Light-emitting diodes, or LEDs, are diodes made from compounds of gallium (Ga), arsenic (As), and phosphorus (P). When electrons and holes recombine, they emit visible light. Therefore, they can be used to create light-emitting diodes. They serve as indicator lights in circuits and instruments, or form text or numerical displays. Gallium arsenide phosphide diodes emit red light, gallium phosphide diodes emit green light, and silicon carbide diodes emit yellow light. These diodes are semiconductor devices that emit light when a certain current is applied, as electrons and holes continuously flow through a PN junction or a similar structure, spontaneously recombine, and emit light.

Light-Emitting Diode Characteristics
Compared to incandescent bulbs and neon lamps, LEDs have the following advantages:
Very low operating voltage (some are only a few volts);
Low operating current (some can emit light with only a few tenths of a milliampere);
Excellent shock and vibration resistance, high reliability, and long life;
The intensity of the light can be easily modulated by adjusting the current flowing through it.

Due to these characteristics, LEDs are used as light sources in some photoelectric control devices and as signal displays in many electronic devices. Its core is shaped like a strip, and seven strips of light-emitting diodes form a 7-segment semiconductor digital display. Each digital display can display Arabic numerals 0 to 9, 10, and partial letters such as A, B, C, D, E, and F (uppercase and lowercase letters are important).

Light-Emitting Diode Principle
This is a type of semiconductor diode that converts electrical energy into light energy. Like ordinary diodes, light-emitting diodes consist of a PN junction and also have unidirectional conductivity. When a forward voltage is applied to a light-emitting diode, holes injected from the P region into the N region and electrons injected from the N region into the P region recombine with electrons in the N region and holes in the P region, respectively, within a few microns near the PN junction, generating spontaneous fluorescence. The energy states of electrons and holes in different semiconductor materials vary. The amount of energy released when electrons and holes recombine varies; the greater the energy released, the shorter the wavelength of the emitted light. Commonly used diodes emit red, green, or yellow light. The reverse breakdown voltage of light-emitting diodes is greater than 5 volts. Its forward volt-ampere characteristic curve is very steep, so a current-limiting resistor must be connected in series to control the current through the diode.

The current-limiting resistor, R, can be calculated using the following formula: R = (E - UF) / IF

Where E is the supply voltage, UF is the forward voltage drop of the LED, and IF is the normal operating current of the LED. The core of a light-emitting diode is a wafer composed of a P-type semiconductor and an N-type semiconductor. Between the P-type and N-type semiconductors lies a transition layer, called a PN junction. In certain semiconductor materials, when injected minority carriers recombine with majority carriers in the PN junction, excess energy is released as light, thereby directly converting electrical energy into light energy. Applying a reverse voltage to the PN junction makes it difficult for minority carriers to be injected, resulting in no light emission. This type of diode, made using the principle of injection electroluminescence, is called a light-emitting diode, commonly known as an LED. When operating in the forward state (i.e., with a forward voltage applied across its terminals), current flows from the LED's anode to its cathode, causing the semiconductor crystal to emit light of varying colors, from ultraviolet to infrared. The intensity of the light is related to the current.