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What is LED?
2011-12-30 18:45:37

What is LED?

2. What is LED?

From a semiconductor crystal on a substrate body with the contact pin and an optical system. Modern LEDs bear little resemblance to the first hull LEDs used for display. The design of a powerful series of LEDs Luxeon, manufactured by Lumileds, is shown schematically in Fig.


3. How does the LED?

Luminescence occurs when the recombination of electrons and holes in pn-junction. So, first of all need a pn-transition, ie the contact between two semiconductors with different types of conductivity. For this near-contact layers of semiconductor crystal dopant impurities different: on one side of the acceptor, on the other - donor.

But not everyone pn-transition emits light. Why? First, the gap in the active region of LEDs should be close to the energy of photons of visible range. Secondly, the probability of radiation in the recombination of electron-hole pairs should be high, for which solid crystals contain small defects, because of which recombination occurs without the radiation. These conditions have to some extent contradictory.

In fact, in order to keep the two conditions, one of the pn junction in the crystal is not enough, and you have to produce multilayer semiconductor structures, the so-called heterostructure for the study of which the Russian physicist Academician Jaures Alferov received the Nobel Prize in 2000.

4. Does this mean that the more current passes through the LED, so it shines brighter?

Of course, yes. The more current, more electrons and holes enter the zone of recombination per unit of time. But the current can not increase indefinitely. Because of the internal resistance of the semiconductor and pn-junction of the diode will overheat and fail.

5. What good LED?

In LEDs, unlike incandescent or fluorescent lamps, electric current is converted directly into light radiation, and in theory it can be done almost without loss. Indeed, the LED (with proper heat sink) is not very hot, which makes it indispensable for some applications. Further, the LED emits in a narrow part of the spectrum, the color is pure, which is especially appreciated by designers, and UV and infrared radiation, as a rule, no. LED is mechanically durable and reliable, its lifetime can reach 100 thousand hours, which is almost 100 times more than incandescent bulbs, and 5 - 10 times greater than that of fluorescent lamps. Finally, the LED - Low-voltage electrical appliance, and therefore, safe.

6. What's wrong with the LED?

Only one - price. So far, the price per lumen, LED emission is 100 times higher than the halogen lamp. But experts say that in the next 2 - 3 years this figure will be reduced by 10 times.

7. When the LEDs begin to be used for lighting?

Initially, the LEDs were used solely for display. To make them suitable for illumination, it was necessary to first learn how to make white LEDs, as well as to increase their brightness, and better light output, which is the luminous flux to the energy consumed.

In the 60's and 70's were created by LEDs based on gallium phosphide and gallium arsenide, emitting in the yellow-green, yellow and red spectral regions. They are used in LEDs, displays, instrument panels of cars and airplanes, advertising displays, different systems of information visualization. For light output LEDs surpassed incandescent bulbs. For durability, reliability, security, they also exceed them. One was bad - there were no blue LEDs, blue-green and white.
By the end of 80 years in the Soviet Union to produce more than 100 million light-emitting diodes in a year, while world production amounted to several tens of billions.

8. What determines the color of the LED?

Exclusively from the band gap, which recombine with electrons and holes, that is, from a semiconductor material, and the dopants. The "blue" LED, the higher the photon energy, and hence the greater should be the width of the gap.

9. What difficulties have had to overcome a rocket scientist to make a blue LED?

Blue LEDs can be made on the basis of semiconductors with wide band gap - silicon carbide, compounds of elements II and IV group or nitrides of Group III elements. (Remember the periodic table?)

In LEDs based on SiC was too low efficiency and low quantum yield (ie the number of emitted photons to recombine a pair). For LEDs based on solid solutions of zinc selenide ZnSe quantum yield was higher, but they are overheated due to high resistance and served briefly. There remained the hope of nitrides.

Gallium nitride GaN melts at 2000 ° C, while the equilibrium vapor pressure is 40 atmospheres of nitrogen, it is clear that raise such crystals is not easy. Similar compounds - nitriles aluminum and indium - also semiconductors. These connections form a ternary solid solutions with a band gap, which depends on the composition, which can be chosen so as to generate the desired light wavelength, including blue. But ... problem could not be resolved before the end of 80 years.

First, back in the 70's, blue LED based on gallium nitride films on sapphire managed to get Professor Jacques Pankov (Jacob Isayevich Panchechnikovu) from the company IBM (USA). The quantum yield was sufficient for practical applications, but executives said: "Well, that's a sapphire - expensive and not very bright, to the same pn-transition is not good ..." - and work Pankov not supported.

Meanwhile, a group Saparinah and Chukicheva from MSU has found that under the electron beam GaN doped with zinc is bright phosphor, and even patented optical memory devices. But then a mysterious phenomenon could not explain.
This made the Japanese - Professor I. Akasaka, and Dr. X. Amano from the University of Nagoya. After processing the film GaN doped with magnesium, an electron beam with the scanning, they were brightly luminescent Officer layer of p-type with a high concentration of holes. But developers of LEDs is not paid due attention to their publication.

Only in 1989, Dr. S. Nakamura, from the firm Nichia Chemical, studying film of nitrides of Group III, was able to benefit from Professor Akasaka. He picked up the doping (Mg, Zn) and heat treatment, replacing its electronic scanning, which could get effectively inject layers p-type GaN-heterostructures. Here's how they had obtained a blue LED.

The firm Nichia patented the key stages of technology and the end of 1997 has already produced 10 - 20 million blue and green LEDs per month and in January 1998 began to produce white LEDs.

10. What is the quantum yield of the LED?

The quantum yield - the number of photons emitted per recombining electron-hole pair. There are internal and external quantum yield. In-house - in the pn-junction, external - for the instrument as a whole (because the light can be lost "on the road" - absorbed, scattered). The internal quantum efficiency for the good of crystals with good heat-diversion of almost 100%, a record external quantum efficiency for red LEDs is 55%, and ddya blue - 35%.

 

The external quantum efficiency - one of the main characteristics of the LED efficiency.

11. How to get white light using LEDs?

There are three ways to get white light from LEDs. First - color mixing technology RGB. At the same matrix tightly placed red, blue and green LEDs, the radiation which is mixed with the optical system, such as lenses. The result is a white light. The second way is that on the surface of the LED emitting in the ultraviolet range (there are), applied three phosphors, which emit, respectively, blue, green and red light. This is similar to how light fluorescent lamp. And finally in the third method, the yellow-green or green plus red phosphor applied on a blue light-emitting diodes, so that two or three light mix to form a white or nearly white light.

12. Which of the three methods is better?

Each method has its advantages and disadvantages. Technology RGB, in principle, allows not only get white, but also move on the color chart when changing the current through the different LEDs. This process can be controlled manually or through the program, can also get different color temperatures. Therefore, RGB-matrix widely used in svetodinamicheskih systems. In addition, a large number of LEDs in the matrix provides a high total luminous flux and a large axial force of light. But the light spot due to aberration of the optical system has a different color in the center and edges, and most importantly, due to uneven heat transfer from the edges of the matrix and of its mid-emitting diodes are heated in different ways, and, accordingly, different changes in color in the process of aging - the total color temperature and color"Float" during operation. This unpleasant phenomenon is quite difficult and expensive to compensate.

White LEDs with phosphors substantially cheaper than LED RGB-matrix (per unit of luminous flux), and give a good white color. And for them, in principle no problem to get to the point with coordinates (of 0.33, 0.33) on the color chart ICE. Disadvantages of the same are as follows: firstly, they have less than RGB-matrix light output due to the conversion of light in the phosphor layer, and secondly, it is difficult to precisely control the uniformity of the phosphor coating in a technological process and, therefore, color temperature, and finally third - also phosphor aging,being faster than the LED.

Industry releases LEDs with a phosphor and RGB-matrix - they have different scope.

13. What are the electrical and optical characteristics of LEDs?

LED - low-voltage apparatus. Normal LEDs used for display, consumes 2 to 4 V DC at a current of 50 mA. LED, which is used for lighting, consumes the same power, but with the current higher - from a few hundred mA to 1 A in the draft. In separate LED modules LEDs can be connected in series and the total voltage is higher (usually 12 or a 24).

By connecting the LED polarity must be observed, otherwise the device may be damaged. The breakdown voltage is indicated by the manufacturer and is usually more than 5 V for a single LED.
The brightness of LEDs is characterized by the luminous flux and the axial luminous intensity and pattern. Existing LEDs emit different designs in a solid angle of 4 to 140 degrees. Color, as usual, determined by chromaticity coordinates and color temperature, and the radiation wavelength.

To compare the efficiency of LEDs with each other and with other light sources used light output: the luminous flux per watt of electrical power. Also interesting marketing feature is the price of one lumen.

14. How responsive LED on the rise in temperature?

Talking about the temperature of the LED, it is necessary to distinguish between the temperature of the crystal surface and in the pn-junction. From the first term of service depends on the second - light output. In general, with increasing temperature pn-transition brightness LEDs is falling, because the internal quantum efficiency decreases due to the influence of lattice vibrations. Therefore, it is important a good heat sink.

Decrease in brightness with increasing temperature is not the same in light-emitting diodes of different colors. It's more a AlGalnP-and AeGaAs-emitting diodes, that is, red and yellow, and less in InGaN, that is, green, blue and white.

15. Why do I need to stabilize the current through the LED?

As seen from the figure, the operating modes, the current is exponentially dependent on voltage and low voltage changes lead to large changes in current. Since the light output is directly proportional to the current, then the brightness of the LED is unstable. Therefore the current necessary to stabilize. In addition, if the current exceeds the allowable limit, then the overheating of the LED can lead to accelerated aging.

typical current-voltage characteristics of LEDs

16. Why LED requires converter?

Converter (in English terminology driver) for the LEDs - the same as the ballast for the lamp. It stabilizes the current flowing through the LED.

17. Can I adjust the brightness of the LED?

The brightness of LEDs very well to regulate, but not by reducing the supply voltage - this is just something you can not do - and so-called method of pulse-width modulation (PWM), which calls for a special control box (actually it can be combined with the block Food and converter, as well ascontroller controls color RGB-matrix). The method of PWM is that the LED is supplied not permanent, and pulse-modulated current, the frequency of the signal must be hundreds or thousands of hertz, while the width of pulses and pauses between them may vary. The average brightness of the LEDs is controlled, at the same time, the LED goes out.

Small changes in color temperature when dimming LED be compared with a similar shift for incandescent lamps.

18. What determines the life of LEDs?

It is believed that the LEDs very durable. But this is not quite true. The more current is passed through the LED in the process of its service, the higher the temperature and the faster the aging occurs. Therefore, the life of a high-power LEDs are shorter than the lower-signal, and is currently 20 - 50 thousand hours. Aging is expressed primarily in the reduction of brightness. When the brightness is reduced by 30% or half the LED must be changed.

19. "Port" Does the color of the LED over time?

The aging of the LED is connected not only with the decrease of its brightness, but with the change of color. There are currently no standards that would allow to quantify change in color of LEDs in the aging process and compare with other sources.

20. Not bad for a LED for the human eye?

The emission spectrum of the LED close to the monochromatic, what is the cardinal difference between the spectrum of the sun or incandescent lamps. Is this good or bad - is not known for certain, because, as far as I know, serious research in this area has not been anywhere. There are no data about the harmful effects of light-emitting diodes on the human eye is missing.

It is hoped that soon the impact of LEDs on the vision will be thoroughly investigated. The problem became interested Academician Mikhail A. Ostrovsky - a great specialist in the field of color vision. The theme for the decision which he took, called: "Psychophysical perception of LED lighting system of rights."

21. When and how superbright LEDs have appeared in Russia?

This is better than any professor will tell Yunovich.
- Luminescence of silicon carbide was first observed by Oleg Vladimirovich Losev in the Nizhny Novgorod Radio Engineering Laboratories in 1923 and showed that it occurs near the pn-junction. The first scientific paper on crystals of gallium nitride has been published by Professor MSU GS Zhdanov in the 30-ies. Luminescence in heterostructures based on gallium arsenide first studied in the laboratory Zh.I. Alferov, in 60-ies. and showed that you can create a structure with an internal quantum efficiency close to 100%. Development of structures and LEDs based on gallium nitride were conducted at the Leningrad Polytechnic and Electrotechnical Institute, in Kaluga, in Zelenograd in 70-ies. But they did not lead to the establishment of efficient blue LEDs.

In 1995, I read the first article of Nakamura and realized that the "blue issue" in principle solved. Then I received a grant from Soros Foundation. In December, the money I was able to go to a conference in the U.S., where Professor Jacques Pankov introduced me to the S. Nakamura. I threw the bait: do I want to append the students of Moscow University to the best achievements in the field of blue LEDs and tell them about such a wonderful invention. Fish peck, and in February I received from Dr. S. Nakamura from Japan packet 10 LED from violet to green. Everything then was simple - the company Nichia Chemical Launches LEDs on the market and was interested in scientific advertising. At MSU, we have investigated them thoroughly, they took off all the features and obtained new scientific results. Dr. S. Nakamura has graciously agreed to a joint publication of our first articles.

Simultaneously, experts from the group of Boris Fera pontovicha Trinchuka in Zelenograd, displayed samples of green LEDs, and the chief of traffic police have received positive feedback. The fact is that this group has done a prototype LED traffic lights, but they did not have a good green LEDs. Traffic lights with new green LEDs with super far superior to traffic lights with lamps, and the Moscow government has made an order for 1000 LED traffic lights to the 850 anniversary of Moscow. Such luck!
Just when we visited the Kyrgyz violinist Raykan Karagulova - a graduate of the Moscow Conservatory, student of my wife, who worked in Japan, the first concertmaster of Symphony Orchestra in Osaka. It turned out that her place of work is far from firm Nichia Chemical! B. F. Trinchuk given her a thousand dollars and asked to buy them and send to my address 200 green LEDs. Among them were made the first traffic lights from the Jubilee thousands. Moscow became the first city in the world with massive use of LED traffic lights.

Our scientists and engineers in a RI "Sapphire" tried to repeat the achievement of the Japanese and produce structures based on nitrides for blue and green LEDs on the epitaxial old installation, which had to be upgraded to reach higher temperatures and pressures. But the initiative died out due to lack of money and interest in leadership.

22. What today are manufacturing technology of LEDs and LED modules?

With regard to crystal growth, the basic technology - Organometallic epitaxy. This process requires very pure gases. In the modern facilities provided for automation and control of gases, their separate streams, precise temperature control of gases and substrates. The thickness of the grown layers are measured and controlled in the range of tens of angstroms to several microns. Different layers must be doped, the donors or acceptors, to create a pn-transition with a high concentration of electrons in the n-region and holes - in the p-region.
During one trial, which lasts several hours, the structure can be grown on 6 - 12 substrates of 50 - 75 mm. It is important to ensure uniformity and control structures on the surface of the substrate. Room facilities for epitaxial growth of semiconductor nitride, developed in Europe (the company Aixtron and Thomas Swan) and the U.S. (Emcore), reaches 1,5 - 2 million dollars. The experience of different companies showed that learn to get to this setup, the competitive structure with the necessary parameters can be a time from one year to three years. It is - the technology requires a high culture.

An important step in technology is planar processing of films: they etching, the establishment of contacts to n-and p-layers of metal films for contact conclusions. Film grown on a substrate can be cut by a few thousand chips in sizes from 0,24 x0, 24 and 1x1 mm2.

The next step is the creation of these LED chips. Necessary to mount the crystal in the body, make contact outputs, to make optical coatings, anti-reflective surface for the output radiation or reflect it. If it's white LED, you need evenly phosphor. It is necessary to ensure the heat from the crystal and the corps, to make a plastic dome, which focuses the radiation in the desired solid angle. About half the cost of LEDs, an ode by these stages of high technology.
The need to increase capacity to increase the luminous flux led to the fact that the traditional form of the corps ceased to satisfy the LED manufacturers due to lack of heat. It was necessary to bring your chip to the heat conducting surface. Therefore, to replace the traditional technology and several more advanced SMD-tech-nology (surface montage details - surface mount parts) comes to the most advanced technology PSB (chip on board). LED, manufactured by technology PSB, shown schematically in Fig.

LEDs made of SMD-and PSB-technology, mounted (glued) directly to a common substrate, which can serve as the radiator - in this case it is made of metal. So are LED modules, which may be linear, rectangular or circular in shape, to be rigid or flexible, in short, are designed to satisfy any whim of the designer.

Appear and LED lamps with the same socket as a low-voltage halogen intended to replace them. And for high-power lamps and spotlights are made LED assembly on the round a massive radiator.

Earlier in the LED assemblies were a lot of LEDs. Now, with increasing power LED becomes smaller, but the optical system, which guides the light flux in the desired solid angle, plays an increasingly important role.


Technology PSB

23. Where today it is expedient to use LEDs?

LEDs are used in virtually all areas of lighting, except for light industrial areas, and there may be used in emergency lighting. The LEDs are indispensable in design lighting due to their pure colors, as well as in svetodinamicheskih systems. Profitable as they used where frequent maintenance is costly, which is necessary to strictly save energy and where high demands on electrical safety.

In Moscow in early 2004 adopted a three-year program of energy efficient lighting based on LED technology. Coordinating Council headed by Professor JB Eisenberg. According to this program are encouraged to use LEDs in the test construction, housing and other areas. For instance, LED lights will be installed in pedestrian subways, stairwells, the elevator platforms, that is, where do not need much light, but require minimal maintenance and energy costs, but also important high vandal.


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