There is provided a thin film transistor having improved reliability. A gate
electrode includes a first gate electrode having a taper portion and a second gate
electrode with a width narrower than the first gate electrode. A semiconductor
layer is doped with phosphorus of a low concentration through the first gate electrode.
In the semiconductor layer, two kinds of n-;-type impurity regions are
formed between a channel formation region and n+-type impurity regions.
Some of the n-;-type impurity regions overlap with a gate electrode,
and the other n-;-type impurity regions do not overlap with the gate
electrode. Since the two kinds of n-;-type impurity regions are formed,
an off current can be reduced, and deterioration of characteristics can be suppressed.
