Semiconductor device, manufacturing method thereof, and display device

Abstract

A multi-gate structure is used and a width (d 1 ) of a high concentration impurity region sandwiched by two channel forming regions in a channel length direction is set to be shorter than a width (d 2 ) of low concentration impurity regions in the channel length direction. Thus, a resistance of the entire semiconductor layer of a TFT which is in an on state is reduced to increase an on current. In addition, a carrier life time due to photoexcitation produced in the high concentration impurity region can be shortened to reduce light sensitivity.

1 . (canceled) 
     
     
         2 . A display device comprising:
 a first conductive layer comprising a first metal element;   a semiconductor layer comprising a first channel forming region and a second channel forming region overlapping the first conductive layer, the first channel forming region and the second channel forming region being electrically connected in series;   a second conductive layer comprising a first gate electrode and a second gate electrode, the first gate electrode and the second gate electrode overlapping the first channel forming region and the second channel forming region, respectively; and   a third conductive layer and a fourth conductive layer over the semiconductor layer, the third conductive layer being in electrical contact with one of a source region and a drain region of the semiconductor layer, and the fourth conductive layer being in electrical contact with the other one of the source region and the drain region of the semiconductor layer,   wherein the first channel forming region entirely overlaps with the first conductive layer and one of the third conductive layer and the fourth conductive layer when seen in a cross sectional view taken along a length direction of the first channel forming region.   
     
     
         3 . A display device comprising:
 a first conductive layer comprising a first metal element;   a semiconductor layer comprising a first channel forming region and a second channel forming region overlapping the first conductive layer, the first channel forming region and the second channel forming region being electrically connected in series;   a second conductive layer comprising a first gate electrode and a second gate electrode, the first gate electrode and the second gate electrode overlapping the first channel forming region and the second channel forming region, respectively; and   a third conductive layer and a fourth conductive layer over the semiconductor layer, the third conductive layer being in electrical contact with one of a source region and a drain region of the semiconductor layer, and the fourth conductive layer being in electrical contact with the other one of the source region and the drain region of the semiconductor layer,   wherein the first channel forming region entirely overlaps with the first conductive layer and one of the third conductive layer and the fourth conductive layer when seen in a cross sectional view taken along a length direction of the first channel forming region, and   wherein the second channel forming region entirely overlaps with the first conductive layer and the other one of the third conductive layer and the fourth conductive layer when seen in a cross sectional view taken along a length direction of the second channel forming region.   
     
     
         4 . The display device according to  claim 3 ,
 wherein the second channel forming region is entirely overlapped by the other one of the third conductive layer and the fourth conductive layer.   
     
     
         5 . A display device comprising:
 a first conductive layer comprising a first metal element;   a semiconductor layer comprising a first channel forming region and a second channel forming region overlapping the first conductive layer, the first channel forming region and the second channel forming region being electrically connected in series;   a second conductive layer comprising a first gate electrode and a second gate electrode, the first gate electrode and the second gate electrode overlapping the first channel forming region and the second channel forming region, respectively; and   a third conductive layer and a fourth conductive layer over the semiconductor layer, the third conductive layer being in electrical contact with one of a source region and a drain region of the semiconductor layer, and the fourth conductive layer being in electrical contact with the other one of the source region and the drain region of the semiconductor layer,   wherein the first channel forming region entirely overlaps with the first conductive layer and one of the third conductive layer and the fourth conductive layer when seen in a cross sectional view taken along a length direction of the first channel forming region,   wherein the second channel forming region entirely overlaps with the first conductive layer when seen in a cross sectional view taken along a length direction of the second channel forming region, and   wherein the second channel forming region does not overlap with the other one of the third conductive layer and the fourth conductive layer.   
     
     
         6 . A display device comprising:
 a semiconductor layer comprising a first channel forming region and a second channel forming region, the first channel forming region and the second channel forming region being electrically connected in series;   a second conductive layer comprising a first gate electrode and a second gate electrode, the first gate electrode and the second gate electrode overlapping the first channel forming region and the second channel forming region, respectively; and   a third conductive layer and a fourth conductive layer over the semiconductor layer, the third conductive layer being in electrical contact with one of a source region and a drain region of the semiconductor layer, and the fourth conductive layer being in electrical contact with the other one of the source region and the drain region of the semiconductor layer,   wherein the first channel forming region entirely overlaps with one of the third conductive layer and the fourth conductive layer when seen in a cross sectional view taken along a length direction of the first channel forming region.   
     
     
         7 . The display device according to  claim 2 ,
 wherein the first conductive layer and the second conductive layer are electrically connected to each other.   
     
     
         8 . The display device according to  claim 3 ,
 wherein the first conductive layer and the second conductive layer are electrically connected to each other.   
     
     
         9 . The display device according to  claim 5 ,
 wherein the first conductive layer and the second conductive layer are electrically connected to each other.   
     
     
         10 . The display device according to  claim 2 ,
 wherein the first conductive layer is continuously formed.   
     
     
         11 . The display device according to  claim 3 ,
 wherein the first conductive layer is continuously formed.   
     
     
         12 . The display device according to  claim 5 ,
 wherein the first conductive layer is continuously formed.   
     
     
         13 . The display device according to  claim 2 ,
 wherein the first channel forming region is entirely overlapped by the one of the third conductive layer and the fourth conductive layer.   
     
     
         14 . The display device according to  claim 3 ,
 wherein the first channel forming region is entirely overlapped by the one of the third conductive layer and the fourth conductive layer.   
     
     
         15 . The display device according to  claim 5 ,
 wherein the first channel forming region is entirely overlapped by the one of the third conductive layer and the fourth conductive layer.   
     
     
         16 . The display device according to  claim 6 ,
 wherein the first channel forming region is entirely overlapped by the one of the third conductive layer and the fourth conductive layer.   
     
     
         17 . The display device according to  claim 2 ,
 wherein any one of the first conductive layer, the second conductive layer, and the third conductive layer has a function of a light shielding layer.   
     
     
         18 . The display device according to  claim 3 ,
 wherein any one of the first conductive layer, the second conductive layer, and the third conductive layer has a function of a light shielding layer.   
     
     
         19 . The display device according to  claim 5 ,
 wherein any one of the first conductive layer, the second conductive layer, and the third conductive layer has a function of a light shielding layer.