What is LED?
The abbreviation for light-emitting diode is LED. In simple words, a light-emitting diode is a semiconductor device that emits light when current passes through it. When the particles (electrons and holes) within the semiconductor material carry the electric current, it produces light.
Led definition- Among all the different kinds of semiconductor diodes available today, light-emitting diodes (LEDs) is one that is most widely used. It emits either visible light or invisible infrared light when connected in forward biased. The invisible infrared light-emitting LEDs are used in remote controls.
When voltage is applied to an optical semiconductor device, and it emits light, it is known as Light Emitting Diode (LED). It is a device which converts electrical energy to light energy.
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Light - Emitting Diode Symbol
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The Led Symbol and normal p-n junction diode are similar, except that it contains an arrow pointing away from the diode that indicates the diode is emitting light.
LEDs come in various colors, and the most common colors of LEDs are red, yellow, green, and orange.
The color of the light being emitted by LED cannot be represented in its schematic symbol; rather the symbol is common for all colors of LEDs. So, based on the symbol, it is not possible to know the color of LED.
Types of LEDs
The basic types of LEDs are:
Through - Hole LEDs
This type of LED is available in different sizes and shapes, but the most common one is 3 mm, 5 mm, and 8 mm. They are available in different colors like red, blue, yellow, white, green, etc.
SMD LEDs
A special package of surface mount or SMD LEDs 9 Surface Mount Light-Emitting Diodes) can be easily mounted on the surface of PCB. Based on the physical dimensions, they are usually differentiated. E.g., the most common SMD LED is 3528 and 5050.
Bi - color LEDs
The LED that can emit two colors is known as Bi-color LEDs. They have three leads, two anodes, and a common cathode. Based on the configuration of the leads, the color gets activated.
RGB LED
Among the designers and hobbyists, RGB LEDs (Red-Blue-Green LED) are the favorite and most popular. Even among the technicians who build computers, it is popular for implementing RGB LEDs in motherboards, rams, computer cases, etc.
There are 3 LEDs in RGB LEDs on a single chip, and with the help of a technique called PWM (Pulse Width Modulation), the output of RGB LEDs to produce a wide range of colors can be controlled.
High Power LEDs
If the power rating of LED is greater or equal to 1 Watt, then it is called High Power LED as normal LEDs have power dissipation of few milli watts.
High-power LEDs are very bright and often used as flashlights, spotlight, automobile headlamps, etc.
Led Construction and Working
One of the processes used to construct LED is to deposit three semiconductor layers on the substrate. The three layers of semiconductor deposited on substrate are n-type, p-type, and active region. The region between n-type and p-type semiconductor layers is known as the active region.
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The brief working of led is shown here:
An LED can generate light because of the arrangement of two semiconductor materials located between its electrodes.
N-type- It is the semiconductor with extra electrons, also known as extra negatively charged particles.
P-type- It is the semiconductor with extra holes, also known as extra positively charged particles.
When N-type semiconductor is connected to negative electrode and P-type semiconductor to positive electrode, it activates the electron flow across the junction from negative to positive layer. As the negatively charged electrons move through the positive charged particles, they emit light.
The conductive material of LEDs is typically aluminum-gallium-arsenide (AIGaAs), however, other kinds are also available. These materials are specifically selected because they produce photons which will be released into the visible portion of the light spectrum. The type of and amount of material chosen alters the color of light since each material generates photons of different wavelengths, affecting how it appears to the human eye.
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Characteristics Curve of LED
The characteristics curve of LED shows that the forward bias of 1 V is sufficient to increase the current exponentially.
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The output characteristics curve shows that radiant power of LED is directly proportional to the forward current in LED.
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FAQs on Light Emitting Diode
1. What is a Light Emitting Diode (LED) and what is its basic working principle?
A Light Emitting Diode or LED is a heavily doped p-n junction diode that operates under forward bias and emits light. Its working principle is based on the phenomenon of electroluminescence. When the diode is forward-biased, electrons from the n-side and holes from the p-side are pushed towards the junction. At the depletion region, these charge carriers recombine, releasing energy in the form of photons (light). The colour of the emitted light depends on the energy band gap of the semiconductor material used.
2. How is the colour of light emitted by an LED determined?
The colour of the light emitted by an LED is determined by the energy band gap of the semiconductor material it is made from. The energy of the emitted photon is approximately equal to the band gap energy. Different materials have different band gaps, resulting in different colours. For example:
- Gallium Arsenide Phosphide (GaAsP) is used for red, orange, or yellow light.
- Gallium Nitride (GaN) is used to produce blue and green light.
- Gallium Arsenide (GaAs) is used for infrared (IR) LEDs.
3. What are the main advantages of using LEDs over traditional incandescent bulbs?
LEDs offer several significant advantages over traditional incandescent light sources. Some key examples include:
- Energy Efficiency: LEDs convert a much higher percentage of electrical energy directly into light, producing very little heat, which makes them far more efficient.
- Long Lifespan: They have a much longer operational life, often lasting tens of thousands of hours.
- Durability: Being solid-state devices, LEDs are highly resistant to shock, vibrations, and external damage.
- Fast Switching: LEDs can be turned on and off almost instantaneously, with no warm-up time required.
4. What is the circuit symbol for a Light Emitting Diode and what do the arrows signify?
The circuit symbol for an LED is similar to that of a standard p-n junction diode, which is a triangle pointing towards a vertical line. However, the LED symbol includes two small arrows pointing away from the diode. These arrows signify the core function of the device: the emission of light or photons when it is in a forward-biased state.
5. What is the fundamental difference between a regular p-n junction diode and a Light Emitting Diode?
The fundamental difference lies in their material and purpose. A regular diode, typically made of silicon or germanium, is designed for rectification or switching. During electron-hole recombination, it releases energy primarily as heat. In contrast, an LED is made from compound semiconductors (like GaAsP) with a direct band gap. This structure ensures that when electrons and holes recombine, the energy is efficiently released as light (photons) rather than heat.
6. Why are LEDs typically made from compound semiconductors instead of elemental ones like Silicon?
This is because of the nature of their energy band gaps. Elemental semiconductors like Silicon and Germanium have an indirect band gap. In these materials, electron-hole recombination requires a change in momentum and releases energy mostly as heat, making them inefficient for light emission. Compound semiconductors like Gallium Arsenide (GaAs) have a direct band gap, where recombination occurs directly and releases energy efficiently as photons, which is the principle behind an LED's light production.
7. What would happen if an LED was connected in reverse bias in a circuit?
If an LED is connected in reverse bias, it will act like a standard diode and will not emit light. In reverse bias, the depletion region at the p-n junction widens, which prevents significant current from flowing through the device. Consequently, there is no electron-hole recombination at the junction, and no photons are produced. Applying a very high reverse voltage beyond its breakdown limit can permanently damage the LED.
8. How do an LED and a photodiode differ in their function and biasing, even though both are optoelectronic devices?
While both are optoelectronic devices, they perform opposite functions and operate under different biasing conditions. The key differences are:
- Function: An LED converts electrical energy into light energy. A photodiode converts light energy into electrical energy.
- Biasing: An LED is always operated under forward bias to facilitate electron-hole recombination and light emission. A photodiode is typically operated under reverse bias to create a wider depletion region for detecting photons and generating a current.
9. What are some key real-world applications of Light Emitting Diodes?
LEDs are used in a vast array of applications due to their efficiency and durability. Common examples include:
- Indicator Lights: Used in almost every electronic device to show power status.
- Digital Displays: Found in digital clocks, calculators, and large-scale public information screens.
- Lighting: General illumination for homes and businesses, automotive headlights, and traffic signals.
- Communications: Used in remote controls and optical fibre communication systems to transmit signals.
