What is a key feature of semiconductor LEDs?

A key feature of semiconductor LEDs (Light Emitting Diodes) is that they emit light through spontaneous emission. This process occurs when electrons in the semiconductor recombine with holes, releasing energy in the form of photons, which constitute visible light. The spontaneous emission is fundamental to how LEDs produce light; when current flows through the LED, electrons move across the junction within the diode, leading to this light emission without requiring any external amplification or specialized conditions.

Regarding other options, the notion that LEDs can only emit coherent light is inaccurate; they emit incoherent light since the light waves produced do not maintain a constant phase relationship. Additionally, while some LEDs may operate at specific voltage levels, they typically do so at relatively low voltages compared to other light sources, thus high voltage is not a requirement in general use. Lastly, while LEDs can be used in optical applications, they are not exclusively used in fiber-optic transmissions, which are primarily dominated by laser technology. Therefore, the primary characteristic defining semiconductor LEDs relates to their mechanism of light emission, accurately captured in the concept of spontaneous emission.