To produce visible light, the PN junction must be optimized and the correct material must be selected. Commonly used semiconductor materials include silicon and germanium, which are simple elements, but PN junctions made from these materials do not emit light. In contrast, compound semiconductors including gallium arsenide, gallium phosphide, and indium phosphide are compound semiconductors, and knots made of these materials do emit light.

Pure gallium arsenide releases energy in the infrared portion of the spectrum, and to bring light emission to the visible red end of the spectrum, aluminum is added to semiconductors to produce aluminum gallium arsenide (AlGaAs), and phosphorus can also be added to emit red light. For other colors, other materials are used. For example, gallium phosphide emits green light, while aluminum indium gallium phosphide is used to emit yellow and orange light, and most light-emitting diodes are based on gallium semiconductors.