A two-terminal electronic device that utilizes the properties of the semiconductor from which it is constructed. In a semiconductor diode without a pn junction, the bulk properties of the semiconductor itself are used to make a device whose characteristics may be sensitive to light, temperature, or electric field. In a diode with a pn junction, the properties of the pn junction are used. The most important property of a pn junction is that, under ordinary conditions, it will allow electric current to flow in only one direction. Under the proper circumstances, however, a pn junction may also be used as a voltage-variable capacitance, a switch, a light source, a voltage regulator, or a means to convert light into electrical power.