Semiconductor device and display device

Abstract

In a transistor including an oxide semiconductor film, field-effect mobility and reliability are improved. A semiconductor device includes a gate electrode, an insulating film over the gate electrode, an oxide semiconductor film over the insulating film, and a pair of electrodes over the oxide semiconductor film. The oxide semiconductor film includes a first oxide semiconductor film, a second oxide semiconductor film over the first oxide semiconductor film, and a third oxide semiconductor film over the second oxide semiconductor film. The first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film include the same elements. The second oxide semiconductor film includes a region having a higher carrier density than the first oxide semiconductor film and the third oxide semiconductor film.

What is claimed is: 
     
         1 . A semiconductor device comprising:
 a first gate electrode;   a first insulating film over the first gate electrode;   an oxide semiconductor film over the first insulating film; and   a pair of electrodes over the oxide semiconductor film,   wherein the oxide semiconductor film comprises:
 a first oxide semiconductor film; 
 a second oxide semiconductor film over the first oxide semiconductor film; and 
 a third oxide semiconductor film over the second oxide semiconductor film, 
   wherein the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film comprise the same elements, and   wherein the second oxide semiconductor film comprises a region having a higher carrier density than the first oxide semiconductor film and the third oxide semiconductor film.   
     
     
         2 . The semiconductor device according to  claim 1 , comprising:
 a second insulating film over the oxide semiconductor film and the pair of electrodes; and   a second gate electrode over the second insulating film.   
     
     
         3 . The semiconductor device according to  claim 1 , comprising:
 a second insulating film over the oxide semiconductor film and the pair of electrodes; and   a second gate electrode over the second insulating film,   wherein the first gate electrode and the second gate electrode are connected through an opening provided in the first insulating film and the second insulating film, and   wherein the first gate electrode and the second gate electrode each comprise a region positioned outside an edge portion of the oxide semiconductor film.   
     
     
         4 . The semiconductor device according to  claim 1 , wherein the second oxide semiconductor film comprises nitrogen. 
     
     
         5 . The semiconductor device according to  claim 1 , wherein the second oxide semiconductor film comprises a region having a higher nitrogen concentration than the first oxide semiconductor film and the third oxide semiconductor film. 
     
     
         6 . The semiconductor device according to  claim 1 ,
 wherein the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film separately comprise indium, M, and zinc, and   wherein M is aluminum, gallium, yttrium, or tin.   
     
     
         7 . The semiconductor device according to  claim 1 ,
 wherein the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film separately comprise a crystal part, and   wherein the crystal part has a c-axis alignment.   
     
     
         8 . A display device comprising:
 the semiconductor device according to  claim 1 ; and   a display element electrically connected to the semiconductor device.   
     
     
         9 . A display module comprising:
 the display device according to  claim 8 ; and   a touch sensor electrically connected to the display device.   
     
     
         10 . An electronic device comprising:
 the display module according to  claim 9 ; and   an operation key or a battery electrically connected to the display module.