A thin film transistor having a high operation speed with a field effect mobility greater than 20 cm2/Vs and a method for manufacturing the same, and a semiconductor device having the same are provided. A thin film transistor in which a gate electrode, a gate insulating film and an oxide semiconductor film are laminated on a substrate, a source region and a drain region are respectively formed in outer portions of the oxide semiconductor film in the width direction, and a channel region is formed in a region between the source region and the drain region; and a source electrode is connected to the source region, while a drain electrode is connected to the drain region. The gate insulating film contains fluorine; and the ratio of the width W of the channel region to the length L thereof, namely W/L is less than 8.