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Overview of LED - Light Emitting Diode

Updated: Apr 3

A Light-emitting diode is a widely used standard source of light in electrical equipment.

What is LED?

A light-emitting diode is a semiconductor device that emits light when an electric current passes through it. At the point when current passes through an LED, the electrons recombine with holes, emitting light in the process. Light-emitting diodes allow the current to flow in the forward direction and block the current in the reverse direction.


LED Symbol

The symbol is similar to the diode, where two small arrows indicate the emission of light.

LED symbol
LED symbol

Construction of LED

LED construction is simple and designed with three semiconductor material layers. The three layers are arranged one by one, where the top layer is a P-type which includes holes, middle layer is active region which includes both holes and electrons and the bottom layer called N-type region which includes electrons.

When the voltage is not applied to the light-emitting diodes, there is no flow of electrons and holes, and they are stable. Once the voltage is applied, the LED will be forward biased, so the electrons in the N-region and holes from the P-region will move to the active region. Another name for this area is the depletion region. Because the charge carriers are holes which have a positive charge, and electrons have a negative charge, so the light can be generated through the recombination of polarity charges.

Working Principles of LED

The foundation of a light-emitting diode is quantum theory, which states that energy is released from a photon when an electron moves from a higher energy zone to a lower energy region. The photon's energy is equal to the difference in energy between these two levels. Current passes through the PN-junction diode when it is in the forward-biased state.

working principle of LED
working principle of LED

The movement of electrons in the current direction and the flow of holes in the opposite direction are what drive current flow in semiconductors. Recombination will thus occur as a result of these charge carriers' movements.

The electrons in the conduction band appear to drop to the valence band as a result of recombination. Electrons produce electromagnetic energy in the form of photons when they move from one band to another; the energy of a photon is equal to the forbidden energy gap.

For example, in quantum theory, the energy of the photon is the product of both the plank constant and the frequency of electromagnetic radiation. The equation will be:

E = hf

where h is the Planck constant and the velocity of electromagnetic radiation is equal to the speed of light which is c. The frequency of radiation is related to the velocity of light as

f = c / λ.

lambda(λ) is denoted as a wavelength of electromagnetic radiation and the above equation will become as

E = hc / λ.

From the above equation, we can say that the wavelength of electromagnetic radiation is inversely proportional to the forbidden gap. In general silicon and germanium semiconductors are not direct-gap semiconductors; rather these are indirect-gap semiconductors. However, the lowest energy level of the conduction band and the greatest energy level of the valence band do not occur at the same instant for electrons in direct-gap semiconductors. As a result, the momentum of the electron band will alter during the recombination of electrons and holes and the migration of electrons from the conduction band to the valence band.

Light Emitting Diode colors

LED Characteristics
LED Characteristics

The wavelength of light emitted from a light-emitting diode determines its real color, which is determined by the semiconductor material actually employed to construct the PN junction during fabrication.

Consequently, the color of an LED's plastic body does not affect the color of light it emits, even though these parts are somewhat colored to improve light output and to show color when an electrical source is not present.

Light-emitting diodes are used extensively as moving light displays and visual indicators because they come in a variety of colors, the most popular being RED, AMBER, YELLOW, and GREEN.

The resultant compound has the crystalline structure of gallium arsenide (GaAs), as can be seen from the above table. Gallium (Ga, atomic number 31) is the primary P-type dopant utilized in the fabrication of Light Emitting diodes, whereas arsenic (As, atomic number 33) is the primary N-type dopant.

As a semiconductor material, gallium arsenide alone has an issue in that when a forward current passes through it, it emits a lot of low-brightness infrared radiation (850–940 nm, roughly).

If we wish to utilize the LED as an indicator light, the amount of infrared light it generates is not particularly beneficial. However, it works well for television remote controls. However, the total wavelength of the radiation released is lowered to less than 680 nm by the addition of phosphorus (P, atomic number 15) as a third dopant, providing visible red light to the human eye. As we have seen above, more improvements in the PN junction's doping process have produced a spectrum of colors that spans visible light as well as infrared and ultraviolet wavelengths.

The following list of LEDs may be created by combining different semiconductor, metal, and gas components.

Types of Light Emitting Diode

  • Gallium Arsenide (GaAs) : infrared.

  • Gallium Arsenide Phosphide (GaAsp) : Red to infra-red, orange.

  • Aluminum Gallium Arsenide Phosphide (AlGaAsp) : High-brightness red, Orange-red, orange, and yellow.

  • Gallium Phosphide (GaP) : red, yellow and green.

  • Aluminum Gallium Phosphide (AlGaP) : green.

  • Gallium Nitride (GaN) : green, emerald green

  • Gallium Indium Nitride (GaInN) : near-ultraviolet, bluish-green and blue.

  • Silicon Carbide (SiC) : blue as a substrate.

  • Zinc Selenide (ZnSe) : blue.

  • Aluminum Gallium Nitride (AlGaN) : ultraviolet.

Light-emitting diodes, like traditional PN junction diodes, are current-dependent devices having a forward voltage drop (VF) that is dependent on the forward-biased LED current as well as the semiconductor compound (the color of the light). The majority of standard LEDs have a forward working voltage requirement of around 1.2 to 3.6 volts and a forward current rating of roughly 10 to 30 ma, with the most common range being 12 to 20 ma.

Depending on the kind of semiconductor material used, the forward working voltage and forward current might vary; however, for a normal red LED, this point is around 1.2V, and for a blue LED, it is approximately 3.6V.

Given the various dopant materials and wavelengths utilized, the precise voltage drop will naturally vary depending on the manufacturer. The initial conduction VF point of the LED will also determine the voltage drop across it at a specific current amount, say 20mA. The forward current-to-voltage characteristic curves of an LED may be displayed for each diode color as they are essentially diodes, as seen below.

Light Emitting Diodes I-V Characteristics

LED and its I-V characteristics
LED and its I-V characteristics

Since light-emitting diodes are current-dependent devices, their light output intensity is exactly proportional to the forward current passing through them before they can "emit" any kind of light.

Since the LED will be connected in a forward bias position across a power source, it needs to be protected from excessive current flow by having its current controlled by a series resistor. An LED will almost immediately burn out if it is connected directly to a battery or power source because too much current will flow through it.

The forward voltage drop across the PN junction of each LED is unique, and this parameter—which depends on the semiconductor material used—is the forward voltage drop for a certain amount of forward conduction current, usually for a forward current of 20mA. This is evident from the table above.

LEDs are often powered by low-voltage DC supplies and are used in conjunction with series resistors (RS) to restrict the forward current to a safe amount. For example, an LED indication with a simple design could consume 5 ma, while higher-brightness LEDs require 30 ma or more of current.

LED Driver Circuits

Having learned about LEDs, we now need a method to turn them "ON" and "OFF". As a result, an LED may be driven by the output stages of both TTL and CMOS logic gates, which can source and drain meaningful current. In sink mode design, most integrated circuits (ICs) can have an output drive current of up to 50 mA; however, in source mode configuration, the internal output current is restricted to around 30 mA.

In either case, as we've already seen, the LED current has to be restricted to a safe value using a series resistor. Examples of controlling light-emitting diodes using inverting integrated circuits are shown below; however, the concept applies to any kind of integrated circuit output, combinational or sequential.

IC Driver Circuit

LED IC driver circuit
LED IC driver circuit

An alternate method of driving the LEDs using either bipolar NPN or PNP transistors as switches is provided below if more than one LED needs to be driven at the same time, such as in large LED arrays where the load current is too high for the integrated circuit, or we may just want to use discrete components instead of ICs. As previously mentioned, in order to limit the LED current, a series resistor, RS, is needed.

Transistor Driver Circuit

LED Transistor driver circuit
LED Transistor driver circuit

It is not possible to simply adjust the current passing through a light-emitting diode to regulate its brightness. An LED that has more current flowing through it will light brighter, but it will also release more heat. LEDs are made to run at a certain forward current of between 10 and 20 ma, in order to create a certain quantity of light.

It could be able to use less current in scenarios where power savings are crucial. Reducing the current to less than, say, 5ma, however, can cause the LED to switch "OFF" entirely or excessively decrease its light output. Pulse Width Modulation, or PWM, is a significantly superior method of controlling LED brightness. PWM involves frequently turning an LED "ON" and "OFF" at different frequencies based on the desired LED light intensity.

LED Light Intensity using PWM

LED light intensity using PWM
LED light intensity using PWM

Higher light outputs can be achieved by using a pulse width-modulated current with a relatively short duty cycle (or "ON-OFF" ratio). This allows for a significant increase in the diode current during the pulses, resulting in a higher output light intensity, while still maintaining the LEDs' "average current level" and power dissipation within safe bounds.

The human eye is not affected by this "ON-OFF" flashing state because, if the pulse frequency is high enough, it "fills" in the spaces between the "ON" and "OFF" light pulses, giving the impression of a continuous light output. Therefore, pulses with a frequency of at least 100 Hz seem brighter to the human eye than a continuous light with the same average intensity.

How LEDs are Different

Compared to other lighting kinds like CFL and incandescent, LED lighting is significantly different. Important variations include the following:

  • Light Source: LEDs are tiny, color-changing lightbulbs that are about the size of a peppercorn. In order to create white light, red, green, and blue LEDs are occasionally combined.

  • Focus: Since LEDs generate light in a focused direction, diffusers and reflectors that trap light are less necessary. LEDs are more effective thanks to this characteristic in a variety of applications, including task lighting and recessed down lights. More than half of the light in other forms of lighting may never exit the fixture and must be reflected in the intended direction.

  • Heat: The heat produced by LEDs is negligible. Comparatively, the energy released by CFLs is around 80%, and that of incandescent bulbs is 90% in the form of heat.

  • Lifespan: Compared to other lighting kinds, LED lighting products usually have a much longer lifespan. A high-quality LED bulb has a lifespan of thirty times longer than an incandescent bulb and three to five times longer than a CFL bulb.

Advantages of LED

  • LEDs have a quick rate of operation.

  • These light sources have a variety of color emissions.

  • Unlike fluorescent bulbs, which employ mercury as a harmful substance, LEDs don't contain it.

  • LED brightness is simply adjustable by changing the current.

  • These have a longer lifespan, are lighter, and are smaller.

  • They use little energy, are widely accessible, and are quite affordable.

  • Hardly enough warm-up time. LEDs emit light in a matter of nanoseconds.

  • Superb color-rendering LEDs create colors that never fade, which makes them ideal for retail applications and displays.

  • Environmentally friendly: mercury and other dangerous materials are not present in LEDs.

  • Controllable: LED light output may be adjusted in terms of both brightness and color.

Applications of LED

  • LED light bulbs are utilized in both households and businesses.

  • In vehicles and motorbikes, light-emitting diodes are employed.

  • LEDs are utilized by cell phones to show the message.

  • LEDs are utilized at traffic light signals.

See below links for more information

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