Thin film transistor and semiconductor device

A thin film transistor comprising:

• a conductive film on an insulating surface;
• a gate insulating film over the conductive film;
• a first semiconductor film which overlaps with the conductive film over the gate insulating film;
• and
• a pair of second semiconductor films over the first semiconductor film, wherein the pair of second semiconductor films comprises a first impurity element imparting one conductivity type,
• wherein the first semiconductor film comprises at least a third semiconductor film comprising a microcrystalline semiconductor and a fourth semiconductor film comprising an amorphous semiconductor,
• wherein the third semiconductor film comprises a layer which is separated from the gate insulating film,
• wherein the layer comprises a second impurity element imparting a same conductivity type as the first impurity element, and
• wherein the fourth semiconductor film is provided between the third semiconductor film and the pair of second semiconductor films.

A semiconductor device comprising:

• a first conductive film on an insulating surface;
• a gate insulating film over the first conductive film;
• a first semiconductor film which overlaps with the first conductive film over the gate insulating film;
• a pair of second semiconductor films over the first semiconductor film, wherein the pair of second semiconductor films comprises a first impurity element imparting one conductivity type;
• and
• a pair of second conductive films over the pair of second semiconductor films, respectively,
• wherein the first semiconductor film comprises a microcrystalline semiconductor,
• wherein the first semiconductor film comprises a layer which is separated from the gate insulating film and the pair of second semiconductor films, and
• wherein the layer comprises a second impurity element imparting a same conductivity type as the first impurity element.

A semiconductor device comprising:

• a first conductive film on an insulating surface;
• a gate insulating film over the first conductive film;
• a first semiconductor film which overlaps with the first conductive film over the gate insulating film;
• a pair of second semiconductor films over the first semiconductor film, wherein the pair of second semiconductor films comprises a first impurity element imparting one conductivity type;
• and
• a pair of second conductive films over the pair of second semiconductor films, respectively,
• wherein the first semiconductor film comprises at least a third semiconductor film comprising a microcrystalline semiconductor, and a fourth semiconductor film comprising an amorphous semiconductor,
• wherein the third semiconductor film comprises a layer which is separated from the gate insulating film,
• wherein the layer comprises a second impurity element imparting a same conductivity type as the first impurity element, and
• wherein the fourth semiconductor film is provided between the third semiconductor film and the pair of second semiconductor films.