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Light emitting diode

Light Emitting Diode

A light-emitting diode (LED) is a semiconductor that emits light from an electric current.The semiconductor material that produces light is formed when the electrons and holes, which carry the current, are combined.

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Table of contents

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The most common source of light is the Light Emitting Diode or LED. There are many uses for LEDs, whether your car’s headlights or daytime running lights or your living room lights.

Like most legacy filament bulbs, LEDs and fluorescent bulbs require a special circuit to work. These are also known as LED Drivers or ballast for fluorescent bulbs.

LEDs will inevitably be part of our daily lives. This makes it a great idea for engineers, driver designers, and others interested in this topic. First, learn the basics of Light Emitting Diode. This article provides a quick overview of LED. It includes an introduction to the electric symbol, types, construction, characteristics, and LED Drivers.

NOTE: This article is also available in a simplified version LED-Light Emitting Diode”, which provides a simplified overview of LEDs without getting into technical details.

Introduction to light emitting diodes

These diodes are the most visible and emit a narrow bandwidth of visible light at different wavelengths, invisible light for remote controls, or laser-type light when forward current passes through them.

The “Light Emitting Diode,” or LED as commonly known, is a specialized diode with very similar electrical characteristics as a PN junction. An LED can pass current in its forward direction but will block current flowing in reverse.

Light-emitting diodes (LEDs) are made of a thin layer of semiconductor material that is very heavily doped. Depending on the material and doping, an LED can emit color at a specific wavelength when forward-biased.

Forward biased diodes allow electrons from the semiconductors conduction band to recombine and release enough energy to create photons that emit monochromatic (single-colored) light. In addition, this thin layer allows for a fair number of photons to leave the junction and radiate away, producing a colored light output.

Two of the most important semiconductor light emitting sources are LED and LASER diodes. LASER diodes operate on stimulated emission while LEDs work on spontaneous emission.

Light Emitting Diodes (LEDs) are the most popular source of light in electronic components. They are used to display the time and other data on certain displays. The Opto-semiconductor LEDs convert electric current into light. The area of an LED is typically very small, and it can have many integrated optical components to design its radiation pattern. Therefore, this LED has the advantage of being more cost-effective and lasts longer than the laser diode.

Two elements are the main components of a semiconductor that make up a light-emitting diode. These are P-type positively charged holes and N-type negatively charged electrons.

The positive P side of the diode connects to the power supply while the N side is connected to the ground. This is known as forwarding bias. It allows electric current to flow through it. The majority and minor charge carriers of the N side and P sides combine to neutralize the charge carriers at the PN junction.

In turn, electrons and holes migrate, which releases photons. This energy is in the form of monochromatic light at a constant wavelength, usually in nm. It is similar to the color of an LED. LED emission has a narrow color spectrum.

It can generally be described as a specific spectrum of electromagnetic wavelengths. Due to the nature and manufacturing of the semiconductor, the LED’s emission range of colors is limited. The most common colors for LED are amber, white, red, yellow, blue, amber, and yellow.
You can combine the light of red, green, and blue colors to create white light with limited brightness. The working voltage for red, green, and amber colors is approximately 1.8 volts. The breakdown voltage of the semiconductor materials used in the manufacture of LED can determine the actual working voltage. The PN junction of semiconductor materials is what determines the color of the LED light.

This is because of the different energy gap band structures of semiconductor materials. As a result, different numbers of photons are emitted at different frequencies. The wavelength of light depends upon the band gap between semiconductor materials at the junction. In addition, the intensity of light is dependent on the amount of energy applied to the diode. Compound semiconductors can maintain the output wavelength, so the required color can still be seen while keeping the output within the visible spectrum.

Electronic means can control light in many ways. For example, electroluminescence, a solid-state process, is used to produce light in light-emitting diodes. Solid-state procedures can produce coherent light under certain conditions, similar to laser diodes.

Construction of Light emitting diode

A Light Emitting Diode’s construction is quite different from a regular signal diode. An LED’s PN junction is protected from vibration and shock by a transparent, hard plastic epoxy resin hemispherical-shaped body.
Surprisingly, LED junctions do not emit much light. The epoxy resin body is designed so that photons of light emitted from the junction are reflected away by the substrate base to which it is attached. Instead, they are focused upwards through a domed top of the LED, which acts as a lens, concentrating the light. This is why the LED’s top appears brighter.

Construction of light emitting diode

Not all LEDs have a dome-shaped epoxy shell. Indicator LEDs may have a rectangular- or cylindrical-shaped construction with a flat top surface, or their bodies are shaped into an arrow or bar. All LEDs have two legs that protrude from the bottom.

Nearly all modern light-emitting diodes ( -) have their cathode terminal, which is identified either by a notch on the body or by the cathode lead that is shorter than the other, as the anode (+ ) lead has a longer length than the cathode(k).

The light-emitting diode generates a “cold” generation of light, which is superior to traditional incandescent lamps or bulbs that produce large amounts of heat when they are illuminated. This results in higher efficiencies than normal “light bulbs” because most of the energy generated radiates outwardly within the visible spectrum. Solid-state LEDs can be very small and last longer than other light sources.

These LEDs were designed to maximize the recombination charge carriers on the surface of the PN junction.

  • The substrate doping concentration can be increased to allow the electrons of the minor charge carriers, which are the electrons that move up the structure, recombine, and emit light from the LED surface.
  • You can increase the diffusion time of charge carriers by increasing L = Dt, where D is the coefficient for diffusion and t the charge carrier lifetime. If the critical value is exceeded, there may be re-absorption of the photons released into the device.

Forward bias connects the diode in a way that allows charge carriers to acquire enough energy to overcome the PN junction’s barrier potential. Forward bias can be applied to inject the minor charge carriers of P-type and N-type across the junction. They then recombine with majority carriers. Radiative or nonradiative, this recombination may occur between majority and minority charge carriers. Radiative recombination emits heat, while nonradiative recombination emits sunlight.

Organic Light-Emitting Diodes (OLED).

The organic light-emitting diodes are made from organic semiconductor materials. An organic semiconductor is an organic material electrically conductive in a part of the molecule or all of it due to the conjugated electron. It can be either in the crystal phase of polymeric molecules. It is a thin material with low cost and low voltage, which allows for high radiation patterns, high contrast, intensity, and maximum radiance.

Light Emitting Diode Colors

Contrary to normal semiconductor signal diodes, which are used to switch circuits, rectifiers, and power electronics circuits made of silicon or germanium semiconductor material, Light Emitting Diodes, which are made from compound semiconductor materials like Gallium Arsenide Phosphide and Silicon Carbide, are mixed in different proportions to create a distinctive wavelength.

Different semiconductor compounds emit different levels of visible light. Therefore, the wavelength and color of the light emitted by the LED depend on the semiconductor material used to make it.

Light Emitting Diode Colours

How does a light-emitting diode get its color? This contrasts with normal signal diodes used for power rectification or detection and is usually made of either silicon or germanium semiconductor materials. Light Emitting DiodesThese are made of exotic semiconductor compounds like Gallium Arsenide, Gallium Phosphide and Gallium Arsenide Phosphide (GaP), Silicon Carbide(SiC), and Gallium Indium Nitride (“GaInN”) all mixed in different proportions to create a unique wavelength of color.

Different LED compounds emit light at different wavelengths of the visible spectrum, producing different intensities. The wavelength and color of light emitted will depend on the exact semiconductor material chosen.

Light Emitting Diode Colours

Light emitting Diode

The wavelength of light emitted by a light-emitting diode determines its color. This is, in turn, determined by the semiconductor compound used during manufacturing to form the PN junction.

Its plastic body does not determine the color of an LED’s light. However, they are slightly colored to enhance their light output and indicate that an electric supply doesn’t illuminate it.

Many colors are available for light-emitting diodes, with the most popular being RED, Green, AMBER, and YELLOW. They are widely used as visual indicators as well as moving light displays.

These LEDs can also be manufactured in blue or white, but they are more costly than standard colors. This is due to the manufacturing costs of mixing two or more complementary colors within the semiconductor compound and injecting nitrogen atoms during the doping process.

The table shows that the main P-type dopant was used to manufacture light Emitting Diodes Gallium (Ga) is atomic number 31, and the main N-type Dopant is Arsenic (3 atomic numbers 33). This gives rise to the crystalline compound Gallium Arsenide (GaAs).

Gallium Arsenide is a semiconductor compound that emits high levels of infrared radiation. It radiates approximately 850nm-940nm. When a forward current flows through it, the junction will be energized.

It produces enough infrared light to be useful for TV remote controls but not enough to be used as an indicator light. The wavelength of the radiation emitted by the LED is now below 680nm when it is enriched with Phosphorus (atomic number 15) as a third dopant. This gives visible red light to our eyes. In addition, the PN junction has been refined further to produce a wide range of colors that span the spectrum of visible and infrared, as well as ultra-violet and infrared wavelengths.

The following list of LEDs is possible by mixing various semiconductor, metal, and gas compounds.

What are the Types of Light emitting Diode?

Two major types of light emitting diodes are available. They can be classified as

  • Visible LEDs
  • Invisible LEDs

Visible LEDs are used primarily for switching, optical displays, and illumination purposes without the need for any photo sensors. With the help of photo sensors, invisible LEDs can be used for applications such as optical switches, analysis, and optical communications.

General Characteristics of Light Sources

Driver Current Vs. Light Output

Due to significant power dissipation, the temperature at the PN junction of semiconductors increases with high forward drive current values. This temperature rise at the junction causes radiative recombination to decrease. This causes the current density to rise further, and internal series resistance will reduce light-emitting efficiency.

Quantum Efficiency

Quantum efficiency is the ratio of the radiative rate of recombination to total recombination. It is described as:


Change Speed

The switching speed of a light source is similar to how fast it can be turned on and off using an applied electrical supply to produce the corresponding optical output. However, LEDs switch at a slower speed than other LASER diodes.

Spectral Wavelength

The wavelength at which maximum light intensity is generated is called the peak spectral wavelength. It is determined using the energy bandgap for the semiconductor material used to manufacture LEDs.

Spectral Width

The spectral width is the range of wavelengths that a light source emits light. Therefore, the narrower spectral width must be met by the light source.

Light Emitting Diodes I-V Characteristics.

Because LEDs are current-dependent devices, the current flowing across them must be sufficient to emit light. Therefore, the LED’s output light intensity is directly proportional to the forward current.

The light-emitting diode must be connected with a forward bias combination across the power supply. It should also be current limited using a resistor in series to prevent excess current flow. An LED should never be connected directly to a battery or power supply as the excess current can flow through it, causing LED damage.

Each LED has a unique forward voltage drop at the PN junction. This parameter is determined by the semiconductor material used to manufacture the LED. Usually, the forward current for each LED is about 20mA.

The driving current of the diode at low forward voltages is dominated by non-radiative radiation current. This is due to the recombination charge carriers along the length of LED chips. However, higher forward voltages will see the diode driving current dominate by the radiative diffusion.

The series resistance limits the diode current even at higher voltages than usual. Permanent damage to the diode can occur if it reaches reverse breakdown voltage for too short a time. Below is a diagram showing the I-V characteristics for different colors of LEDs..

Light Emitting Diodes I-V Characteristics.

What is Efficacy in Led Lighting?

It is based on its luminous efficacy that light-emitting diodes are rated. It can be described as the ratio between luminous flux and the electrical input power to the diode. The response of the eye at different wavelengths is called luminous flux.

Red620 – 645720.39
Green520 – 550930.15
Blue460 – 490370.35
Cyan490 – 520750.26
Red – Orange610 – 620980.29

LED Series Resistance

The series resistor value RS can be calculated simply by using Ohm’s Law. This allows you to know the forward current IF required by the LED, the supply current VS across the combination, and the expected forward voltage drop for the LED. In addition, this will allow you to calculate the current limiting resistance.

LED Series Resistor Circuit

Light emitting diode Series Resistor Circuit

Light Emitting Diode No1

A 5.0v DC power supply stabilized with amber LEDs should be connected to the LED. Calculate the series resistor value to reduce the forward current to 10mA using the circuit. If a 100O series resistor was used, you could also calculate the current flowing through the diode.

1). series resistor required at 10mA.


2). with a 100Ω series resistor.

As we know from our Resistors tutorials that resistors can be purchased in standard preferred values. The 300O resistor would be required to stop the current flowing through LEDs from exceeding 10mA. This is evident in the first calculation. The E12 series resistors do not have a 300O resistor. Therefore, we will need to select the 330O value. Quick calculations show that the current forward value has been recalculated to 9.1mA. This is fine.

Connecting LEDs in Series

To increase the number of LEDs required or increase the light level for displays, we can connect LEDs in series. Like series resistors, LED’s connected in series have the same forward current I F as a single one. Therefore, it is best to have the same type or color of LEDs in series, as they all pass the same current.

Light emitting diode

The current flowing through the LED series chain is the same. Still, the series voltage drop between them must be taken into account when calculating resistance to the current limiting resistor R S. Assuming that each LED is subject to a voltage drop of 1.2 volts when illuminated, the voltage drop across them all will be 3×1.2v = 3.6 Volts.
Assume that all three LEDs will be lit from the same 5V logic device or supply with a forward current of approximately 10mA. This will calculate the voltage drop across R S as well as its resistance value.

Again, in the E12 (10% tolerance) series of resistors there is no 140Ω resistor so we would need to choose the next highest value, which is 150Ω.

Multi-color Light Emitting Diode

There are many LEDs on the market, with different shapes, sizes, colors, and light output intensities. The most popular LED is the Gallium Arsenide phosphide red-colored Led, which has a diameter of 5mm. It is also very affordable to make. Multiple color-emitting light-emitting diodes are now being produced. They are often available in multiple packages. Most of them contain two to three LEDs.

Bi-Color Light Emitting Diodes

The bi-color light-emitting diodes are LEDs similar to single-color LEDs, with an additional one more LED chip enclosed in the package. The bi-color LEDs may have either two or three leads for connecting; it depends on the method used. In general, the two LED leads are connected in inverse parallel combinations. The anode of one LED is connected to the cathode of another LED and vice versa. When the supply is given to either of the anodes, only one LED will glow. We can also turn on both LEDs at the same time with dynamic switching at high speed.

Light emitting diode

Tri-colored light-emitting diode

Three lead LEDs usually have a common cathode, which is the lead that connects to the ground. The other two LED chips are then connected internally. If both one or two of the LEDs must be turned on, it is important to connect the common cathode with the ground. To control the current, the anodes are each connected with current limiting resistors.

It is possible to use single- or bicolor LED illumination by connecting the power supply to each anode individually or simultaneously. These tri-colored LEDs consist of single RED AND GREEN LED chips that are connected to the same cathode. This type of diode can produce additional colors from the primary colors by switching on the two LEDs at different rates of forwarding current.


Circuits for LED Drivers

The light-emitting diodes can be driven using integrated circuits, either sequential or combinational. Integrated circuits can be used to switch the light-emitting diodes on and off. TTL and CMOS logic gates’ output stages can drive light-emitting diodes in two configuration modes. These are the source and sink configurations.
In sink mode configuration, integrated circuits can produce a current of about 50 mA. In source mode configuration, the forward current is about 30 mA. The resistor connected in series should limit the current generated by the light-emitting device.ibus leo.

LED Driver Ciruit

If more than one LED needs to be driven at once, such as in large arrays of LEDs, or if the load current is too high for the integrated circuit, we want to use discrete parts instead of ICs. An alternative method of driving LEDs using either bipolar PNP or NPN transistors as switches are provided below in these cases. To limit the current, and R S series resistor is needed.

Driving an LED using Transistor

The LEDs can also be driven using discrete components, such as NPN transistors and bipolar PNP. In addition, these components can be used to drive multiple LEDs, such as large LED array structures.
A smaller number of applications only use one LED to perform their functions. Junction transistors drive current across multiple light-emitting diodes so that the forward current driven LED by it is approximately 10-20 mA. The series resistor acts as a current source if NPN transistors are used to drive the LED. The series resistor acts as a current sink if PNP transistors are used to drive LEDs.

Driving an LED Using Transistor

Most applications require multiple LEDs, such as for backlighting, street lights, or replacing fluorescent lamps or incandescent lamps. However, driving multiple single LEDs at once causes uneven current sharing between them. Even then, all LEDs are rated to the same forward voltage drop.
You can overcome a single LED failure by connecting parallel Zener diodes (SCRs) to each LED in a series. SCRs are a smart choice as they consume less power when they need to work around the failed LED.
A parallel combination that includes a driver for each string will cost more than several drivers with the appropriate output power.

PWM Control of the LED Light Intensity

Current through the LED controls the intensity of the light it emits. The current that flows through it can vary the brightness of the light. The LED light will glow brighter if a lot of current passes through it.

The maximum current value is greater than the limit. This causes the LED to emit more heat and increases the intensity. The forward current limit for designing LEDs is between 10 and 40 mA. There may be an opportunity to turn off the LED if the required current is lower.

Pulse width modulation used to repeatedly turn the LED ON or OFF according to the intensity of the light required is used in such cases. PWM drivers are not used as linear control devices because they don’t deliver any power. Instead, they dissipate excess energy as heat.

A PWM oscillator will be required first to inject PWM pulses into the LED circuits. There are many types of PWM generators.

LED Displays

A single package contains multiple Light-emitting diodes in different colors, including bicolor, multicolor, and single color. These diodes can be used for backlighting, stripping, and bar graphs. The visual numeric display is an essential requirement for digital display devices. Seven segments display is an example of a single package with multiple LEDs.

The seven-segment display is, as its name implies, seven LEDs in a single package. It can display the information.

Display information can be displayed in the digital data format of numbers, letters, characters, and alphanumeric characters. A seven-segment display typically has eight input connections—one for each LED and one for the common connection for all internal LEDs.

The individual segments will be illuminated if the anodes of all LEDs are connected, and a logic HIGH signal is applied. Likewise, the individual segments will be illuminated if the anodes from all LEDs are connected and applied a logic HIGH signal.

A Typical Seven Segment LED Display

Seven segments display can be described as a type of electronic display device for displaying Numerical decimal numbers. This is an alternative to the more complicated dot matrix displays.


Another valuable use of light-emitting diodes can be found in the following: Opto-coupling. Opto-coupler, or optoisolator, is an electronic device consisting of an optocoupler, or optoisolator, or a combination of a photodiode or phototransistor. It provides an optical signal path between the input and output connections while maintaining electrical isolation between them.

Opto-isolators are made of light-proof plastic bodies with a breakdown voltage between the input (photodiode) and the output (phototransistor of approximately 5000 volts. This is especially useful when the signal from a low-voltage circuit, such as a computer, battery, or microcontroller, is needed to control or operate another circuit that operates at a dangerous mains voltage.

Photo-diode and Photo-transistor Opto-couplers

Opto-isolator consists of an optical transmitter, such as an infrared emitting Gallium arsenide LED, and an optical receiver. They are optically coupled and use light (or light) to transmit signals and information. This allows data to be transferred from one circuit to another without the need for an electrical connection or standard ground potential.
Opto-isolators can be digital or switching devices. They transmit either control signals (ON-OFF) or data. An analog signal can be shared by using frequency or pulse-width modulation.

LED Advantages, Disadvantages and Applications


  • Low cost and small size of the chips
  • Longevity
  • High energy efficiency
  • Low temperatures
  • Design flexibility
  • Many colors
  • Eco-friendly
  • High switching speed
  • High level of luminous intensity
  • It is designed to direct its light in a specific direction.
  • Damages less severe
  • Radiated heat is less
  • Greater resistance to vibrations and thermal shock
  • UV Rays are not allowed to be present


  • Radiant output power and wavelength depend on ambient temperature
  • Sensitivity for damage by excessive voltage and current
  • Theoretical overall efficiency can only be achieved in cool or pulsed conditions.


  • Bicycle lights and motor vehicles
  • Traffic light indicators, signs, and signals
  • Data displaying boards
  • For toys and medical applications
  • Applications that are not visual
  • Light bulbs and many other items
  • Remote controls


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