Patent ID: 8063403

Claim:
A semiconductor device comprising: a first conductive layer on an insulating surface; a gate insulating film over the first conductive layer; a first semiconductor layer which overlaps with the first conductive layer over the gate insulating film; a second semiconductor layer over the first semiconductor layer; a third semiconductor layer over the second semiconductor layer; a pair of fourth semiconductor layers over the third semiconductor layer; and a pair of second conductive layers over the pair of fourth semiconductor layers, respectively, wherein the pair of the fourth semiconductor layers comprises a first impurity element imparting one conductivity type, wherein the second semiconductor layer comprises a microcrystalline semiconductor, wherein the third semiconductor layer comprises an amorphous semiconductor, wherein a concentration of a second impurity element in the first semiconductor layer is lower than a concentration of the second impurity element in the second semiconductor layer, and wherein the second semiconductor layer comprises the second impurity element imparting a same conductivity type as the first impurity element.