A semiconductor device with a TFT includes a substrate, an island-shaped semiconductor film serving as an active layer of the TFT on or over the substrate, a pair of source/drain regions formed in the semiconductor film, and a channel region formed between the pair of source/drain regions in the semiconductor film. The pair of source/drain regions is thinner than the remainder of the semiconductor film other than the source/drain regions. The thickness difference between the pair of source/drain regions and the remainder of the semiconductor film is in a range from 10 angstrom (.ANG.) to 100 angstrom. The total process steps are reduced and the operation characteristic and reliability of the device are improved.