1. Light-emitting diode (LED) is a light-emitting display device made of semiconductor materials such as gallium phosphide (GaP) that can directly convert electrical energy into light energy. When a certain amount of electric current passes through it, it will glow. Figure 4-21 shows the common circuit diagram.

Light-emitting diodes are also made of PN structure like ordinary diodes, and also have one-way conductivity. It is widely used in various electronic circuits, home appliances, instruments and other equipment, as a power indicator or level indicator.

2. Light-emitting diodes are classified in a variety of ways.

According to the materials used, it can be divided into gallium phosphide (GaP) light-emitting diodes, gallium phosphate-arsenide (GaAsP) light-emitting diodes, gallium arsenide (GaAs) light-emitting diodes, indium Gallium phosphate-arsenide (GaAsInP) light-emitting diodes and gallium aluminide (GaAlAs) light-emitting diodes.

According to its package structure and package form, it can be divided into metal package, ceramic package, plastic package, resin package and leadless surface package, and can also be divided into additive dispersion package (D), colorless scattering package (W), colored transparent package (C) and colorless transparent package (T).

According to its package shape can be divided into round, square, rectangle, triangle and combination shape, etc. Figure 4-22 shows the shape of several light-emitting diodes.

According to the color of the tube body, the plastic LED is divided into red, amber, yellow, orange, light blue, green, black, white, transparent and colorless. The outer diameter of the circular LED is from ¢2 to ¢20mm, which is divided into various specifications.

According to the luminous color of the light-emitting diode, it can be interpreted as colored light and infrared light. Colored light is divided into red light, yellow light, orange light, green light and so on.

In addition, light-emitting diodes can also be divided into ordinary monochromatic light-emitting diodes, high-brightness light-emitting diodes, ultra-high brightness light-emitting diodes, color-changing light-emitting diodes, flicker light-emitting diodes, voltage controlled light-emitting diodes, infrared light-emitting diodes and negative resistance light-emitting diodes.

3. Ordinary single-color LED Ordinary single-color LED has the advantages of small size, low operating current, small working current, uniform and stable light, fast response speed, long life, etc., can be used in a variety of DC, AC, pulse and other power sources. It belongs to the current control type semiconductor device, which needs to be connected with a suitable current limiting resistance in series when used.

Figure 4-23 shows the application circuit of a common LED.

The color of the ordinary monochromatic LED is related to the wavelength of the light, and the wavelength of the light depends on the semiconductor material used to manufacture the LED. The wavelength of red LED is generally 650~700nm, the wavelength of amber LED is generally 630~650 nm, the wavelength of orange LED is generally about 610~630 nm, and the wavelength of yellow LED is generally about 585 nm. The wavelength of green LED is generally 555~570 nm.

Commonly used domestic ordinary monochrome LED series BT (factory standard model), FG (ministry standard model) series and 2EF series, see Table 4-26, Table 4-27 and Table 4-28.

Commonly used imported ordinary monochrome leds are SLR series and SLC series.

4. High-brightness monochromatic leds and ultra-high-brightness monochromatic leds use different semiconductor materials than ordinary monochromatic leds, so the intensity of the light is also different.

Usually, high-brightness monochromatic light-emitting diodes use materials such as gallium aluminide as arsenic (GaAlAs), ultra-high-brightness monochromatic light-emitting diodes use materials such as indium gallium phosphorus arsenide (GaAsInP), and ordinary monochromatic light-emitting diodes use materials such as gallium phosphide (GaP) or gallium phosphorus arsenide (GaAsP).

The main parameters of commonly used high-brightness red light-emitting diodes are shown in Table 4-29, and the main parameters of commonly used ultra-high-brightness monochromatic light-emitting diodes are shown in Table 4-30.

5. Color-changing light-emitting diodes are light-emitting diodes that can change the color of the light. The color types of light-emitting diodes can be divided into two-color light-emitting diodes, three-color light-emitting diodes and multi-color (red, blue, green, white four colors) light-emitting diodes.

According to the number of pins, color-changing leds can be divided into two-end color-changing leds, three-end color-changing leds, four-end color-changing leds and six-end color-changing leds.