What causes a light-emitting diode (LED) to emit light?

A light-emitting diode (LED) emits light when it is forward-biased, which means that a forward current flows through it. In this state, electrons from the n-type material move toward the p-type material, where they recombine with holes. This recombination process releases energy in the form of photons, which is perceived as light. The specific wavelength, or color, of the emitted light depends on the materials used in constructing the LED and the energy bandgap of the semiconductor.

In contrast, reverse current does not cause the LED to emit light, as it can lead to breakdown if the reverse voltage exceeds the diode's limit but does not facilitate recombination that results in light emission. Capacitively-coupled and inductively-coupled RF signals operate based on different principles that are not relevant to the functioning of an LED, which relies on direct electrical current to create light. Thus, the reason forward current is the correct answer is that it is the mechanism necessary for the light-emitting process in LEDs.