Semiconductor device comprising lightly doped drain (LDD) region between channel and drain region

A semiconductor device having TFT using an oxide semiconductor comprising:

• the oxide semiconductor including a channel region, a source region, a drain region, and a transition region between the channel region and the source region and between the channel region and the drain region, wherein
• a resistivity of the transition region is smaller than that of the channel region, and larger than that of the source region or the drain region,
• a source electrode is formed overlapping the source region, and a drain electrode is formed overlapping the drain region,
• a thickness of the transition region of the oxide semiconductor is larger than a thickness of the channel region of the oxide semiconductor,
• a first gate insulating film covers the oxide semiconductor,
• a second gate insulating film covers the first gate insulating film,
• a through hole is formed in the second gate insulating film at a place corresponding to the transition region of the oxide semiconductor, and
• a gate electrode is formed covering the second gate insulating film and the through hole.