In recent years, display devices on which a sensor that detects light (also referred to as a “photosensor”) is mounted have attracted attention.  By providing a display device with a photosensor, input of information can be performed on a display screen.  For example, a display device having an image capturing function can be given (for example, see Patent Document 1). 
In addition to the above display device, as a semiconductor device including a photosensor, an imaging device which is used in an electronic appliance such as a scanner or a digital still camera can be given. 
In a semiconductor device including a photosensor such as the above display device or the imaging device, the photosensor detects light reflected by an object or light emitted from an object; thus, the semiconductor device can detect the presence of the object around a region in which the photosensor is provided. 
[Reference]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No.  2001-292276