Semiconductor device

Abstract

An oxide semiconductor layer in which “safe” traps exist exhibits two kinds of modes in photoresponse characteristics. By using the oxide semiconductor layer, a transistor in which light deterioration is suppressed to the minimum and the electric characteristics are stable can be achieved. The oxide semiconductor layer exhibiting two kinds of modes in photoresponse characteristics has a photoelectric current value of 1 pA to 10 nA inclusive. When the average time τ 1 until which carriers are captured by the “safe” traps is large enough, there are two kinds of modes in photoresponse characteristics, that is, a region where the current value falls rapidly and a region where the current value falls gradually, in the result of a change in photoelectric current over time.

1 . A semiconductor device, comprising:
 an oxide semiconductor layer at least a part of which overlaps with a gate electrode with a gate insulating layer provided therebetween,   wherein the oxide semiconductor layer has a channel formation region, and   wherein photoresponse characteristics of the channel formation region of the oxide semiconductor layer have two kinds of modes after light irradiation is performed and a light source is turned off.   
     
     
         2 . The semiconductor device according to  claim 1 , wherein the oxide semiconductor layer has a photoelectric current value of greater than or equal to 1 pA and less than or equal to 10 nA. 
     
     
         3 . The semiconductor device according to  claim 1 , wherein the oxide semiconductor layer comprises at least one of In, Zn, and Ga. 
     
     
         4 . The semiconductor device according to  claim 1 , wherein a photoelectric current value of the oxide semiconductor layer after 100 seconds of light irradiation is greater than or equal to 400 aA/μm and less than or equal to 0.1 pA/μm. 
     
     
         5 . The semiconductor device according to  claim 1 , wherein the two kinds of modes comprise a region where a photoelectric current value of the oxide semiconductor layer falls rapidly and a region where the photoelectric current value falls gradually, in a result of a change in photoelectric current over time. 
     
     
         6 . A semiconductor device comprising:
 a pixel porting including a transistor,   wherein a channel formation region of the transistor is formed in an oxide semiconductor layer whose photoresponse characteristics have two kinds of modes after light irradiation is performed and a light source is turned off.   
     
     
         7 . The semiconductor device according to  claim 6 , wherein the oxide semiconductor layer has a photoelectric current value of greater than or equal to 1 pA and less than of equal to 10 nA. 
     
     
         8 . The semiconductor device according to  claim 6 , wherein the oxide semiconductor layer comprises at least one of In, Zn, and Ga. 
     
     
         9 . The semiconductor device according to  claim 6 , wherein a photoelectric current value of the oxide semiconductor layer after 100 seconds of light irradiation is greater than or equal to 400 aA/μm and less than or equal to 0.1 pA/μm. 
     
     
         10 . The semiconductor device according to  claim 6 , wherein the two kinds of modes comprise a region where a photoelectric current value of the oxide semiconductor layer falls rapidly and a region where the photoelectric current value falls gradually, in a result of a change in photoelectric current over time. 
     
     
         11 . A semiconductor device, comprising:
 an oxide semiconductor layer at least a part of which overlaps with a gate electrode with a gate insulating layer provided therebetween,   wherein the oxide semiconductor layer has a channel formation region, and   wherein photoresponse characteristics of the channel formation region of the oxide semiconductor layer have at least a first mode and a second mode after light irradiation is performed and a light source is turned off.   
     
     
         12 . The semiconductor device according to  claim 11 , wherein the first mode is a region where a photoelectric current value of the oxide semiconductor layer falls rapidly and the second mode is a region where the photoelectric current value of the oxide semiconductor layer falls gradually, in a result of a change in photoelectric current over time.