A semiconductor device comprising a circuit that has been formed on a substrate and that includes a thin-film diode and a protection circuit for protecting the thin-film diode, the protection circuit having a protection diode, the thin-film diode being an in-circuit diode that is indispensable to allow the circuit to perform its intended function, wherein the thin-film diode comprises: at least one semiconductor layer, which is arranged on the substrate and which has a first region, a second region and a channel region that is located between the first and second regions; a gate electrode, which is arranged so as to overlap with the channel region; a gate insulating layer, which is arranged between the gate electrode and the semiconductor layer; a first electrode, which is arranged on the first region and which is electrically connected to the first region and the gate electrode; and a second electrode, which is arranged on, and electrically connected to, the second region; and wherein (a) the conductivity type of the thin-film diode is N-type and the anode electrode of the protection diode is connected to a line that is connected to either the gate electrode or the first electrode of the thin-film diode, the cathode electrode of the protection diode is connected to a line leading to a VDD power supply, and the protection circuit includes no other diodes whose cathode electrodes are connected to the line; or wherein (b) the conductivity type of the thin-film diode is P-type and the cathode electrode of the protection diode is connected to the line that is connected to either the gate electrode or the first electrode of the thin-film diode, the anode electrode of the protection diode is connected to a line leading to a VSS power supply, and the protection circuit includes no other diodes whose anode electrodes are connected to the line; and wherein the protection diode and the thin-film diode are not connected in parallel. at least one semiconductor layer, which is arranged on the substrate and which has a first region, a second region and a channel region that is located between the first and second regions; a gate electrode, which is arranged so as to overlap with the channel region; a gate insulating layer, which is arranged between the gate electrode and the semiconductor layer; a first electrode, which is arranged on the first region and which is electrically connected to the first region and the gate electrode; and a second electrode, which is arranged on, and electrically connected to, the second region; and wherein (a) the conductivity type of the thin-film diode is N-type and the anode electrode of the protection diode is connected to a line that is connected to either the gate electrode or the first electrode of the thin-film diode, the cathode electrode of the protection diode is connected to a line leading to a VDD power supply, and the protection circuit includes no other diodes whose cathode electrodes are connected to the line; or wherein (b) the conductivity type of the thin-film diode is P-type and the cathode electrode of the protection diode is connected to the line that is connected to either the gate electrode or the first electrode of the thin-film diode, the anode electrode of the protection diode is connected to a line leading to a VSS power supply, and the protection circuit includes no other diodes whose anode electrodes are connected to the line; and wherein the protection diode and the thin-film diode are not connected in parallel. wherein (a) the conductivity type of the thin-film diode is N-type and the anode electrode of the protection diode is connected to a line that is connected to either the gate electrode or the first electrode of the thin-film diode, the cathode electrode of the protection diode is connected to a line leading to a VDD power supply, and the protection circuit includes no other diodes whose cathode electrodes are connected to the line; or wherein (b) the conductivity type of the thin-film diode is P-type and the cathode electrode of the protection diode is connected to the line that is connected to either the gate electrode or the first electrode of the thin-film diode, the anode electrode of the protection diode is connected to a line leading to a VSS power supply, and the protection circuit includes no other diodes whose anode electrodes are connected to the line; and wherein the protection diode and the thin-film diode are not connected in parallel.