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Jun 15 2020

The Basic Introduction to Light-emitting Diode

I Introduction

The light-emitting diode is a commonly used light-emitting device, which emits energy through the recombination of electrons and holes. It is widely used in the field of lighting. Light-emitting diodes can efficiently convert electrical energy into light energy. Light-emitting diodes have a wide range of uses in modern society, such as lighting, flat panel displays, and medical devices.

Light Emitting Diode (LED) Working Principle

This kind of electronic component appeared as early as 1962. In the early days, it could only emit low brightness red light. Later, other versions of monochromatic light were developed. The light that can be emitted today has spread to visible light, infrared light, and ultraviolet light. Initially,  the light-emitting diodes were used as indicator lights, display panels, etc. With the continuous progress of technology, light-emitting diodes have been widely used in displays and lighting.

Catalog

I Introduction

II Parameters of light-emitting diode

2.1 Significance of limit parameters

2.2 Significance of electrical parameters

III Working principle of light-emitting diode

IV Types of light-emitting diode

V Materials of a light-emitting diode

5.1 Wafer

5.2 Bracket

5.3 Silver glue 

5.4 Gold wire 

5.5 Epoxy resin

VI Applications of light-emitting diode

6.1 Display screen and communication signal display

6.2 Automotive industry

6.3 LCD backlight

6.4 LED lighting

6.5 Other applications

VII The trends of LED lighting technology

II Parameters of light-emitting diode

2.1 Significance of limit parameters

(1) Allowable power consumption Pm: the maximum value of the product of the forward DC voltage applied to both ends of the LED and the current flowing through it. If this value is exceeded, the LED becomes hot and damaged.

(2) Maximum forward DC current IFm: the maximum forward DC current allowed to be added. Exceeding this value can damage the diode.

(3) Maximum reverse voltage VRm: the maximum reverse voltage allowed to be applied. Above this value, the light-emitting diode may be damaged by the breakdown.

(4) Working environment topm: the ambient temperature range where the LED can work normally. Below or above this temperature range, the light-emitting diode will not work properly and the efficiency is greatly reduced.

2.2 Significance of electrical parameters

(1) Spectral distribution and peak wavelength: the light emitted by a certain light-emitting diode is not a single wavelength, and its wavelength is generally shown in Figure.

Spectral distribution and peak wavelength

Figure 1. Spectral Distribution and Peak Wavelength

It can be seen from the figure that the light intensity of a certain wavelength λ0 in the light emitted by the light-emitting tube is the largest, and this wavelength is the peak wavelength.

(2) Luminous intensity IV: The luminous intensity of the light-emitting diode usually refers to the luminous intensity in the direction of the normal. When the radiation intensity in this direction is (1/683) W / sr, it emits 1 candela (symbol cd). Since the general LED has a low luminous intensity, the luminous intensity is usually measured in candela (mcd).

(3) Spectral half-width Δλ: It represents the spectral purity of the light-emitting tube. It refers to the interval between the two wavelengths corresponding to the 1/2 peak light intensity in Figure 3.

(4) Half value angle θ1 / 2 and viewing angle: θ1 / 2 refers to the angle between the direction in which the luminous intensity value is half of the axial intensity value and the luminous axis (normal direction).

Two times the half-value angle is the viewing angle (or half-power angle).

Angle distribution of luminous intensity of LED

Figure 2. Angle Distribution of Luminous Intensity of the LED

Figure above shows the angular distribution of the luminous intensity of two different types of LEDs. The coordinates of the perpendicular (normal) AO are the relative luminous intensity (ie, the ratio of the luminous intensity to the maximum luminous intensity). Obviously, the relative luminous intensity in the normal direction is 1. The larger the angle away from the normal direction, the smaller the relative luminous intensity. From this graph, the half-value angle or viewing angle value can be obtained.

(5) Forward working current If: It refers to the forward current value when the light-emitting diode is normally emitting light. In actual use, the IF should be selected below 0.6·IFm.

(6) Forward working voltage VF: The working voltage given in the parameter table is obtained under a given forward current. Generally measured at IF = 20mA. The forward working voltage VF of the light-emitting diode is 1.4 ~ 3V. When the outside temperature increases, VF will decrease.

(7) V-I characteristics: The relationship between the voltage and current of the light-emitting diode can be represented in the figure below.

The relationship between the voltage and current of the light-emitting diode

Figure 3. The Relationship between the Voltage and Current of the LED

When the forward voltage is less than a certain value (called a threshold), the current is extremely small and does not emit light. When the voltage exceeds a certain value, the forward current increases rapidly with the voltage and emits light. From the V-I curve, parameters such as the forward voltage, reverse current, and reverse voltage of the light-emitting tube can be obtained. The forward leakage current IR of the LED tube is less than 10μA.

III Working principle of light-emitting diode

The core part of the light-emitting diode is a wafer composed of a P-type semiconductor and an N-type semiconductor. There is a transition layer between the P-type semiconductor and the N-type semiconductor, called a PN junction. In the PN junction of some semiconductor materials, the injected minority carriers and majority carriers will release excess energy. The energy is in the form of light, thereby directly converting electrical energy into light energy. The reverse voltage is added to the PN junction. Minority carriers are difficult to inject, so they do not emit light. This kind of diode is called a light-emitting diode, commonly known as LED. When it is in the forward working state (that is, the forward voltage is applied to both ends), and when the current flows from the anode to the cathode of the LED, the semiconductor crystal emits light of different colors from ultraviolet to infrared. The intensity of the light is related to the current.

The principle of light-emitting diode needs to be analyzed from the following three situations:

When no voltage is applied across the diode, the electrons in the N-type material will move along the PN junction between the layers. The electrons fill the holes in the P-type material and form a depletion region. In the depletion region, the semiconductor material returns to its original insulation state. That is, all holes are filled, so there is neither free electrons nor space for electrons to move in the depletion region, and the charge cannot flow.

No Voltage is Applied Across the Diode

Figure 4. No Voltage is Applied Across the Diode

When a forward voltage is applied to the light-emitting diode, the holes injected from the P region to the N region. And the electrons injected from the N region into the P region within a few microns near the PN junction. They recombine with electrons in the N region and holes in the P region respectively. Therefore, they produce spontaneously emitted fluorescence. Different semiconductor materials have different energy states for electrons and holes. The more energy released, the shorter the wavelength of the emitted light. Diodes that emit red, green, or yellow light are commonly used.

Apply a forward voltage to the LED

Figure 5. Apply a Forward Voltage to the LED

When a reverse voltage is applied to the light-emitting diode, the P-type terminal is connected to the negative electrode of the circuit, and the N-type terminal is connected to the positive electrode. And the current will not flow. Negatively charged electrons in N-type materials will be attracted to the positive electrode. Positively charged holes in P-type materials will be attracted to the negative electrode. Since holes and electrons move in the wrong direction. No current will flow through the PN junction, and the depletion region will also expand.

Apply a Reverse voltage to the LED

Figure 6. Apply a Reverse Voltage to the LED

 

LED material

Color

AlGaAs GaAsP AlGaInP GaP:ZnO

red and infrared

 

InGaN/GaN GaP AlGaInP AlGaP

green

GaAsPAlGaInP AlGaInP GaP

high brightness orange red, orange, yellow, green

GaAsP

red, orange, yellow

GaP ZnSe InGaN SiC

red, yellow, green

GaN

green, emerald green, blue

InGaN

near ultraviolet, blue green, blue

SiC

blue

Si

blue

Al2O3

blue

ZnSe

blue

C

ultraviolet

AlN AlGaN

UV with wavelengths far to near

Table: Inorganic Semiconductor Materials Used in Light-emitting Diodes and the Colors They Emit

IV Types of light-emitting diode

There are various types of light-emitting diodes according to different aspects. According to the materials used, there are gallium phosphide (GaP) light-emitting diodes, phosphorous gallium arsenide (GaAsP) light-emitting diodes, gallium arsenide (GaAs) light-emitting diodes, phosphorous indium gallium arsenide (GaAsInP) light-emitting diodes and gallium arsenide aluminide (GaAlAs) light-emitting diodes.

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

kinds of light-emitting diode

Figure 7. Kinds of Light-emitting Diode

According to its package shape, it can be divided into circular, square, rectangular, triangular, and combined shapes. The figure shows the shape of several light-emitting diodes.

Plastic light-emitting diodes are divided into red, amber, yellow, orange, light blue, green, black, white, transparent, and colorless according to the color of the tube body. The outer diameter of the round light-emitting diode is from ¢ 2 to ¢ 20mm. According to the light-emitting color of the light-emitting diodes, it can also be colored light and infrared light. Colored lights include red light, yellow light, orange light, green light, and so on. In addition, light-emitting diodes can be divided into ordinary monochrome light-emitting diodes, high-brightness light-emitting diodes, ultra-high-brightness light-emitting diodes, color-changing light-emitting diodes, flashing light-emitting diodes, voltage-controlled light-emitting diodes, infrared light-emitting diodes, and negative resistance light-emitting diodes.

V Materials of a light-emitting diode

Wafer, bracket, silver glue, gold wire, and epoxy resin are the five LED raw materials of light-emitting diode

5.1 Wafer

The structure of the wafer: it consists of a gold pad, P-pole, N-pole, PN junction, and a back gold layer (the double-pad wafer has no back gold layer). The wafer is composed of P-layer semiconductor elements and N-layer semiconductor elements. They are rearranged and combined by electron movement. It is this change that enables the wafer to be in a relatively stable state. When the positive electrode is applied to the wafer with a certain voltage, the holes in the positive P region will continue to swim toward the N region. And the electrons in the N region will move to the P region relative to the holes. While the electrons and holes move relatively, the electron holes pair with each other, exciting photons and generating light energy.

The main classification of wafer: surface-emitting type: most of the light is emitted from the surface of the wafer. Five-sided light-emitting type: there is more light emitted on the surface and side.

5.2 Bracket

The structure of the bracket is 1 layer of iron, 1 layer of copper (good conductivity, fast heat dissipation), 1 layer of nickel (anti-oxidation), 1 layer of silver (good reflectivity, easy to solder wire)

5.3 Silver glue 

Take H20E as an example. Silver glue is also called white glue, milky white. The role of silver glue is to conduct and bond materials (baking temperature: 100°C / 1.5H). Storage conditions: Silver glue manufacturers generally store silver glue at -40°C, and application units generally at -5°C. Single-agent is 25°C / 1 year (dry, ventilated place), mixed agent 25°C / 72 hours. Baking conditions: 150°C / 1.5H. Stirring conditions: Mix for 15 minutes in one direction.

5.4 Gold wire 

Take φ1.0mil as an example. The gold wires used for LEDs are φ1.0mil and φ1.2mil. The material of the gold wire is generally 99.9% of the gold content of the gold wire for LED. The purpose of the gold wire: with its characteristics such as high gold content, soft, easy to deform, good conductivity and good heat dissipation, a closed circuit is formed between the wafer and the bracket. (Conversion relationship: 1 mil = 0.0254mm, 1 in = 25.4mm)

5.5 Epoxy resin 

Ttake EP400 as an example. Composition: A and B two parts: Glue A: it is the main agent, which consists of epoxy resin + defoamer + heat resistance agent + thinner. Agent B: it is a curing agent, consisting of acid, mold release agent, and accelerator.

VI Applications of light-emitting diode

6.1 Display screen and communication signal display

LED lamp has the characteristics of shock resistance, fast light response, power-saving, and long life, etc. It is widely used in various indoor and outdoor displays. It is divided into full-color, three-color, and monochrome displays that are developed and produced by more than 100 units nationwide. Traffic lights mainly use ultra-high-brightness red, green, and yellow LEDs. Because LED lights are energy-saving and reliable, the traffic lights are gradually being replaced throughout the country and the speed of promotion is fast.

traffic lights

Figure 8. Traffic Lights

6.2 Automotive industry

Automotive lamps include dashboards, audio indicators, backlights for switches, reading lights and external brake lights, tail lights, sidelights, and headlights. Incandescent lamps for automobiles are not resistant to shock that require frequent replacement. Due to the fast response speed of the LED, the driver can be reminded of the brakes early to reduce the rear-end accident. In developed countries, the central rear high-position brake light made of LED has become a standard part of the car. The LED car tail light model launched by the American HP company in 1996 Groups can be combined into various car taillights at will. In addition, the light sources in the car dashboard and other various lighting parts can be used as ultra-high brightness light-emitting lamps. In recent years, an annual output value of 1 billion yuan will be formed, and within 5 years, an annual output value of 3 billion yuan will be formed.

automotive headlight

Figure 9. Automotive Headlight

6.3 LCD backlight

LED backlights are most noticeable with high-efficiency side-emitting backlights. As an LCD backlight application, LEDs have the characteristics of long life, high luminous efficiency, no interference, and high-cost performance. They have been widely used in electronic watches, mobile phones, BP, computers, electronic calculators, and credit card machines. With the increasing miniaturization of portable electronic products, the LED backlight has more advantages.  The backlights manufacturing technology will be developed to thinner, low power consumption, and uniformity. LED is the key device of a mobile phone. An ordinary mobile phone needs about 10 LED devices, while a color screen and a mobile phone with camera function need to use about 20 LED devices. At present, the amount of mobile phone backlight is very large. 3.5 billion LED chips are used a year.

6.4 LED lighting  

Early lighting products have low luminous efficiency. The light intensity can only reach a few to dozens of mcd, which is suitable for indoor occasions, such as home appliances, instrumentation, communication equipment, microcomputers, and toys. At present, the direct goal is to replace incandescent lamps and fluorescent lamps with LED light sources. This substitution trend has started to develop from local application fields. To save energy, Japan is planning to replace the incandescent light-emitting diode project (called "illuminate Japan"). The budget for the first five years is 5 billion yen. If LED replaces half of the incandescent and fluorescent lamps, the annual savings can be equivalent to the energy of 6 billion liters of crude oil. That is equivalent to the power generation of five 1.35 × 106kW nuclear power plants. It can reduce the production of carbon dioxide and other greenhouse gases and improve the living environment.

6.5 Other applications

LED Christmas lights

Figure 10. LED Christmas Lights

Other applications such as a kind of flash shoes popular with children, the built-in LED will flash when walking; the power indicator of electric toothbrushes; the popular LED Christmas lights. Due to the novel shape, rich colors, unbreakable and low-voltage safety Sexuality, LED Christmas light is generally welcomed by people. It is threatening and replacing the existing Christmas market of electric bulbs.

1. The mainstream light source of lighting will be converted to LED. At present, the luminous efficiency of high-power led commercial products has reached more than 150lm / W. The life span has reached more than 30,000 hours. The comprehensive performance of LEDs has exceeded other light sources. At the same time, in addition to large power, the price of LED lamps is close to lamps composed of other light sources. Therefore, LED already has certain advantages. At the same time, the light efficiency of LED laboratory products has reached more than 300lm / W. Through a reasonable heat dissipation design, the life span of more than 50,000 hours can be fully achieved. In principle, there is not much room for other light sources to improve light efficiency and life. Therefore, it can be expected that LED becoming the mainstream light source in the lighting market.

2. The research of LED device technology will mainly focus on the improvement of green LED efficiency. The core device of semiconductor technology-LED, currently the red LED and the blue LED has high photoelectric efficiency, but the photoelectric efficiency of the green LED is very low. This limits the substantial application of LED spectral flexibility. Therefore, improving the light efficiency of green LEDs is the most important research topic of LED devices.

3. Another study of LED technology will be the study of narrow-spectrum LED devices. The narrow spectral width of a single LED will help to truly achieve unlimited flexibility in assembling the LED spectrum. At the same time, one of the main applications of LEDs at present-LED displays, if the spectral width of monochromatic LEDs can be reduced, a larger color gamut space can be achieved.

4. White LEDs for general lighting will gradually shift to RGB mode. The light sources used for general lighting are presented in the form of white light or near-white light. At present, the most comprehensive and cost-effective white LEDs on the market are also implemented by blue LEDs and YAG phosphors. However, the RGB method has higher light efficiency. It will enable the lamp to be dimmed, toned, and even adjusted the color rendering index. Therefore, with the improvement of green LED light efficiency, it is believed that the RGB method (further extended to 3 or more monochromatic LED mixed colors) will become the mainstream white LED mode.

5. The characterization of the color rendering of the light source will be a long-term debate. The characterization in the form of the spectrum may be the ultimate result. The current parameter that characterizes the color rendering of the light source is the color rendering index. However, this is based on the limited spectral form of traditional light sources. The infinite flexibility of the LED spectrum will make this characterization problematic. It can be predicted that if the LED achieves true spectrum unlimited flexibility. Any single-parameter color rendering will actually be flawed, and the ultimate characterization method should be the spectrum itself.

6. The application of light sources will include two basic aspects of visual applications and non-visual applications. The flexibility of LEDs in spectral assembly makes it possible to achieve various visual and non-visual optimized applications through spectral assemblies, such as agricultural lighting, medical lighting, LED Visible light communication, etc. But relatively speaking,  in the non-visual field, there are still many problems to be solved, including what kind of lighting conditions are optimized, the price of LED needs to be further reduced. The former is a scientific problem, while the latter is an industrialization problem.

7. Lighting technology will be deeply integrated with information technology to create smart lighting. As a semiconductor device, LED is a good physical carrier of information technology because LED is easy to control plus the relevance of lamps and human life. Therefore, LED can be deeply integrated with information technology, and smart lighting will be born. This will be an important development direction for future LED lighting technology.

8. The appearance of the lamps and lanterns will have great innovation potential. At present, most LED lamps and lanterns include LED bulbs, LED downlights, LED tubes, etc., all of which are designed to cater to people's consumption habits. Whether this is the best mechanical form of the lamp or not remains to be studied. At least, there is a lot of room for innovation in the future mechanical form of LED lamps.

9. OLED will occupy an important position in the display field. Relative to LEDs that emit light similar to point light sources, OLEDs emit light in the form of surface light sources. Therefore, after LED occupies the vast majority of lighting applications in the future, OLED still has certain advantages in the display field.

10. Natural lighting will be valued. In recent years, energy-saving lighting, health lighting, and ecological lighting have received great attention. The use of sunlight to achieve lighting naturally becomes the best choice. Therefore, with the advancement of technology, natural light lighting will receive more and more attention.

Due to the great flexibility of LEDs in the three dimensions of scale, spectrum, and time, many innovative applications and concepts may be born in the future of LED development. This is the development trend of lighting based on artificial light sources. At the same time, as people continue to deepen the concept of green environmental protection, natural lighting will also get greater development.

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