Patent ID: 7235810

Claim:
A semiconductor device comprising: a semiconductor layer on an insulating surface; a gate insulating film formed to be in contact with the semiconductor layer; and a gate electrode formed to be in contact with the gate insulating film; wherein: the gate electrode includes: a first layer of the gate electrode made of a semiconductor film formed to be in contact with the gate insulating film; and a second layer of the gate electrode formed to be in contact with the first layer of the gate electrode, wherein a part of the first layer of the gate electrode extends beyond at least one of side edges of the second layer of the gate electrode; the semiconductor layer includes: a channel formation region; a first impurity region of one conductivity type in contact with the insulating surface; and a second impurity region of the one conductivity type sandwiched between the channel formation region and the first impurity region of the one conductivity type and being in contact with the channel formation region, the second impurity region having a first portion and a second portion, wherein the second portion of the second impurity region overlaps with at least a portion of the part of the first layer of the gate electrode, that extends beyond at least one side edge of the second layer of the gate electrode, wherein the first portion of the second impurity region does not overlap with the first layer of the gate electrode, wherein the first portion and the second portion of the second impurity region are in contact with the insulating surface, wherein a first material included in the first layer of the gate electrode is different from a second material included in the second layer of the gate electrode, and wherein a concentration of the impurity element of the one conductivity type in the second impurity region is lower than a concentration of the impurity element of the one conductivity type in the first impurity region.