Light-emitting diodes are called leds for short. It is made of compounds containing gallium (Ga), arsenic (As), phosphorus (P), nitrogen (N), etc. When electrons combine with holes, they emit visible light, which can be used to make light-emitting diodes. To act as an indicator in circuits and instruments, or to form a text or digital display. Gallium arsenide diode red light, gallium phosphide diode green light, silicon carbide diode yellow light, gallium nitride diode blue light. Because of the chemical properties of organic light-emitting diode OLED and inorganic light-emitting diode LED.