Display Device And Manufacturing Method Thereof

Abstract

Disclosed is a display device including a transistor showing extremely low off current. In order to reduce the off current, a semiconductor material whose band gap is greater than that of a silicon semiconductor is used for forming a transistor, and the concentration of an impurity which serves as a carrier donor of the semiconductor material is reduced. Specifically, an oxide semiconductor whose band gap is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, more preferably greater than or equal to 3 eV is used for a semiconductor layer of a transistor, and the concentration of an impurity which serves as a carrier donor included is reduced. Consequently, the off current of the transistor per micrometer in channel width can be reduced to lower than 10 zA/μm at room temperature and lower than 100 zA/μm at 85° C.

1 . (canceled) 
     
     
         2 . A display device comprising:
 a transistor; and   a pixel electrode electrically connected to the transistor,   wherein the transistor comprises:
 a semiconductor film comprising a channel formation region; 
 a source electrode over and electrically connected to the semiconductor film; and 
 a drain electrode over and electrically connected to the semiconductor film; 
   a gate insulating film over the semiconductor film, the source electrode and the drain electrode; and   a gate electrode over the gate insulating film;   wherein the transistor has a characteristic that a current flowing between the source electrode and the drain electrode is lower than 10 zA/μm at room temperature when a voltage of 3.1 V is applied between the source electrode and the drain electrode,   wherein the current is normalized by a channel width of the transistor,   wherein the semiconductor film comprises indium, zinc, and oxygen, and   wherein an energy gap of the semiconductor film is larger than or equal to 2 eV.   
     
     
         3 . The display device according to  claim 2 , wherein the semiconductor film comprises at least one metal element selected from the group consisting of gallium, aluminum, manganese, and cobalt. 
     
     
         4 . The display device according to  claim 2 , comprising an insulating film, wherein the semiconductor film, the source electrode and the drain electrode are over and in contact with the insulating film. 
     
     
         5 . The display device according to  claim 4 , wherein the source electrode and the drain electrode are in contact with the gate insulating film. 
     
     
         6 . The display device according to  claim 2 , comprising a liquid crystal layer over the pixel electrode. 
     
     
         7 . The display device according to  claim 6 , comprising a counter electrode over the liquid crystal layer. 
     
     
         8 . The display device according to  claim 2 , wherein the semiconductor film is intrinsic or substantially intrinsic. 
     
     
         9 . The display device according to  claim 2 , wherein the semiconductor film comprises a crystalline region. 
     
     
         10 . The display device according to  claim 9 , wherein the crystalline region has a c-axis orientation with respect to a surface of the semiconductor film. 
     
     
         11 . A display device comprising:
 a transistor; and   a pixel electrode electrically connected to the transistor,   wherein the transistor comprises:
 a semiconductor film comprising a channel formation region; 
 a source electrode over and electrically connected to the semiconductor film; and 
 a drain electrode over and electrically connected to the semiconductor film; 
   a gate insulating film over the semiconductor film, the source electrode and the drain electrode; and   a gate electrode over the gate insulating film;   wherein the transistor has a characteristic that a current flowing between the source electrode and the drain electrode is lower than 10 zA/μm at room temperature when a voltage of 3.1 V is applied between the source electrode and the drain electrode,   wherein the current is normalized by a channel width of the transistor.   
     
     
         12 . The display device according to  claim 11 , wherein the semiconductor film comprises at least one metal element selected from the group consisting of gallium, aluminum, manganese, and cobalt. 
     
     
         13 . The display device according to  claim 11 , comprising an insulating film, wherein the semiconductor film, the source electrode and the drain electrode are over and in contact with the insulating film. 
     
     
         14 . The display device according to  claim 13 , wherein the source electrode and the drain electrode are in contact with the gate insulating film. 
     
     
         15 . The display device according to  claim 11 , comprising a liquid crystal layer over the pixel electrode. 
     
     
         16 . The display device according to  claim 15 , comprising a counter electrode over the liquid crystal layer. 
     
     
         17 . The display device according to  claim 11 , wherein the semiconductor film is intrinsic or substantially intrinsic. 
     
     
         18 . The display device according to  claim 11 , wherein the semiconductor film comprises a crystalline region. 
     
     
         19 . The display device according to  claim 18 , wherein the crystalline region has a c-axis orientation with respect to a surface of the semiconductor film.