Semiconductor device and method for manufacturing the same

Abstract

An object is to provide a semiconductor device including a thin film transistor with excellent electrical characteristics and high reliability and a method for manufacturing the semiconductor device with high mass productivity. A main point is to form a low-resistance oxide semiconductor layer as a source or drain region after forming a drain or source electrode layer over a gate insulating layer and to foiiii an oxide semiconductor film thereover as a semiconductor layer. It is preferable that an oxygen-excess oxide semiconductor layer be used as a semiconductor layer and an oxygen-deficient oxide semiconductor layer be used as a source region and a drain region.

1 . (canceled) 
     
     
         2 . A display device comprising:
 a first conductive layer;   a first insulating film over the first conductive layer;   a second conductive layer over the first insulating film; and   a second insulating film over the second conductive layer,   wherein the first conductive layer and the second conductive layer are provided outside a pixel portion,   wherein the first conductive layer is formed using a same material as a gate electrode of a transistor in the pixel portion,   wherein the first insulating film is formed using a same material as a gate insulating film of the transistor,   wherein the second conductive layer is formed using a same material as a source electrode and a drain electrode of the transistor, and   wherein a potential of the first conductive layer is in a floating state.   
     
     
         3 . The display device according to  claim 2 , wherein a width of the first conductive layer is larger than a width of the second conductive layer. 
     
     
         4 . The display device according to  claim 2 , further comprising a third conductive layer electrically connected to the second conductive layer. 
     
     
         5 . The display device according to  claim 2 , wherein the transistor comprises a semiconductor layer. 
     
     
         6 . The display device according to  claim 5 , wherein the semiconductor layer comprises indium, gallium, and zinc.