Types and principles of LED light-emitting diodes

Light-emitting diode is a semiconductor electronic components can be converted into light energy, its electrical symbols as shown in the following figure (1), the figure with the emission of the LED, usually the positive (anode) pin with A, and the negative (cathode) pin with K said. Light-emitting diode this electronic component appeared as early as 1962, the early can only emit low-luminosity red light, after the development of other monochromatic light version, to date, can emit light has been spread throughout the visible, infrared and ultraviolet light, luminosity is also increased to a considerable degree of luminosity. At present, light-emitting diodes have been widely used in indoor and outdoor LED lighting, LED display, traffic signal lights, automotive lights, TFT
LCD display backlight, lighting, fiber optic communications and so on.
2, light-emitting principle
LED and ordinary diode is composed of a PN junction, also has a unidirectional conductivity, just LED can emit light! Its light-emitting principle can be described as:
Electron (negatively charged) more N (-: negative) type semiconductor and hole (positively charged) more P (+: positive)
(+: positive) semiconductor with many holes. When a positive voltage is applied to the semiconductor, the electrons and holes move and combine again at the bonding site, and it is during this combination that a large amount of energy is generated, which is released in the form of light, which is the mystery of its luminescence. Compared with the previous light sources that first converted electrical energy into heat energy and then into light energy (e.g..
Early Edison’s invention of incandescent lamps), because the LED can directly convert electrical energy into light energy, so it can not waste light energy, high efficiency to obtain light.

3, the structure of the LED
Below, we take a common LED white light as an example to illustrate the structure of the LED. The LED is mainly composed of the following parts:
Chip (role: light source light-emitting)
Stands: including substrate and heat sink base, pins, etc. (Role: heat dissipation, conductive)
Gold wire (role: conductive)
Transparent resin (role: to protect the grain, light transmission)

4, the types of LED
According to the different packages of LED, light-emitting surface and different characteristics, LED can be roughly divided into the following types:
Plug-in LED
Need to point out:
Plug-in type LEDs in addition to the common 2-legged single-color type LEDs, including 3-pin dual-color type LEDs, in addition to 4-pin RGB full-color LEDs.

Usually SMD LEDs are divided into 0402, 0603, 0805, 1206 and other packages, and the colors are single-color, dual-color and RGB full-color.

High Power LED
This kind of LED is usually used for lighting purposes, and most of them are white-colored LEDs, and the power can be more than several watts.
LED Digital Tube
By making multiple LEDs into individual fields to form characteristic letters or combinations, they can display 0~9 or meter type (English characters can be displayed) or Bar bar segment type for indicating progress or proportion.

LED Dot Matrix Screen
The LED will form a matrix, can display Chinese and English letters, such as manufacturers of LED display screen is a combination of these dot matrix screen module. According to the color, it can be single color, double color or RGB full color.

“Smart” LED
This type of LED not only contains LED wicks, but also built-in control circuit IC, which can realize specific functions, such as flashing Flash
Or bus addressing to control the color of each point, etc., representative models such as: WS2812, WS2811, etc..

Special LED
The reason why this LED is special is that it can emit human eyes can not see the light, such as infrared, ultraviolet and so on. In our daily life, the remote control we use is this kind of LED light.

5, the main parameters of the LED
5.1 Forward voltage VF
Refers to the voltage generated between the anode A and cathode K when there is a positive current flow. The unit is V (volts). In the forward voltage is less than a certain value (called threshold), the current is very small, not light. When the voltage exceeds a certain value, the forward current increases rapidly with the voltage and emits light. And beyond the normal operating voltage, the LED may be breakdown. Usually, the normal forward voltage is different for LEDs of different luminous colors, such as red light is usually 1.8~2.0V
between, while blue and white lights are between 2.8~3.3V.
5.2 Forward Current
Refers to the current generated between anode A and cathode K when a positive current flows. The unit is A (Ampere). Usually, the forward current of LED should be controlled within 20mA, too much current will easily lead to the junction temperature Tj between P and N of LED is too high and lead to LED damage or reduce the service life. In addition, LED
As with ordinary diodes, the current will rise to a large extent when the voltage increases, so the general LED light-emitting diodes are connected in series with a protective resistor.
5.3 Luminous intensity
Indicates the brightness observed from a particular direction. The unit cd (candela, corresponding to a smaller unit is millicandela, or mcd), one unit of luminous intensity, and one unit of luminous intensity.
(i.e., mcd), a unit of three-dimensional angle within a stream of bright light is called a candela. Special attention needs to be paid to the pointing angle when comparing light intensity. Luminous intensity is the luminous flux emitted per unit steradian angle. Lens as a component of the LED package can concentrate light output in a specific direction (light collection by lens), and even if the light output is small, it can collect light, so the light intensity becomes large. It is important to note that the IV value usually needs to specify at what forward current IF the LED was measured.
Luminous Flux Φv [lm]
is the total amount of light emitted from a light source. The unit is lm (lumens).
Main wavelength λD [nm]
LEDs generally use the wavelength to indicate the color. The main wavelength corresponds to the wavelength corresponding to the color seen by the eye, and differs from the peak wavelength of the light-emitting wavelength in nm (nanometers).
The following are the wavelength parameters of various light-emitting color LEDs:
■Violet: 400 to 435nm
■Blue: 435~480nm
■Blue-green: 480 to 500nm
■Green: 500~560nm
■Yellow-green: 560~580nm
Yellow: 580~595nm
Orange: 595~610nm
■Red: 610~760nm
■White, usually represented by the following color coordinates. Or it is simply indicated by warm white, positive white, and cool white.
The above wavelength ranges and color markers vary slightly according to the literature.
Interestingly and wonderfully, the inorganic semiconductor materials used in conventional light-emitting diodes are related to the colors emitted by them, as shown below:

5.5 Chromaticity Coordinates
x, y
It refers to the actual value of the color of the LED light emitted in terms of a two-dimensional orthogonal coordinate system () x and y), as shown in the figure below:

Color Temperature
Color temperature means when the color of the light emitted by a light source is the same as the color radiated by a blackbody at a certain temperature, the temperature of the blackbody is called the color temperature of that light source. The lower the color temperature, the more the color leans toward orange, and the higher the color temperature, the more the color leans toward blue.

5.7 Luminous Angle of View
In the luminous intensity distribution graph , the luminous intensity is equal to the maximum intensity of half of the angle constituted by the angle known as the half-value angle. The thickness of the chip, the package mold strip form factor, the depth of the stent reflective cup and the stent insertion depth in the mold cavity have a direct impact on the size of the half-value angle. Manufacturing, according to customer requirements, through the selection of different materials and choose a different package size to get a different size of the half-value angle.

From the luminous intensity of the angular distribution graph to be divided into three categories:
A high directivity:
Generally for the pointed epoxy package, or with a metal reflection cavity package, and do not add scattering agent. Half-value angle of 5 ° ~ 20 ° or less, with a high directivity, can be used as a local illumination light source, or with the light detector associated with the composition of the automatic detection system.
B standard type:
Usually used as indicator light, its half-value angle of 20 ° ~ 45 °.
C scattering type:
This is an indicator with a larger angle of view and a half-value angle of 45° to 90° or greater.
Operating Temperature
The operating temperature is an important parameter that affects the diode’s operation and has a great influence on parameters such as light intensity and current.
People call LED light source as long life lamp. It is a solid cold light source, epoxy resin package, there is no loose part of the lamp body, there is no filament luminescence easy to burn, heat deposition, light attenuation and other shortcomings, in the appropriate current and voltage, the service life of up to 60,000 to 100,000 hours, more than 10 times the life of traditional light sources.
6, about white LED
There are many ways to get white light from LED. Here only introduce the representative two light-emitting methods.
Blue LED + yellow phosphor
Blue LED and its auxiliary color, i.e. yellow phosphor combination, to obtain white light.
Compared with other methods, this method has a simple structure and high efficiency, so it has become the mainstream now.

Red LED + Green LED + Blue LED
White light is obtained by combining three-color LEDs.
This method is more used for full color LED display devices than for lighting purposes.

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