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Light-emitting diodes (LEDs) part I

LEDs or light emitting diodes are the most common and most commonly used semiconductor diodes today. They emit visible light in various narrow wavelengths, as well as infrared and laser beams. LEDs are a specific type of PN transition diodes made of a very thin layer of highly doped semiconductor alloys. When the semiconductor electrons recombine with the valence zone holes, enough energy is released to emit photons emitting a narrow light spectrum. Then it can be said that LEDs are a semiconductor “device” that converts electrical energy into lightning energy. The color depends on the wavelength, which on the other hand depends from the width of the prohibited semiconductor zone:

 

where: λ is the length of the light wave;

 

h – Planck’s constant;

 

DW3 – the width of the prohibited semiconductor zone.

 

An important feature of LEDs is their small inertia – from 10ns to 1ns. This allows the LEDs to operate in pulse mode at 100MHz, and is also one of the biggest advantages of LED lamps in front of CFL lamps where the maximum luminous flux is produced with some delay.

 

The construction of the LED differs too much from that of a normal diode. The PN transition is surrounded by a transparent hard plastic with a hemispherical shape that protects the LED from vibrations and strokes. In fact,  the LED itself does not emit much light, so the transparent housing of the diode, which can also be of epoxy resin, is designed in such a way that the photons emitted by the LEDs are reflected from the surface of the base to which is attached diode and focused upward through the domed peak of the LED. For this purpose, the body of the diode is designed in the form of a lens to achieve concentration and a higher brightness at the tip of the LED.

 

However, much of the LEDs are not made with a hemispherical shape. Many LEDs are made with a cylindrical structure with a flat top, others are rectangular or even angled. For some applications, the LED must be in a metal housing.

 

Unlike ordinary incandescent lamps that generates large amounts of heat associated with lighting, LEDs generate cold light, resulting in high efficiency, nearly 5 times higher because most of the light is emitted into the visible spectrum. On the other hand, as the LEDs are semiconductor, they can be extremely small and durable and provide much longer lamp life.

 

How do LEDs glow with different colors? Unlike ordinary diodes made of germanium or silicon, the LEDs are made of exotic semiconductor materials such as gallium arsenide, gallium phosphite, gallium arsenide-phosphite, silicon carbide, gallium indium nitrite, all mixed in different ratios to obtain light of different wavelength, and hence different color. The basic P-type additive in LED production is gallium (Ga, chemical element of atomic number 31), and the main N-type additive is arsenic (As, chemical element of atomic number 33), the chemical compound is GaAs with crystalline structure. The different compounds emit light in a different part of the light spectrum and therefore produce different levels of light intensity. The color of the light emitted usually depends on the color of the LED housing as well.

 

On the other hand this is done to enhance the radiating color of light and also to distinguish the LEDs when they do not glow. LEDs are available in a wide range of colors, the most common being red, yellow, amber and green.

 

The newer LEDs are blue and white, which are also used today. To be more specific in the early 1990s, Isamah Akasaki and Hiroshi Amano of the University of Nagoya as well as Suzy Nakamura, regardless of them, found a cheap blue LED, which in 2014, receive the Nobel Prize in Physics.

 

Soon after their discovery, these LEDs were much more expensive than others due to higher production costs, due to the need to mix two or more chemical elements in their exact proportions and the cost of the ingredients themselves.

 

The first column of the table gives the wavelengths in nanometers that determines the radiating color.

 

At the top of table, the above-mentioned gallium arsenide is used. The problem with it is that it emits a portion of its light in the infrared spectrum. When such LED is used for television remote controls, the composite is left in this form, but if we want the light to be in the visible part of the spectrum, we add phosphorus.

 

The ratio between the different chemical elements – semiconductors and the color light emitted by their compounds is as follows:

 

– gallium arsenide – infrared

– gallium arsenide phosphate – red to orange (depending on the ratio)

– aluminum gallium arsenide phosphate – high-purple red, orange-red, orange, to dark yellow

-glium phosphate – red, yellow and green

-aluminium gallium phosphate – green

– gallium nitrite – green, emerald green

– gallium indium nitrite – blue, blue-green, ultraviolet

– silicon carbide – blue azure

– zinc selenide – bright blue

-aluminum gallium nitride – ultraviolet

 

Like conventional diodes with PN transition, LEDs are current dependent devices, but they also depend on the voltage drop on them. The voltage at which light production starts is about 1.2V for a standard red LED and reaches about 3.6V for a blue LED. The specific voltage drop depends on the various additives in LED production. LEDs are non-linear elements, as can be seen from their volt-ampere characteristic. Let’s look at the voltage values ​​at a current of 20mA. Since the LED is a type of diode, the V-A features look the same as the other diodes, but depending on the color, they can vary for each color. It is noteworthy that large changes in the voltage result in large current changes during the transition. The opposite is also true – the voltage on the transition remains almost constant with large changes in the current flowing through it.

 

LEDs have several key features that are:

 

Maximum power dissipated – typically in mW, but there are already powerful LEDs scattering tens of watts;

Powerful LED diode with distracted power of 50W.

This is the power that LED can distribute without damage and depends on the semiconductor alloy and the LED housing;

Light intensity – in mcd (milliwanders), depends on the type and properties of the semiconductor material and on the quality of the lens in front of the LED;

Maximum continuous forward current – indicated in mA and represents the maximum current that passes through the transition for a long time without causing damage to the LED;

Maximum peak (pulse) current in the forward direction – also in mA. It is important to indicate, in addition to its size, what is the time it passes through the LEDs, and the pause time before it re-runs, i. impulse fill factor;

Directional voltage – determined in V (volts) this is the voltage drop between the two LEDs when a current is flowing through a certain value;

Maximum voltage in the opposite direction – when the LED is turned on in the opposite direction, virtually no current flows through it, but if the maximum voltage in the reverse direction is exceeded – the LED burns;

Color of light.

 

In the next part of the article dedicated to LEDs, we will look at the connection patterns, the use of current limiters and LEDs with more than one color.

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