High-brightness light-emitting diodes and their applications in the field of illumination

1 Introduction

LEDs were introduced in the early 1960s. Early LEDs had very low luminous efficiency (0.1 lm/W), low luminous flux (a few thousandths of lumens), and a single color (red), so they could only be used as indicators. Use, such as the power indicator on the tape recorder. With the development of semiconductor technology, the current L ED performance has been a breakthrough development, the luminous efficiency has reached tens of lumens per watt, the luminous flux reaches several lumens, the color is more diverse, there are red, orange, yellow, green Various colors such as blue and white have been greatly expanded in the field of application. In particular, the successful development of white LEDs has made it possible to use L ED in the field of lighting. In the past 100 years, lighting sources have undergone three important development stages. Incandescent lamps are the first generation of light sources, fluorescent lamps are the second generation of light sources, and high-intensity gas discharge lamps are the third generation of light sources. Now many people in the lighting industry are very optimistic about the prospects of LED, that this is the fourth generation of light source. This article will discuss some aspects of the application of L ED in the field of lighting, and look forward to its development prospects.

2 LED technology development

2.1 Materials

Light-emitting diodes (LEDs) were introduced in the 1960s. In 1964, the III-V luminescent material GaAsP was successfully developed, and red LEDs appeared, with a peak wavelength of about 650 nm. Although the driving current is 20 mA, the luminous flux of a single L ED is only a few thousandths of a lumen, and the corresponding luminous efficiency is only 0.1 lm / W, but the all-solid-state light source is beginning to be accepted, mainly used in the field of indicator lights.

In the 1970s, material research was more active and the first climax in the history of L ED development. The quality of GaAsP/GaAs has been improved, and the epitaxial materials have been fabricated by vapor phase epitaxy (VPE) and liquid epitaxy (LPE), which not only improves the light efficiency to 1 lm/W, but also covers the color from yellow-green to infrared. The spectral range (565 to 940 nm) has also begun to enter the display field.

After the 1980s, the application level gradually developed, packaging technology was gradually improved, and peripheral support conditions were relatively formed, which led to breakthroughs in L ED technology. For example, the GaAlAs epitaxial layer is fabricated by LPE technology to produce high-brightness red LEDs and infrared diodes (IL ED) with wavelengths of 660, 880, and 940 nm, respectively. With the development of metal organic chemical vapor phase epitaxy (MOVPE), a 780 nm semiconductor laser diode was produced; the red and yellow L ED light effect made with the new chip material Al InGaP can reach 10 lm/W, if transparent lining is used. At the bottom, the light efficiency can exceed 20 lm / W. In 1984, the third-generation semiconductor material GaN developed by MOVPE made the blue and green LEDs achieve a luminous efficacy of 10 lm/W, achieving full colorization of LEDs.

In the 1990s, technology development focused on white LEDs. There are roughly three ways to make white LEDs: a small amount of fluorescent material is applied to the surface of the blue chip, part of the blue light excites the coating material to form yellow light, and the blue light emitted by the transmitted chip itself constitutes white light; red, green and blue are used. The chips are packaged together; a single-chip white LED is fabricated by using ZnSe as a base. After being energized, the blue light emitted by the ZnSe active layer and the yellow light emitted from the center of the substrate are mixed into white light.

After entering the 21st century, L ED is known as the “new light source of the 21st century” and is the “fourth generation light source” after incandescent lamps, fluorescent lamps and high-intensity discharge lamps (HIDs). With the continuous development and innovation of materials technology and packaging technology, red and orange L ED light effects can reach 100lm / W, and green L ED can reach 50 lm / W. The luminous flux of a single LED also reaches tens of lumens. The high efficiency of LED, ultra-high brightness and full colorization have made its application field more and more popular, and gradually move toward outdoor lighting.

Usually, people use a light intensity of 1 cd as the demarcation point between a general L ED and a high-brightness LED. Currently, materials for making high-brightness LEDs are mainly AlGaAs, AlGa InP, and Ga InN. AlGaAs is suitable for high-brightness red and infrared LEDs, made of LPE; AlGa InP is suitable for high-brightness red, orange, yellow and yellow-green LEDs, made with MOVPE; Ga InN for high-brightness dark green, blue, violet and ultraviolet L ED, manufactured with high temperature MOVPE.

2.2 Chip structure

Taking the commonly used Al InGaP chip as an example, the substrate of Al InGaP L ED produced by Hewlett Packard is a 200 μm thick GaAs layer (see Figure 1). The top three layers of the substrate are each composed of a 1 μm thick Al InGaP layer doped with different impurities. This structure is called a heterojunction structure. The intermediate layer and the bottom layer form a pn junction. Among them, the intermediate layer is also referred to as an active layer, and since the above three layers are doped differently, electrons are mainly recombined in the active layer. This structure allows for an increase in the maximum drive current.

The standard Al InGaP L ED has a maximum current intensity of 50 mA at the junction, compared to 30 mA for conventional L ED technology. High current densities can cause local overheating, which can damage the crystal structure of the LED substrate. In these areas, dark spots are formed and no longer emit light, which degrades the performance of L ED . The active layer of the heterostructure can withstand higher current densities and reduce the possibility of impaired L ED performance.

A 45 μm thick GaP layer is placed over the three-layer Al InGaP layer as an interface to the metal. Most of the surface of the Al InGaP substrate is free of metallic materials, and only the 3 μm Al InGaP layer has aluminum on the side. Therefore, in a humid environment, there is almost no surface that will be oxidized. Al InGaP technology demonstrates its superior attenuation resistance under a variety of harsh conditions such as high drive currents, sudden temperature changes and humidity.

There are two main types of substrate materials for Al InGaP: absorption of light by a GaAs substrate, and absorption of photons generated in the active layer when it reaches the GaAs substrate, so the technique is called AS-Al InGaP technology; A process called "chip coupling" that replaces the absorbed GaAs substrate with a transparent GaP substrate (see Figure 2), a technique known as TS-Al InGaP technology. Applying a reflective film behind the GaP substrate, the photons are reflected back to increase efficiency, which is about twice the efficiency of AS-Al InGaP L ED.

In terms of increasing the light output of the chip, people's research is not only to change the amount of impurities in the material, lattice defects and dislocations to improve internal efficiency, but also how to improve the chip structure and increase the probability of photon emission inside the chip. The problem. Since the refractive index of the semiconductor material is greater than that of air, partial photons will undergo total reflection when they reach the chip interface. The traditional chip is usually a cube, and the chip is like a totally reflective right-angle prism. Part of the photon is reflected multiple times inside the crystal, and eventually it is lost due to the absorption of the crystal, and the crystal cannot be escaped. Therefore, even if many photons can be generated inside the chip, such as the inability to effectively increase the escape rate, the effect of increasing the luminous efficiency of the LED cannot be well reflected. Lumileds uses a chip structure with a trapezoidal cross-section (as shown in Figure 3), which allows more photons to exit in the direction of the bevel, greatly increasing the photon escape rate.

2.3 Package

A typical LED package structure is shown in Figure 4. The positive pole of the chip is connected to the pin through a spherical contact point and a gold wire, and the negative electrode is connected to the other pin through the reflector cup and the lead frame. The reflector cup is mainly used to collect the light emitted from the side and bottom surfaces of the chip and reflect it in a desired direction angle. Epoxy resin mainly plays three roles: one is to protect the chip from external corrosion; the other is to use different shapes to act as a lens to control the divergence angle of light; the third is because the refractive index of the chip is compared with the refractive index of air. Too large, the light inside the chip is prone to total reflection, resulting in excessive light loss. Therefore, the epoxy resin is used as a transition to improve the light-emitting efficiency of the chip.

In general, the light output of L ED increases with increasing current. However, most current L ED drive currents are limited to 20 mA, which limits the output luminous flux of a single L ED . In fact, 20mA is not the maximum current value for driving L ED. Now there are many high-power L EDs with drive currents of 70 mA, 100 mA or even 1 A. The key is to improve the drive current, you must first solve the problem of heat conduction. Since the operating temperature of the pn junction has a maximum value of about 120 °C, therefore, to increase the driving current and obtain an increased output luminous flux of a single device, it is necessary to improve the package structure, for example, using a large wafer structure and selecting a good thermal conductivity. Silver paste and methods for increasing the surface area of ​​metal stents. In addition, in application design, the thermal conductivity of PCB boards and the like is also important.

3 white LED

The electroluminescence mechanism of the semiconductor pn junction determines that LEDs are unlikely to produce white light with a continuous spectrum, and that it is impossible for a single LED to produce more than two types of high-brightness monochromatic light. Therefore, in order to generate white light, the semiconductor light source can generate blue light first, and then indirectly generate a broadband spectrum by means of the fluorescent substance to synthesize white light. Figure 5 shows the structure of such a white LED, and Figure 6 shows its output spectrum. For comparison, Figure 6 also shows the output spectrum of an ordinary incandescent lamp.

Now, for white LEDs using phosphors, by changing the chemical composition of the phosphors and adjusting the concentration of the phosphor layers, various white light with a color temperature of 3 500 to 10 000 K can be obtained to meet the requirements of different lighting applications. In addition to the method of using a phosphor, a method of encapsulating several chips of different colors of light may be employed, and a mixture of these colored lights constitutes a white light-emitting L ED . Table 1 lists the current types of white L ED and their luminescence principles.

4 LED as an illumination source

As a novel semiconductor light source, LED is mainly characterized by the following aspects.

4.1 Long life

LEDs can last up to 100,000 hours, and traditional light sources cannot be compared to this. Generally speaking, the life of an ordinary incandescent lamp is about one thousand hours, and the life of a fluorescent lamp or a metal halide lamp is not more than 10,000 hours. The high-pressure sodium lamp has a long life in an electrode with a discharge lamp of more than 20,000 hours. Or the life of the microwave-excited electrodeless discharge lamp discharge tube can reach 60,000 hours, but the life of the whole lamp is subject to the life of the electronic components and the microwave oscillation tube in the excitation circuit. Therefore, in some occasions where maintenance and lamp replacement are difficult, the use of LEDs as a light source can greatly reduce labor costs.

4.2 Short start-up time

The steady output of the gas discharge light source from start-up to light radiation takes tens of seconds to several tens of minutes, which is determined by the characteristics of the gas discharge source itself, because the working substance of most gas discharge lamps is liquid or solid at normal temperature. A heating and gasification process is required after startup to achieve a stable working condition. The incandescent lamp is a heat radiation source, which gives the impression that it is bright at a point. In fact, the incandescent lamp also has a rise time of about a few tenths after starting. The response time of L ED is only a few tens of nanoseconds, so in some situations where fast response or high speed motion is required, it is suitable to use LED as the light source.

4.3 Firm structure

The LED is a semiconductor light-emitting solid-state light source encapsulated with epoxy. The structure does not contain fragile components such as glass and filament. It is a solid all-solid structure and can withstand vibration and impact without causing damage. This feature of L ED makes it suitable for use in harsh and harsh conditions.

4.4 Low power consumption

LEDs consume less energy and are an energy-saving light source. At present, the luminous efficacy of white LEDs has reached 25 lm / W, exceeding the level of ordinary incandescent lamps, and now the technology of L ED is developing very fast. It is expected that by 2005, the efficacy of white LEDs can reach 50 lm / W, and From 2010 to 2015, with the breakthrough of key technologies, the light efficiency of white LEDs may reach 150-200 lm / W, which greatly exceeds the light efficiency of all current lighting sources, and has attractive application prospects in lighting.

4.5 illuminator close to point source

The LED's illuminator chip size is small, and it can basically be regarded as a point source when designing the luminaire, which can bring a lot of convenience to the luminaire design. The illuminant of an incandescent lamp is a filament, which has a certain length, and the size of the fluorescent tube is larger. These illumination sources cannot be regarded as a point source. In the design of the luminaire, a radiation model of the source is first established, which has certain difficulty in processing. The light source radiation model of the point source is the simplest, which is beneficial to the LED lamp design.

4.6 can be made into thin lamps

Conventional illumination sources illuminate in almost every direction of space. In order to improve the efficiency of light utilization when designing lighting fixtures, curved reflectors are usually used to collect light and illuminate it in the desired direction. Since the reflector has a certain distance from the light source, the curved surface of the reflector has a certain curvature, so the entire luminaire has a certain thickness. The direction of LED illumination is very strong. In many cases, it is only necessary to use a lens to collimate and deflect the light emitted by the lens without using a reflector. The designed lamp has a small thickness and can be made into a thin and beautiful lamp. Especially suitable for occasions where there is not much installation space for lamps.

5 LED application and prospects in lighting

The technical level of LED has made great progress compared with decades ago, and has replaced traditional incandescent light sources in some fields, especially signal display. In China, one area of ​​LED application is traffic light. In the past, one signal light used a 100 W light bulb, while with LED, one signal light consumed more than 10W, and its energy saving effect was very obvious. At the same time, LED signal lamps are also maintenance-free, thin, and a wide selection of lighting materials. In addition, LED can also be applied to road traffic signs, railway signal lights, building aviation obstacle lights, navigation lights, car signal lights, instrument backlighting, and architectural lighting decoration. With the continuous development of new application fields, LED light sources are receiving more and more attention.

With the advancement of human civilization, people's requirements for lighting are no longer blind pursuit of brightness. At present, many countries in the world pay attention to environmental protection issues in lighting. The energy of lighting is mainly derived from the light energy converted by electric energy, which in turn comes from the burning of fossil fuels. The reserves of fossil fuels such as coal, natural gas and petroleum on the earth are limited. With the continuous exploitation of human beings, their reserves are increasingly The world energy situation is not optimistic. The use of energy-efficient and efficient light source can save electricity and protect energy, and energy saving is of great significance to environmental protection. Fluorescent lamps, mercury lamps, and other light sources that are widely used today contain mercury that is harmful to human health, and the production process of the light source and the discarded lamps cause environmental pollution. LEDs are a source of light that meets the requirements of green lighting and will play an increasingly important role in green lighting projects.

At present, the biggest obstacle for LEDs to be widely used in the field of lighting is cost. White lighting LEDs are used for general lighting. Since the luminous efficacy of white LEDs is not high enough, the power is not large enough and the price is relatively expensive, which limits its application in the field of lighting. General lighting is large, traditional fluorescent lamps and HID lamps have established a dominant position in this field, and are widely used, where price is an important factor. We can compare the price required for fluorescent and white LEDs to produce 1 000 lm of light. The luminous flux of a normal 40 W fluorescent lamp is about 3 000 lm, and the price of a lamp and an electrical accessory is about 10 yuan, so it takes only a few dollars to produce 1 000 lm of light. A 20 mA white LED has a luminous flux of about 2 lm and a price of 8 yuan. It takes 500 LEDs to produce 1 000 lm of light. The price is 4,000 yuan, which is 1,000 times that of fluorescent lamps. At present, there are two general situations in which LEDs are used for general illumination. First, the requirements for luminous flux are not high, mainly for local lighting and decoration, but the pursuit of new occasions, such as garden lights in some villas; second, in high-grade, expensive products, such as certain aircraft Passengers read the lights. White light L ED should be widely used as a general illumination source. Its development goal is to achieve a single input power of 10 W, luminous efficiency of 100 lm / W, output luminous flux of 1 000 lm, and the price is appropriate. To achieve such a goal is not a very distant matter. Table 2 gives some statistics and predictions.

In addition, in terms of the light-emitting characteristics of the light source itself, LEDs have advantages over incandescent lamps and fluorescent lamps. Generally, LEDs have a small illumination angle and strong directivity. Therefore, LEDs are a good choice when it is necessary to perform localized illumination, which is suitable for small area lighting applications. However, if a room or a large area is to be illuminated, the light distribution angle of the lamp is required to be large, and the self-illumination angle of the incandescent lamp or the fluorescent lamp is large, and the luminous loss caused by the lamp is small and the effect is good. Therefore, today, many foreign manufacturers use white LEDs to make small reading lamps, spotlights and other small lamps, which are used in aircraft, automobiles and interior decoration lighting.

It can be seen that LED is a promising illumination source. Although it is currently limited by the technical level and price, it cannot compete with traditional light sources in the field of illumination, but its development potential is enormous. With the advancement of technology, LEDs will surely make full use of their energy-saving and high-efficiency characteristics in the field of lighting, thus gaining more and more applications.

Edit: Cedar