that it penetrates through the gate electrode and a gate insulating film into a semiconductor layer.

A novel semiconductor device including an oxide semiconductor is provided. In particular, a planar semiconductor device including an oxide semiconductor is provided. A semiconductor device including an oxide semiconductor and having large on-state current is provided. The semiconductor device includes an oxide insulating film, an oxide semiconductor film over the oxide insulating film, a source electrode and a drain electrode in contact with the oxide semiconductor film, a gate insulating film between the source electrode and the drain electrode, and a gate electrode overlapping the oxide semiconductor film with the gate insulating film. The oxide semiconductor film includes a first region overlapped with the gate electrode and a second region not overlapped with the gate electrode, the source electrode, and the drain electrode. The first region and the second region have different impurity element concentrations. The gate electrode, the source electrode, and the drain electrode contain the same metal element.

A polysilicon thin film transistor, a manufacturing method thereof, an array substrate involve display technology field, and can repair the boundary defect and the defect state in polysilicon, suppress the hot carrier effect and make the characteristics of TFTs more stable. The polysilicon thin film transistor includes a gate electrode, a source electrode, a drain electrode and an active layer, the active layer comprises at least a channel area, first doped regions, second doped regions and heavily doped regions, and the first doped regions are disposed on two sides of the channel area, the second doped regions are disposed on sides of the first doped regions away from the channel area; the heavily doped regions are disposed on sides of the second doped regions opposed to the first doped regions; and dosage of ions in the heavily doped regions lies between that in the first doped regions and that in the second doped regions.