Light Emitting Diode (LED): Definition, Symbol, Working Principle & Applications
Quick Exam Notes
- Definition: An LED (Light Emitting Diode) is a semiconductor device that emits light when forward biased.
- Working Principle: electroluminescence: electron–hole recombination releases energy as photons.
- Symbol: Diode symbol with arrows indicating light emission.
- Construction: Made of semiconductor materials like GaAs, GaP, GaAsP, with a dome-shaped transparent cover.
- Efficiency: Depends on bandgap energy, material quality, and operating current.
- Applications: Used in displays, indicators, optical communication, lighting, and lasers.
Principle
Light-emitting diodes (LEDs) work on the principle of electroluminescence, which is the emission of light when a current passes through a diode and minority and majority charge carriers recombine at the junction.
Figure 1. Working principle of Light Emitting Diode
Theory
When an LED is forward biased, current flows through it and the majority charge carriers move from p to n and from n to p region. The excess minority carriers at the junction recombine with the majority carriers. The energy released in the form photon when they combine at the junction and which produces light or infrared radiation. The voltage at which the LED begins to emit light is called the threshold voltage. As the forward voltage rises, the light intensity rises, eventually reaching a maximum.
The wavelength of the light produced depends on the energy band gap of the semiconductors used. Band gap of semiconductors plays an important role in LED. It decides the color of the emitted light. Mathematically, the light wavelength and energy gap are related by
\[ E_g = \frac{hc}{\lambda} \] \[ \lambda = \frac{hc}{E_g} \]Table 1. Band gap and corresponding wavelength of some direct band gap material
| Material | Band gap (eV) | Wavelength (nm) |
|---|---|---|
| GaAs | 1.44 | 860 |
| GaAsP | 1.98 | 626 |
| ZnSe | 2.6 | 476 |
Advantages of LED
- Converting a greater percentage of electrical energy into light
- Long Lifespan
- Durability
- Compact Size
- Operates at very low voltage.
- Operated at a wide range of temperatures (0 - 70°C).
Disadvantages of LED
- Power output is low.
- The light cannot travel over larger distances.
- The efficiency is low at higher temperatures
- The output is not coherent light
Applications of LED
- Burglar alarms systems
- Traffic signals
- Camera flashes
- Sensors
- Toys
For a hands-on approach to understanding the LED experiment, visit our experiment page.
Summary
A Light Emitting Diode (LED) is a semiconductor device that emits light when forward biased. It works on the principle of electroluminescence, where electron–hole recombination releases photons. LEDs are constructed using materials like gallium arsenide or gallium phosphide, which determine the emitted light’s color. The symbol of an LED is similar to a diode with arrows showing light emission. LED efficiency depends on bandgap energy and material quality. Widely used in indicators, displays, lighting, optical communication, and lasers, LEDs are more efficient and durable compared to traditional light sources.
MCQs on LED (Light Emitting Diode)
-
The full form of LED is:
- a) Light Electrical Device
- b) Light Emitting Diode
- c) Low Energy Diode
- d) Light Energy Detector
Answer
b) Light Emitting Diode
-
The working principle of LED is based on:
- a) Photoelectric effect
- b) Electroluminescence
- c) Thermionic emission
- d) Electromagnetic induction
Answer
b) Electroluminescence
-
Which of the following materials is commonly used to make LEDs?
- a) Gallium Arsenide (GaAs)
- b) Silicon
- c) Germanium
- d) Aluminium
Answer
a) Gallium Arsenide (GaAs)
-
The LED symbol is represented as:
- a) A battery with arrows
- b) A diode with arrows pointing outward
- c) A resistor with arrows
- d) A transistor with arrows
Answer
b) A diode with arrows pointing outward
-
Which of the following is NOT an application of LEDs?
- a) Display systems
- b) Optical communication
- c) Voltage amplification
- d) Lighting
Answer
c) Voltage amplification