The present invention has a photodiode and a circuit used to amplify the output of the photodiode. Two terminals are formed over the photodiode and circuit with an insulating layer interposed therebetween, and a dummy electrode with a larger area than that of either of the two terminals is formed thereover, adjacent to the two terminals. The dummy electrode is not connected to the photodiode or to the circuit of the semiconductor device. Because the dummy electrode has a wide area, damage due to electrostatic discharge occurs in the dummy electrode more easily than in the two terminals; thus, damage due to electrostatic discharge can be prevented from occurring in the semiconductor device.