Led, device, principle of work

Led, device, principle of work.





 

 

Led is a semiconductor with electron-hole transitions, whose task is to transform (convert) an electric current directly into visible light.

 

The led and its device

The principle of operation of the led

Characteristics of the LEDs

Types of LEDs

The advantages of LEDs

Colors and materials of LEDs

 


The led and its device:

Led is a semiconductor with electron-hole transitions, whose task is to transform (convert) an electric current directly into visible light. Its official name is light-emitting diod (short – LED), which translated into Russian accurately describes its purpose is “diodethat emits light”. It consists of:

chip – a semiconductor chip;

electrodes (cathode and anode)

– thin wire contact connecting the anode (in some designs also the cathode) with a chip (a semiconductor crystal),

– substrate on which is placed the chip (semiconductor chip);

– building equipped with the terminals;

– optical system.

Optical radiation occurs as a result of passing direct electric current through the crystal, and the emitted color depends on the material (chemical composition), from which he made, as well as possible inclusion in the chip (crystals) of various additives. Most LEDs has a single semiconductor chip, but there are diodes with two or more chips. Such devices are manufactured, if you want to increase their power or get a colored glow.

Usually the led is connected to the mains through a resistor installed on the input. The resistor protects the led from voltage spikes and high current. In the absence of resistor the led might burn out.

 

Working principle of led:

A distinctive feature of the LEDs from the more familiar to us lighting devices (incandescent lamps, fluorescent lamps) is the lack of filament and fragile bulb filled with gas.






The light in the led is formed by p-n-transition, which transmits electric current. So, semiconductor materials are p-type have the ability to accumulate charges with positive particles and the semiconductor materials are n-type with a negative one. Materials n-type represent the “cumulative stock” of electrons, whereas in p-type be vacant space (hole) where no electrons. At the moment when the diode is supplied through the contacts of the electric current, the electrons start to move, rushing to the electron-hole transition, where injections directly to p-type. At the same time in the n-type, representing the negative terminal, also there is such a movement.

When electric current flows through the p-n junction in the forward direction, the charge carriers (electrons and holes) recombine, i.e. there is a disappearance of a pair of free carriers of opposite charge with a release of energy in the form of radiation photons (due to the transition of electrons from one energy level to another).

However, not all semiconductor materials to effectively emit light (photons) in the recombination. The best emitters are to direct bandgap semiconductors. Diodes are made from semiconductors napryazhennykh, the light almost does not emit.

Changing the composition of semiconductor materials, it is possible to create LEDsthat emit light in the visible spectrum and in the ultraviolet and middle infrared range.

 

Characteristics of the LEDs:

LED devices have several basic parameters. This:

– current consumption;

– power consumption;

– rated voltage;

– colour temperature;

the strength of the light flow.

Almost all of these characteristics are noted on the appliance, but there are other indicators that are specific.

The power supply current. The strength of the input current determines the brightness of the led. The current consumption of the led is measured in amperes and most often corresponds to 0,02 A. Is the parameter of a single crystal. If a few chips, and the number is increasing: 0.04 And with two crystals, 0.06 A with three, etc. to take into Account the increased current consumption should be to select the resistor installed on the input. If the indicators do not correspond to each other, high current will overcome the resistance of the led and it will burn out, and almost instantly. The resistor also protects the device from current surges in the network that occur at different voltage drops.

The resistance of the led. This figure can change, because it is nonlinear and varies depending on the inclusion in the chain. When you turn one way it can reach approximately one of the kilo Ohm (kω) and the other is to increase up to several megohms (Mω). Accordingly, the higher the voltage, the diode experiences, the less the resistance they have shown.

Rated voltage. This characteristic of the led depends on its color, and the last parameter is the material, chosen for its production and the inclusion of different additives.

The infrared glow is characteristic of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). In this case, when a current of 20 mA, the voltage range is 1.1-1.6 V and the typical voltage value of 1.2 V.





Red, orange and yellow colors of the diode is achieved through the solid solutions of gallium arsenide-gallium phosphide (GaAsP), gallium phosphide (GaP) and aluminum phosphide gallium indium (dual color mini -). The voltage range at the same amperage of 20 mA are:

the red led is 1.5-2.6 V;
orange led is 1.7-2.8 V;
the yellow led is 1.7-2.5 V.

Typical value voltage of all the colors is 2.0 V.

A green led is obtained thanks to the content of gallium phosphide (GaP) and indium nitride gallium (InGaN). Under the same nominal 20 mA voltage range is 1.7-4.0 V and typical voltage of 2.2 V.

The blue hue of the diode allows to obtain a binary compound of zinc and selenium – zinc selenide (ZnSe) and the nitride indium gallium InGaN. For this color at a current of 20 mA, the voltage range is defined within 3.2 to 4.5 V, the typical voltage is 3.6 V.

For obtaining white light using blue or ultraviolet diodes coated with phosphor or a combination of the three LEDs of the primary colors (red, blue, green). Their voltage at a current of 20 mA in the range of from 2.7 to 4.3 V, typical value of voltage corresponds to 3,6 V.

The power consumption of the LEDs. This parameter is needed to select a power supply of the appliance, equipped with a certain number of LEDs. Each led is individual and varies in the range from 0.5 to 3.0 watts.

The color temperature of the led. This characteristic is measured in Kelvins (K) and has several indicators. The basic division is presented by such shades of light:

from 2700 K To 3500 K – warm light;
3500-5300 To – neutral;
5300-7000 To – cold glow.

Light output and beam angle of the led. Brightness (light intensity) of the led is directly proportional to the current flowing through it an electric current, that is, the voltage will be higher, the greater will be the brightness of the led. The unit of measurement of luminous flux is the lumen (LM).

The light output of light source (luminous efficiency) is the ratio of the emitted source light to its power consumption. The light output is measured in lumens per watt (LM/W). It is an indicator of the effectiveness and efficiency of the light sources.

The power and angle of the luminous flux of the led can vary because they are dependent on shape and materialselected for fabrication of the illumination device. However, the angle cannot exceed 120 degrees. To increase the angle of dispersion can be used special lenses and/or reflectors. So, with proper selection of such devices, to increase the strength of the luminous flux of the led power 3 watt it is possible to 300-350 lumens.

 

Types of LEDs:

LEDs by purpose are divided into two main types:

indicator;





lighting.

Indicator are the weak brightness and power elementsused most often in various electronic appliances as indicators of on/off of a function: the backlight of the instrument panel in a vehicle, LCD TV, computer power supply and so on. Their distribution is quite wide, because these low-power LED devices do not require costly equipment to manufacture, but because their cost is small.

Lighting diodes are elements with high power and brightness, the main scope of which – lighting electric appliances.

Also , the LEDs are distinguished by the type of case. These include:

DIP (Direct-In-line Package) is one of the first invented elements. Today considered obsolete. The main area of use – toys with elements of electronics and light Board;

– Superflux or “piranha” – analogue of the previous generation, but having four contact. They improved fastener that allows the diodes to stay safer, reduced heat emission. The most common use – vehicle dashboard in your car;

– SMD (Surface Mounted Device, which translated from English. means “a device for surface mounting”) is the most popular housing type for use in lighting appliances;

– COB (Chip On Board) – improved model SMD.

 

The advantages of LEDs:

more accessible, economical and safe alternative to the usual incandescent and fluorescent lamps;

working in a mode of direct conversion of electric current into visible radiation, the LEDs emit virtually no heat, which distinguishes them from classic incandescent lamps and fluorescent lamps;

– emitted by the LEDs of visible radiation occurs in the narrow part of the spectrum, making the color itself is very clean, and infrared and ultraviolet radiation are missing almost entirely;

due to the dense top coat, the led is virtually impossible to break or accidentally break, making it safe, as well as the fact that he is low;

a life well LED devices can reach 100 thousand hours, which is ten times greater than this parameter from the fluorescent lamps and 100 times of incandescent lamp.

Unfortunately, LEDs are prone to “aging”: they do not burn out immediately, but gradually and for a long time reduces the brightness of his illumination, which is a great indication how soon the device must be replaced. Ironically, the smallest life in white diodesthat are the most popular for consumers. The problem of reduced exploitation is the material used to obtain diodes with white light: in crystals is applied to the phosphor is a special composition that reduces thermal characteristics of the diode, contributing to its stronger heating and, consequently, more rapid combustion. However, even with these parameters, these LED devices are able to serve 30-50 thousand hours, an average of 25-30 years;

LEDs cause minimal harm to the environment. In led lamp no mercury and lead, not glass, which is reflected in the ways of their utilization;

consumption of electric power LEDs ten times less than the lamp filament.

 

Colors and materials of led:

The table below presents the colors and materials of the semiconductor led.

Color wavelength, nm The semiconductor material of the led
Infrared λ > 760 Gallium arsenide (GaAs)
Aluminum arsenide gallium (AlGaAs)
Red 610 < λ < 760 Aluminum arsenide gallium (AlGaAs)
Arsenide-gallium phosphide (GaAsP)
Aluminium phosphide gallium indium (AlGaInP)
Gallium phosphide (GaP)
Orange 590 < λ < 610 Arsenide-gallium phosphide (GaAsP)
Aluminium phosphide gallium indium (AlGaInP)
Gallium phosphide (GaP)
Yellow 570 < λ < 590 Arsenide-gallium phosphide (GaAsP)
Aluminium phosphide gallium indium (AlGaInP)
Gallium phosphide (GaP)
Green 500 < λ < 570 The nitride indium gallium (InGaN) / gallium Nitride (GaN)
Gallium phosphide (GaP)
Aluminium phosphide gallium indium (AlGaInP)
Aluminium phosphide-gallium (AlGaP)
Blue 450 < λ < 500 Zinc selenide (ZnSe)
The nitride indium gallium (InGaN)
Silicon carbide (SiC) as substrate
Purple 400 < λ < 450 The nitride indium gallium (InGaN)
Purple The mixture of several spectra Double: blue/red diode,
blue with red phosphor,
or white with purple plastic
Ultraviolet λ < 400 Diamond (λ = 235 nm).
Boron nitride (UNINTERRUPTED) (λ = 215 nm)
The aluminum nitride (AlN) (λ = 210 nm)
The aluminum nitride gallium (AlGaN)
The nitride of aluminum-gallium-indium (AlGaInN) (λ < 210 nm)
White A wide range of A combination of three LEDs of the primary colors (red, blue, green)

A blue or ultraviolet led coated with phosphor

 

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