What is a semiconductor device that emits light when current flows through it?

A light-emitting diode (LED) is a semiconductor device that emits light when an electric current passes through it. This phenomenon is known as electroluminescence, which occurs when electrons recombine with holes within the device, releasing energy in the form of photons, or light. The materials used in LEDs typically have a bandgap that allows them to emit light at a specific wavelength, which corresponds to different colors.

In contrast, a transistor is primarily used for switching or amplifying electrical signals and does not emit light during its operation. A photovoltaic cell is designed to convert light into electrical energy rather than emit light itself. A varistor is a voltage-dependent resistor used to protect circuits from voltage surges but does not have the property of emitting light. Therefore, the light-emitting diode stands out as the correct answer due to its specific function of light emission through electrical current.