What primarily determines the visible color emitted by an LED junction?

The visible color emitted by an LED junction is primarily determined by the materials used to construct the device. Different semiconductor materials have unique band gaps, which influence the energy of the photons produced when electrons recombine with holes in the semiconductor. This energy corresponds to specific wavelengths of light, thus establishing the color of the emitted light.

For instance, when using materials like Gallium Arsenide Phosphide (GaAsP), the emitted colors can range from red to green, depending on the specific composition and structure of the semiconductor. The precise selection of materials allows for fine-tuning the color output of the LED, making this a critical factor in the LED's color characteristics.

Although the applied current, voltage across the junction, and size of the LED can have roles in LED performance and efficiency, they do not directly determine the inherent color emitted, which is fundamentally linked to the material properties.