The principle of the LED: how the lamp works, technical characteristics and the history of its creation

The LED is a two-wire semiconductor light source. When a suitable current is applied to the terminals, the electrons are able to recombine with the electron holes inside the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light is determined by the energy gap of the semiconductor.

What is LED

A light emitting diode is an optoelectronic device capable of emitting light when an electric current is passed through it. Light emitting diode only passes electric current in one direction and produces incoherent monochromatic or polychromatic radiation from the conversion of electrical energy.

He has several derivatives:

• OLED.
• AMOLED.
• FOLED.

Due to their luminous efficiency, LEDs currently represent 75% of the indoor and automotive lighting market. They are used in the construction of flat-screen TVs, namely, to backlight LCD screens or to provide a source of electricity. Used as the main lighting in OLED TVs.

The first LEDs to go on sale produced infrared, red, green, and then yellow light. The output of the blue LED, associated with technical and installation progress, allows the coverage of the radiation wavelength range from ultraviolet (350 nm) to infrared (2000 nm). nm) that meets many needs. Many devices are equipped with composite LEDs (three in one component: red, green and blue) to display many colors.

LED lamp

LED lamps are lighting products for household, industrial and street lighting, in which the light source is LED. Basically it is a set of LEDs and power circuits for converting mains power to low voltage direct current.

The LED luminaire is a separate and independent device. Its body is most often individual in design and specially designed for various light sources. A large number of lamps and their small size make it possible to place them in different places, assemble panels, and use them to illuminate displays and televisions.

General purpose lighting requires white light. The principle of operation of an LED lamp is based on the emission of light in a very narrow wavelength range: that is, with the color characteristic of the energy of the semiconductor material that is used to manufacture LEDs. To emit white light from an LED lamp, you must mix the emissions from red, green and blue LEDs or use a phosphor to convert parts of the light into other colors.

One of the methods - RGB (red, green, Blue), is the use of several LED matrices, each of which emits different wavelengths, in close proximity, to create an overall white colors.

The history of the creation of the first lamps

The first emission of light by a semiconductor dates back to 1907 and was discovered by Henry Joseph Round. In 1927, Oleg Vladimirovich Losev filed the first patent for what would later be called a light-emitting diode.

In 1955, Rubin Braunstein discovered infrared radiation from gallium arsenide, a semiconductor that would later be used by Nick Holonyak, Jr. and S. Bevaccoy to create the first red LED in 1962. For several years, researchers have limited themselves to a few colors, such as red (1962), yellow, green and later blue (1972).

Contribution of Japanese scientists

In the 1990s, research by Shuji Nakamura and Takashi Mukai of Nichia in InGaN semiconductor technology allowed to create high brightness blue LEDs and then adapt to white by adding yellow phosphor. This advance has allowed for major new applications such as lighting and backlighting for television screens and LCD screens. On October 7, 2014, Shuji Nakamura, Isamu Akasaki and Hiroshi Amano received the Nobel Prize in Physics for their work on blue LEDs.

How the device works

When the diode is biased forward, electrons move rapidly through the junction. They are constantly uniting, removing each other. Soon after the electrons begin to move from n-type to p-type silicon, the diode connects to the holes and then disappears. Hence, it makes the entire atom more stable and gives a small pulse of energy in the form of a photon of light.

The principle of the formation of a light wave

To understand how the LED works, you need to learn about its materials and their properties. An LED is a specialized form of PN junction that uses a composite connection. The compound must be the semiconductor material used for the connection. Commonly used materials, including silicon and germanium, are simple elements, and a compound made from these materials does not emit light. As for semiconductors such as gallium arsenide, gallium phosphide and indium phosphide - they are composite, and compounds from these materials emit light.

These composite semiconductors are classified according to the valence bands that they occupy. Gallium arsenide has a valency of three and arsenic has a valency of five. This is called a group III-V semiconductor. There are a number of other semiconductors that fit this category. There are semiconductors that are formed from materials of group III-V.

A light emitting diode emits light when it is biased forward. When voltage is applied to the junction to make it biased forward, current flows, as with any PN junction. Holes from the p-type region and electrons from the n-type region enter the junction and recombine like a normal diode to allow current to flow. When this happens, energy is released.

It was found that most of the light comes from the transition region closer to the P-type region. The design of the diodes is made in such a way that this area is located as close as possible to the surface of the device in order for the structure to absorb a minimum amount of light.

To get the light that can be seen, the connection must be optimized and the materials must be correct. Pure gallium arsenide gives off energy in the infrared part of the spectrum. To induce light emission, aluminum is added to the semiconductor in the visible red spectrum, followed by the production of gallium arsenide arsenide (AlGaAs). Phosphorus can be added to produce red light. Different materials are used for other colors. For example, gallium phosphide produces green light, while calcium aluminum phosphide is used to produce yellow and orange light. Most LEDs are based on gallium semiconductors.

Quantum theory

The current flow in semiconductors is due to both flows of free electrons in the opposite direction. Hence, there will be recombination due to the flux of these charge carriers.

Recombination shows that the electrons in the conduction band go down to the valence band. When they jump from one band to another, they emit electromagnetic energy in the form of photons, and the photon energy is equal to the forbidden energy gap.

Mathematical equation displayed:

Eq = hf

H is known as Planck's constant, and the speed of electromagnetic radiation is equal to the speed of light. Frequency radiation is related to the speed of light as f = c / λ. λ is denoted as the wavelength of electromagnetic radiation, and the equation becomes:

Eq = he / λ

Based on this equation, you can understand how an LED works, based on the fact that the wavelength of electromagnetic radiation is inversely proportional to the band gap. In general, the total radiation of an electromagnetic wave during recombination is in the form of infrared radiation. It is impossible to see the wavelength of infrared radiation because it is outside the visible range.

Infrared radiation is called heat because silicon and germanium semiconductors are not direct gap semiconductors, but are indirect intermediate varieties. But in semiconductors with a direct gap, the maximum energy level of the valence band and the minimum energy level of the conduction band do not occur simultaneously with electrons. Therefore, during the recombination of electrons and holes, electrons migrate from the conduction band to the valence band, and the momentum of the electron band will be changed.

Advantages and disadvantages

Like any device, the LED also has a number of its features, main advantages and disadvantages.

Main advantages look like this:

• Small size: for example, you can make LEDs as small as pixels (which opens up the possibility of using diodes to create high-resolution screens).
• Ease of PCB assembly, traditional or CMS (Surface Mount Component).
• Electricity consumption is lower than that of an incandescent lamp and of the same order of magnitude as that of a fluorescent lamp.
• Excellent mechanical stability.
• By assembling multiple LEDs, good lighting can be achieved with innovative shapes.
• Life expectancy (approximately 20,000 to 50,000 hours), which is much longer than a conventional incandescent lamp (1,000 hours). hours) or a halogen lamp (2 thous. hours). The same order of magnitude as for fluorescent lamps (from 5 thous. up to 70,000 hours).
• Very low voltage, safe and easy to transport. For vacationers, there are LED flashlights powered by a simple hand dynamo slow motion ("crank lamp").
• Light inertia is almost zero. The diodes turn on and off in a very short time, which makes it possible to use when transmitting signals of near (optocouplers) or far (optical fiber) signals. They immediately reach their nominal luminous intensity.
• Thanks to their power, the classic 5mm LEDs barely get hot and cannot burn your fingers.
• RGB (red-green-blue) LEDs enable color enhancements with unlimited variation.

Of the shortcomings the following can be noted:

• LEDs, like any electronic component, have maximum operating temperature limits, as well as some passive components that make up their power circuit (for example, chemical capacitors that heat up depending on rms current). The heat dissipation of LED light bulb components is a limiting factor in increasing their wattage, especially in multi-chip assemblies.
• According to the manufacturer Philips, the luminous efficiency of some LEDs drops rapidly. Temperature accelerates the decline in luminous efficiency. Philips also points out that the color may change on some white LEDs and glows green as they get older.
• The LED manufacturing process is very energy intensive. Knowing the main characteristics of LEDs, their advantages and disadvantages, you can make a choice - either purchase them, or refuse to buy and use ordinary incandescent lamps. However, given the cost-effectiveness of such lighting, it is worth considering that it can become a good alternative to the usual, cheaper light sources.