In an embodiment, by doping impurities at a lower concentration than the ion implantation process, a low-concentration doped region may be formed between the channel region CR and the source region SR, and between the channel region CR and the drain region DR. The low-concentration doped region may act as a buffer in the active layer ACT, thereby improving electrical properties of the thin film transistor.

Referring to FIG. 13, an insulation interlayer 150 may be formed on the gate electrode GE, and a source contact hole CHS and a drain contact hole CHD may be formed.

First, the insulation interlayer 150 covering the gate electrode GE may be formed on the gate insulation layer 140. The insulation interlayer 150 may include an inorganic insulation layer, an organic insulation layer, or a combination thereof. For example, the insulation interlayer 150 may include silicon oxide, silicon nitride, silicon carbide, or a combination thereof, or may include an insulating metal oxide, such as aluminum oxide, tantalum oxide, hafnium oxide, zirconium oxide, titanium oxide, or the like. When the insulation interlayer 150 includes an organic insulation layer, the insulation interlayer 150 may include polyimide, polyamide, acrylic resin, phenol resin, benzocyclobutene (BCB), or the like.