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Light Emitting Diode, Symbol, Circuit, Working Principle and Significance

Light Emitting Diode

Light Emitting Diode (LED) is a commonly utilized primary light source in electrical devices. Its applications span from mobile phones to expansive advertising billboards. LEDs are predominantly employed in devices that indicate time and showcase various forms of data. The Light-emitting diode, or LED, has emerged as a groundbreaking illumination technology with a profound impact on various industries and our daily lives. This article delves into the fascinating world of LEDs, shedding light on their evolution, applications, and their promise for a more energy-efficient and sustainable future.

What is LED?

A light-emitting diode (LED) is a semiconductor device that emits light as a result of an electric current passing through it. This occurs when electrons recombine with holes, producing light. LEDs permit current to flow in the forward direction while impeding it in the reverse direction.

Light-emitting diodes are highly doped p-n junctions. Depending on the type of semiconductor material and the level of doping, an LED will emit light of a specific color at a particular spectral wavelength when it is forward biased. As depicted in the illustration, an LED is encased with a transparent cover to allow the emitted light to be released.

Background on Discovery of Light Emitting Diode

Early Discoveries

1907-1927: The phenomenon of electroluminescence was first observed in 1907 by the English experimenter H. J. Round at Marconi Labs. He utilized a crystal of silicon carbide and a cat’s-whisker detector. Russian inventor Oleg Losev documented the development of the inaugural LED in 1927. Although Losev’s research findings were disseminated in scientific journals in Soviet, German, and British publications, the practical application of this discovery was not realized for many decades. This delay was, in part, attributed to the notably inefficient light-emitting properties of the silicon carbide semiconductor employed by Losev.

1936: In 1936, Georges Destriau made the significant observation that electroluminescence could be generated by suspending zinc sulphide (ZnS) powder in an insulator and applying an alternating electrical field to it. In his writings, Destriau frequently attributed this luminescence phenomenon to Losev-Light. It’s worth noting that Destriau conducted his research in the laboratories of Madame Marie Curie, who herself was an early luminescence pioneer with notable work on radium.

1939: Hungarian inventors Zoltán Bay and György Szigeti were early pioneers in LED lighting, securing a patent in 1939 for a lighting device utilizing silicon carbide, with an additional option for boron carbide. This device emitted light that could appear white, yellowish-white, or greenish-white, depending on the impurities present.

1951-53: In 1951, Kurt Lehovec, Carl Accardo, and Edward Jamgochian elucidated the workings of these initial LEDs. They conducted experiments utilizing SiC crystals in conjunction with a power source like a battery or a pulse generator. In 1953, they furthered their research by comparing the results with those obtained from a variant, pure crystal.

1955: In 1955, Rubin Braunstein, associated with the Radio Corporation of America, documented the occurrence of infrared emission from semiconductor alloys, particularly gallium arsenide (GaAs). Braunstein’s experiments revealed the infrared emission generated by straightforward diode configurations employing materials like gallium antimonide (GaSb), gallium arsenide (GaAs), indium phosphide (InP), and silicon-germanium (SiGe) alloys. These emissions were observed both at room temperature and at 77 kelvins.

1957: In 1957, Braunstein went on to showcase that these basic devices could be employed for non-radio communication over a short range. As highlighted by Kroemer, Braunstein “stablished a straightforward optical communication connection: Music playing on a record player was utilized, with the aid of appropriate electronics, to modulate the forward current of a GaAs diode. The emitted light was then picked up by a PbS diode situated some distance away.”

1961: J. W. Allen and R. J. were the pioneers who successfully demonstrated the first visible-spectrum (red) LED.

Physics behind light production and emission of LED

In a light-emitting diode, the process of electron-hole recombination within a semiconductor generates light, which can be in the form of infrared, visible, or UV radiation. This phenomenon is known as “electroluminescence.” The specific wavelength of the emitted light is determined by the energy band gap of the semiconductors utilized. Due to the high refractive index of these materials, special design elements such as optical coatings and die shape are necessary to effectively release light.

In contrast to a laser, the light produced by an LED lacks spectral coherence and is not highly monochromatic. While its spectrum is narrow enough to be perceived by the human eye as a pure, saturated color, it does not possess the spatial coherence required for the very high intensity typically associated with lasers.

Light Emitting Diode

Symbol of Light Emitting Diode

The symbol for a Light Emitting Diode (LED) is typically represented as a triangle with a line indicating the direction of light emission. The longer line within the triangle represents the anode (positive terminal) and the shorter line, often accompanied by a flat edge, represents the cathode (negative terminal). This symbol is used in electronic circuit diagrams to denote the presence of an LED.

The LED symbol is the familiar diode symbol, augmented with two tiny arrows indicating the emission of light.

Circuit of Light Emitting Diode

The setup comprises an LED, a power source, and a resistor to control both current and voltage.

Light Emitting Diode

Working Principle of Light Emitting Diode

Light Emitting Diode

The operation of a Light Emitting Diode (LED) relies on a process known as electroluminescence. This occurs when a voltage is applied across the diode, causing electrons to combine with electron holes within the semiconductor material. As a result, photons (light particles) are emitted. The emitted light’s color is determined by the energy band gap of the semiconductor.

Unlike traditional incandescent bulbs, which generate light through heat, LEDs work on a purely electronic process. This makes them highly energy-efficient and durable. Additionally, LEDs are capable of emitting light in a specific direction, allowing for focused and directional illumination.

Furthermore, LEDs offer instant illumination, without the warm-up time required by incandescent bulbs. They also have a longer lifespan and can last tens of thousands of hours, making them a sustainable and cost-effective lighting solution. Overall, the working principle of an LED exemplifies its efficiency, versatility, and importance in modern lighting technology.

What determines the colour of an LED?

The color of an LED is dictated by the specific semiconductor material employed. The main materials utilized in LEDs are aluminum gallium indium phosphide alloys and indium gallium nitride alloys. Aluminum alloys produce red, orange, and yellow light, while indium alloys yield green, blue, and white light. Even minor adjustments in the composition of these alloys can lead to variations in the emitted light’s color.

Types of Light Emitting Diode

  • Colored LEDs: These LEDs emit various hues like red, green, and blue. They find common applications in light shows, video displays, and status indicators.
  • Miniature LEDs: These LEDs are typically small and come in a single color. They are widely used in devices like cell phones and calculators.
  • Household LEDs: These LEDs are commonly used in our homes and are available in various shapes and sizes. They are also referred to as LED lamps, such as the Edison bulb.
  • High Power LEDs: Also known as high-output LEDs, they produce a greater lumen output (a unit for measuring emitted light) compared to standard lights. These LEDs are employed in automobile headlights and high-powered lamps.
  • Flash LEDs: Flash LEDs closely resemble regular LEDs but emit light in periodic flashes. They are utilized in vehicles, signboards, and similar applications.

Advantages and Disadvantages of LEDs

The advantage and disadvantage of Light Emitting Diode have been mentioned below:

Advantage Disadvantage
– LEDs can be arranged into compact numeric and alphabetic displays.
– LEDs are both eco-friendly and cost-effective.
– LEDs are robust and can endure shocks and vibrations.
– LEDs can function within a wide range of temperatures.
– LED response times (both turning on and off) are less than a millisecond, making them highly suitable for dynamic operations in various arrays.
– LEDs are offered in an array of colors, including red, yellow, green, and blue.
– They consume moderate power, making them ideal for use in low DC power environments.
– LEDs occupy minimal space.
– LED devices can be powered by Transistor-Transistor Logic (TTL), while gas discharge devices require additional transistor stages for 5V TTL operation.
– LEDs can be susceptible to damage from overvoltage or overcurrent.
– They possess a broader optical bandwidth in comparison to LASERs (approximately 10nm).
– Their operating temperature is contingent on the radiant output power and wavelength.
– LEDs may not be the best choice for large-area displays due to their higher cost. For larger displays, devices utilizing gas-filled technology are preferred.
– LEDs have low reverse voltage ratings, for instance, a typical LED may have a maximum reverse voltage rating of 3V. Consequently, applying a reverse voltage greater than 3V can potentially damage the LED. It is important to exercise caution when using LEDs with a high level of reverse bias.

Applications of Light Emitting Diode

LEDs have extensive applications owing to their compact size, low energy consumption, prolonged lifespan, and adaptability for various purposes. Some widely utilized Light Emitting Diode applications include:

  • Seven Segment Display: This type of display serves as a more straightforward alternative to dot matrix displays, particularly for displaying decimal numerals. They are commonly found in digital watches, meters, scoreboards, etc. The small size of LEDs makes them a perfect fit for this purpose.
  • TV Remote Controls: Infrared LEDs play a crucial role in TV remote controls, working on the principle of sending and receiving signals. The remote control contains an LED that emits rapid flashes of infrared light to transmit messages picked up by the TV. In this scenario, the remote serves as the transmitter, while the TV acts as the receiver.
  • Picturephones: LEDs are employed in the image sensing circuits of devices known as ‘picturephones.’
  • Computers: LEDs are utilized to provide power to LASERs, facilitating the input of information into optical computer memories.

Quantum dots and LEDs

Nanoparticles and quantum dots are essential and are used in LED lights which is the innovation of Nobel Prize in Chemistry for 2023 winners Moungi Bawendi, Louis Brus and Alexey Ekimov for their work on quantum dots, a fundamental discovery in nanotechnology. The discovery and synthesis of quantum dots, illuminate computer monitors and television screens and are used by doctors to map tumors.


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