Exploring the inner workings of modern light sources.
Light Emitting Diodes, commonly known as LEDs, are semiconductor devices that produce light when an electric current passes through them. Unlike traditional incandescent bulbs that generate light by heating a filament, or fluorescent lamps that use gas discharge, LEDs produce light through a process called electroluminescenceThe emission of light from a material when an electric current passes through it.. This fundamental difference makes LEDs significantly more energy-efficient, durable, and versatile.
First demonstrated in the early 20th century, practical LEDs emerged in the 1960s, initially emitting only low-intensity red light. Significant advancements in material science and semiconductor technology over the following decades led to the development of LEDs emitting various colors, including high-brightness blue and green LEDs in the 1990s. The invention of efficient blue LEDs was particularly crucial as it enabled the creation of white LEDs, revolutionizing lighting technology and earning the inventors the Nobel Prize in Physics in 2014. Today, LEDs are ubiquitous, found in everything from indicator lights and displays to general illumination and automotive headlights.
LEDs are semiconductor devices that convert electrical energy directly into light via electroluminescence, offering high efficiency and longevity compared to traditional lighting.
To understand how an LED works, we must first grasp some basic concepts of semiconductor physics, particularly the behavior of materials like silicon, germanium, or the compound semiconductors used in LEDs.
Semiconductors are materials with electrical conductivity between that of a conductor (like copper) and an insulator (like glass). Their conductivity can be significantly altered by adding impurities, a process called dopingAdding impurities to a semiconductor to change its electrical properties..
A P-N junction is formed when a p-type semiconductor is brought into contact with an n-type semiconductor. At the junction, some free electrons from the n-side diffuse into the p-side and recombine with holes, and similarly, holes from the p-side diffuse into the n-side and recombine with electrons. This recombination creates a region near the junction depleted of free charge carriers (electrons and holes), known as the depletion regionA region near the P-N junction depleted of free charge carriers.. The diffusion of charges also creates an internal electric field across the depletion region, forming a potential barrier that opposes further diffusion.
In solid materials, electrons occupy specific energy levels, which group together to form energy bands. The two most important bands for semiconductors are the valence bandThe highest range of electron energies in which electrons are normally present at absolute zero temperature. (where electrons are bound to atoms) and the conduction bandThe lowest range of electron energies that is partially or completely filled with electrons and allows them to move freely, conducting electricity. (where electrons are free to move and conduct electricity). These two bands are separated by an energy gap called the band gapThe energy difference between the top of the valence band and the bottom of the conduction band. (Eg).
In an n-type semiconductor, the Fermi level (a conceptual energy level representing the probability of an electron occupying a state) is closer to the conduction band. In a p-type semiconductor, it's closer to the valence band. When a P-N junction is formed, the Fermi levels align, causing the energy bands to bend near the junction, creating the potential barrier.
Explore the P-N junction under different bias conditions.
Red circles represent holes (absence of electrons), blue circles represent electrons.
No Bias: Depletion region forms, internal field opposes charge movement.
Forward Bias: External voltage reduces the depletion region width and potential barrier, allowing charges to flow and recombine.
Reverse Bias: External voltage increases the depletion region width and potential barrier, preventing significant charge flow.
The magic of an LED lies in the process of electroluminescence within its P-N junction when it is operated under a specific condition called forward bias.
Electroluminescence is the phenomenon where a material emits light in response to the passage of an electric current or to a strong electric field. In semiconductors like those used in LEDs, this happens when electrons and holes recombine.
For an LED to emit light, it must be connected to a voltage source in forward biasConnecting the positive terminal of the voltage source to the P-type side and the negative terminal to the N-type side.. This means the positive terminal of the voltage source is connected to the p-type side (anode) and the negative terminal to the n-type side (cathode).
Applying a forward bias voltage pushes electrons from the n-type material towards the junction and holes from the p-type material towards the junction. If the applied voltage is greater than the built-in potential barrier of the depletion region, it effectively reduces the width of the depletion region and lowers the potential barrier. This allows electrons and holes to cross the junction and enter the opposite type of semiconductor material.
Once electrons from the n-side cross into the p-side and holes from the p-side cross into the n-side, they become minority carriersCharge carriers that are not the majority type in a semiconductor region (e.g., electrons in p-type material). in the respective regions. These minority carriers are now in close proximity to the majority carriers of the opposite type.
In the active region of the LED (near the junction), electrons from the conduction band of the n-type material encounter holes in the valence band of the p-type material. An electron "falls" from the higher energy level in the conduction band to a lower energy level in the valence band to fill a hole. This process is called electron-hole recombination.
In certain semiconductor materials (called direct band gap semiconductors), when an electron recombines with a hole, the excess energy is released in the form of a photon – a particle of light. The energy of the emitted photon, and thus the color (wavelength) of the light, is approximately equal to the band gap energy (Eg) of the semiconductor material.
The relationship between the energy of a photon (E) and its frequency (ν) or wavelength (λ) is given by Planck's equation: \(E = h\nu = hc/\lambda\), where \(h\) is Planck's constant and \(c\) is the speed of light. Therefore, by choosing semiconductor materials with different band gap energies, LEDs can be made to emit light of different colors.
Visualize electron-hole recombination and light emission.
Click the visualization area to simulate electron-hole recombination and photon emission under forward bias.
Blue circle: Electron in Conduction Band. Red circle: Hole in Valence Band.
When an electron falls to fill a hole, energy is released as a photon.
A typical LED is more than just a simple P-N junction. It's a carefully engineered structure designed to efficiently generate and extract light.
While designs vary, a common LED structure includes:
The color of the light emitted by an LED is determined by the band gap energy of the semiconductor material used in its active region. Different materials or alloys of materials are used to achieve different colors:
The development of efficient blue LEDs using InGaN was a major breakthrough because blue light, when combined with red and green, can produce any color, including white.
Explore the different parts of a common LED package.
Simplified diagram showing the main components of a typical through-hole LED package.
As discussed, the primary color of light emitted by an LED chip is determined by the band gap of the semiconductor material. But how do we get a full spectrum of colors, especially white light?
Individual LED chips are manufactured to emit a specific, narrow range of wavelengths (a specific color). By using different semiconductor materials, LEDs can be made to emit red, green, blue, yellow, orange, infrared, or ultraviolet light.
Generating white light with LEDs is slightly more complex than generating single colors. There are two primary methods:
See how a blue LED and phosphor create white light.
A blue LED chip excites a yellow phosphor coating. The combination of transmitted blue light and emitted yellow light creates white light.
LEDs have rapidly replaced many traditional lighting technologies due to their numerous benefits, though they also have some limitations.
The unique properties of LEDs have led to their widespread adoption across countless applications:
Flowchart illustrating the key properties of LEDs and how they contribute to various applications.