Science Behind Light Emitting Diodes or LED

Summary

Of all the tiny marvels of modern technology, few are as widespread and underappreciated as the light-emitting diode.

You can glance at the clock on your microwave, check the time on your smartwatch, or look at the indicator light on your laptop charger, and you're interacting with one.

Light is a fundamental part of our world, and for over a century, we created it by heating a thin wire until it glowed white-hot—a brilliant but incredibly wasteful process.

The light bulb was a revolution, but like any technology, it was destined to be improved upon.

What is a Light Emitting Diode (LED)?

Before we can understand how an LED works, we need to break down its name. At its heart, it is a type of diode. In the world of electronics, a diode acts like a one-way street or a valve for electricity.

It allows electric current to flow through it easily in one direction but blocks it almost completely from flowing in the opposite direction. This property is fundamental to controlling electricity in countless circuits.

The "light-emitting" part is what makes it special. Unlike a standard diode, which just manages current flow, an LED has the remarkable ability to produce light when that current passes through it correctly.

The key to this ability lies in its construction. An LED is not made of metal wires or glass bulbs but is a semiconductor device.

A semiconductor is a unique material, like silicon or gallium arsenide, that has an electrical conductivity somewhere between a conductor (like copper, which lets electricity flow freely) and an insulator (like rubber, which blocks it).

By carefully engineering these materials, scientists can create amazing properties, including the ability to generate light.

How Does an LED Work?

To understand the magic inside an LED, imagine you have two different types of the same semiconductor material.

One type has been treated, or "doped," so it has an excess of free electrons (the tiny particles that carry a negative charge and make up electric current).

This is called the n-type (for negative) material. The other type is doped to have a deficit of electrons, creating "holes" where electrons should be.

These holes act like positive charges, so this is called the p-type (for positive) material.

An LED is created by sandwiching a piece of p-type and n-type material together. The area where these two materials meets is called the p-n junction, and this is where all the important action happens.

When the diode is off, a natural barrier forms at this junction, preventing the electrons on the n-side from crossing over to fill the holes on the p-side.

To get the LED to light up, we need to apply an electrical voltage in a specific way, a condition known as Forward Bias.

This means connecting the positive terminal of a power source (like a battery) to the p-type side and the negative terminal to the n-type side.

This applied voltage acts like a push, giving the electrons on the n-side enough energy to overcome the barrier and jump across the junction to meet the holes on the p-side.

When a free electron from the n-side meets and fills a hole on the p-side, the two recombine. It is this moment of recombination that is the source of the light. 

The Science of Electroluminescence 

This process of generating light directly from electricity is called electroluminescence. It’s a fascinating phenomenon that sets LEDs apart from traditional light sources.

In an old-fashioned incandescent bulb, light is a byproduct of immense heat. You run electricity through a thin wire (a filament), and it gets so hot that it glows. Over 90% of the energy consumed by an incandescent bulb is wasted as heat, not light. 

In an LED, the process is profoundly different and far more elegant. When an electron from the n-side drops into a hole on the p-side, it has to release its excess energy.

Think of it like a ball rolling off a high step and falling to a lower one; the potential energy it had at the top has to go somewhere when it lands. In a standard silicon diode, this energy is released as heat, in the form of tiny vibrations in the atomic structure. 

However, in the special LED Materials used to make LEDs, this energy is released in a different form: as a tiny packet of light energy called a photon.

A photon is the fundamental particle of light. So, every time an electron-hole pair recombines inside an LED, a single photon of light is created and emitted.

A constant stream of electric current creates billions of these recombination events every second, which our eyes perceive as a steady, continuous glow.

This direct conversion of electrical energy into light energy, with very little heat, is what makes LEDs so incredibly efficient. 

Materials Used in LEDs and Their Colors 

A common question is: how do LEDs produce so many different colors? The color of the light is not created by a filter or a dye. Instead, it is determined at a fundamental level by the specific LED Materials used to build the p-n junction. 

The key is a property called the "band gap energy" of the semiconductor. The band gap is essentially the amount of energy that an electron releases when it falls from the n-side into a hole on the p-side.

Different materials have different band gaps. The energy of the released photon is directly proportional to this band gap energy.

Since the color of light is determined by its energy (and wavelength), controlling the band gap allows for precise control over the color of the emitted light. 

For example: 

• Gallium arsenide (GaAs) has a relatively small band gap and produces infrared and red light. 
• Aluminum gallium indium phosphide (AlGaInP) alloys can be tweaked to produce red, orange, and yellow light. 
• Indium gallium nitride (InGaN) alloys have a larger band gap and are used to create green, blue, and white light. 

The invention of the efficient blue LED in the 1990s was a monumental breakthrough because, with red, green, and blue LEDs, it became possible to mix their light to create any color, including pure white.

Many white LEDs today work by using a blue LED chip and coating it with a yellow phosphor.

The blue light from the chip excites the phosphor, causing it to glow yellow, and the combination of blue and yellow light appears as white to our eyes. 

LED Symbol and LED Circuit

To work with any electronic component, it's essential to understand its representation in circuit diagrams and the basic principles of how to power it safely.

For the LED, this involves recognizing its unique schematic symbol and knowing the fundamental circuit required for its operation.

These simple diagrams are the language of electronics, providing a clear and universal guide for engineers and hobbyists alike. 

The symbol for an LED is built upon the standard symbol for a diode, which consists of a triangle pointing towards a vertical line.

To signify its light-producing function, two small arrows are added pointing away from the diode, visually representing the emission of photons or light.

A basic LED circuit consists of three key parts: the LED itself, a power source such as a battery, and a current-limiting resistor.

The resistor is a crucial component that protects the LED by reducing the electrical current from the power source to a safe level, preventing the LED from burning out. 

Properties of LED Light 

The light generated by an LED has distinct characteristics that differentiate it from other light sources like incandescent bulbs or lasers. These properties are a direct result of the quantum-mechanical process of electroluminescence within the semiconductor. 

Monochromatic Nature

The light from an LED is considered quasi-monochromatic, meaning it is composed of a very narrow band of wavelengths. This results in the emission of a single, pure color, unlike the broad spectrum of white light that contains all the colors of the rainbow. 

Directionality

While not as perfectly focused as a laser beam, light from an LED is inherently more directional than light from a bulb. It emits from a tiny point on the semiconductor chip and is typically shaped by an integrated lens, allowing its output to be precisely aimed. 

Incoherence

The light produced by an LED is incoherent, which means the individual light waves are generated at random and are not in phase with one another. This is a key difference from laser light, where all the light waves are perfectly synchronized in space and time. 

Applications and Uses of LEDs 

The unique properties of LEDs i.e. their efficiency, long lifespan, durability, and small size have allowed them to be used in a wide range of technologies. 

• LEDs are now the leading technology for general illumination, providing energy-efficient lighting for our homes, offices, and city streetlights. 
• The technology is fundamental to modern digital displays, powering everything from the numbers on a digital watch to massive, high-resolution video billboards. 
• In televisions and smartphones, LEDs provide the backlighting that illuminates the screen, which allows for thinner designs and improved picture contrast. 
• The automotive industry relies on LEDs for headlights, brake lights, and dashboard indicators, as they light up instantly for increased safety and are extremely durable. 
• Infrared LEDs are essential components in remote controls for televisions and other appliances, transmitting commands via invisible light signals. 
• They play a critical role in optical communication systems, sending high-speed data through fiber optic cables that form the backbone of the internet. 
• In the medical field, LEDs are utilized in advanced devices such as surgical lamps, tools for phototherapy treatments, and the sensors in pulse oximeters that measure blood oxygen levels. 

Advantages of LEDs over Traditional Light Sources 

When compared to older lighting technologies, the superiority of the LED becomes clear. 

• Energy Efficiency: LEDs convert up to 80% of their electrical energy into light, whereas incandescent bulbs waste over 90% as heat. This translates directly to lower electricity bills and a smaller carbon footprint. 
• Long Lifespan: A typical LED can last 25,000 to 50,000 hours or more. An incandescent bulb may only last 1,000 hours, and a compact fluorescent (CFL) around 8,000 hours. 
• Durability: Because LEDs are solid-state devices made from a block of semiconductor material encapsulated in epoxy or plastic, they are extremely rugged and resistant to shock, vibrations, and impact, unlike fragile glass bulbs with delicate filaments. 
• Size and Design Freedom: LEDs can be incredibly small, allowing them to be used in ways that were never possible with bulky bulbs. This has given engineers and designers newfound freedom in creating innovative lighting fixtures and displays. 
• Instant On: LEDs reach their full brightness instantly, with no warm-up time required, unlike many fluorescent lamps. 

Types of LEDs 

LEDs are available in different types, each designed and optimized for different applications. Some of them are mentioned below: 

• Miniature and Through-Hole LEDs: These through-hole LEDs are the classic, small indicator lights with long wire leads, commonly used in electronic circuit boards and as status indicators on various devices. 
• Surface Mount Devices (SMD LEDs): These are compact, low-profile LEDs designed to be mounted directly onto the surface of circuit boards, making them ideal for modern, space-constrained electronics like smartphones, laptops, and backlighting systems. 
• High-Power LEDs: Engineered to handle significantly more current and dissipate more heat, these LEDs produce a much brighter light output, making them suitable for applications like flashlights, automotive headlights, and powerful residential and commercial lighting. 
• RGB (Red, Green, Blue) LEDs: These packages contain three separate LED chips—one red, one green, and one blue. By controlling the intensity of each chip, they can be blended to produce millions of different colors, forming the basis for color-changing light strips and advanced digital displays. 
• COB (Chip on Board) LEDs: This technology involves mounting multiple bare LED chips directly onto a substrate to form a single, unified module. This results in a very dense and intense light source that appears as a continuous panel of light, often used in spotlights and downlights. 
• Organic LEDs (OLEDs): This is a fundamentally different technology that uses organic carbon-based compounds that emit light when an electric current is applied. OLEDs can be made into ultra-thin, flexible sheets and are used in high-end television and smartphone screens, where each pixel generates its own light. 

 

 

Conclusion 

From a simple one-way street for electricity to the source of light in our homes and on our screens, the journey of the Light Emitting Diode is a testament to human ingenuity.

It's a technology born from the abstract principles of quantum mechanics and material science, yet its impact is tangible, practical, and all around us.

The next time you flick on a light switch and your room is instantly filled with clear, bright light, take a moment to appreciate the silent, elegant dance of electrons and holes happening within that tiny device.

The science behind the LED is not just about illuminating our world; it's about doing so more efficiently, more durably, and more brilliantly than ever before.