1 LED basic structure and characteristics
1.1 The basic structure of LED and the principle of illumination
The conductivity of a semiconductor is between the conductor and the insulator. But it has its special function. It has been known for half a century ago that semiconductor materials can produce light.
An impurity semiconductor can be obtained by a diffusion process by incorporating a small amount of a suitable impurity into a pure semiconductor. The incorporation of a pentavalent element is an N-type semiconductor, and the incorporation of a trivalent element is a P-type semiconductor. LED is an abbreviation of English light emitting diode (light emitting diode), which is a solid state semiconductor device. In a P-type semiconductor, a hole (positively charged) dominates as a majority carrier, and an electron (negatively charged) is called a minority carrier. In an N-type semiconductor, electrons (negatively charged) dominate the majority carriers, and holes (positively charged) are minority carriers. After power-on, a thin layer with special conductive properties appears at the interface between the P-type semiconductor and the N-type semiconductor, which is also known as a PN junction (PNJunction Transistors). The PN junction can generate resistance to the diffusion motion of majority carriers in P-type semiconductors and N-type semiconductors. When a forward voltage is applied to the PN junction, current flows from the anode of the LED to the cathode, while minority carriers in the PN junction Most of the carriers are recombined, and the excess energy is converted into light energy and released. The semiconductor crystal emits light of different colors from ultraviolet to infrared. The intensity of the light is related to the current. For example, when the current is small, the LED is red. As the current increases, it can turn orange, yellow, and finally green. Through the research on the luminescent materials, people have gradually developed various LED components with higher light color and higher light efficiency.
1.2 LED characteristics
The intrinsic characteristics of the LED determine its advantages and disadvantages; such as:
(1) Small size: LED is basically a small chip packaged in epoxy resin, so it is very small, each unit LED small piece is 3 ~ 5mm square, very light, can be made into various shapes The device is suitable for a variable environment and is easy to modularize.
(2) Low power consumption: using low-voltage DC drive, the power supply voltage is between 6 and 24V, depending on the product, so it is a safer power source than the high-voltage power supply, especially suitable for public places. Ultra-low power consumption (single tube 0.03 ~ 0.06W), electro-optical power conversion is close to 100%. The same lighting effect is more than 80% energy saving than traditional light sources.
(3) Good earthquake resistance and impact resistance.
(4) Good stability: the light decay is 50% of the initial, which is more than 10 times longer than the life of the traditional light source.
(5) Response time: The response time of the incandescent lamp is millisecond, and the response time of the LED lamp is nanosecond.
(6) Environmental protection: LED is made of non-toxic materials, no harmful metal mercury, unlike mercury-containing fluorescent lamps, it can cause pollution, and LED can also be recycled. There is no ultraviolet light or infrared light in the spectrum, no heat, no radiation, small glare, cold light source, safe touch, and it is a typical green light source.
(7) Multi-color: The principle of red, green and blue can be used. Under the control of computer technology, the three colors have 256 levels of gray and can be mixed arbitrarily, which can produce 256×256×256=16777216 colors, forming different The combination of light and color, varied, and achieve a variety of dynamic changes and various images.
(8) Advanced technology: Compared with the monotonous lighting effect of traditional light sources, LED light sources are low-voltage microelectronic products. It successfully combines computer technology, network communication technology, image processing technology, embedded control technology, etc., so it is also a digital information product, a high-tech technology for semiconductor optoelectronic devices, with online programming, unlimited upgrades, flexible and changeable. Features.
(9) The price is relatively expensive.
1.3 LED classification
1.3.1 According to the luminous color of the luminous tube
According to the color of the luminous tube, it can be divided into red, orange, green (also subdivided into yellow-green, standard green and pure green), blue light and so on. In addition, some light-emitting diodes include chips of two or three colors.
According to the light-emitting diode light-emitting portion with or without a scattering agent, colored or colorless, the above-mentioned various color light-emitting diodes can also be divided into four types: colored transparent, colorless transparent, colored scattering and colorless scattering.
1.3.2 According to the characteristics of the light-emitting surface of the light-emitting tube
According to the characteristics of the light-emitting surface of the light-emitting tube, it can be divided into a round lamp, a square lamp, a rectangular shape, a surface light-emitting tube, a lateral tube, a surface-mounted micro tube, and the like. The circular lamps are divided into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm, and φ20mm according to the diameter. In foreign countries, a φ3 mm light-emitting diode is generally referred to as T-1; φ5 mm is referred to as T-1 (3/4); and φ4.4 mm is referred to as T-1 (1/4). The angular distribution of the circular luminous intensity can be estimated from the half value angle size.
There are three categories from the angular distribution of luminous intensity:
(1) High directivity. It is usually a pointed epoxy package or a metal reflective cavity package with no scattering agent. The half-value angle is 5°-20° or less, has high directivity, can be used as a local illumination source, or combined with a light detector to form an automatic detection system.
(2) Standard type. Usually used as indicator light, its half value angle is 20 ° ~ 45 °. (3) Scattering type. This is an indicator light with a large viewing angle, the half value angle is 45° to 90° or more, and the amount of the scattering agent is large.
(3) Scattering type. This is an indicator light with a large viewing angle, the half value angle is 45° to 90° or more, and the amount of the scattering agent is large.
1.3.3 according to the structure of the LED
According to the structure of the light-emitting diode, there are structures such as a full epoxy encapsulation, a metal base epoxy package, a ceramic base epoxy package, and a glass package.
1.3.4 According to luminous intensity and working current
LEDs with ordinary brightness according to luminous intensity and working current (lighting intensity 100mcd); high-intensity light-emitting diodes with luminous intensity between 10 and 100mcd. Generally, the operating current of LEDs is between ten mA and tens of mA, while the operating current of low-current LEDs is below 2 mA (the brightness is the same as that of ordinary LEDs).
In addition to the above classification methods, there are methods for classifying by chip material and classifying by function.