Introduction to Light-emitting Diode

Table of Contents

I. Introduction

The light-emitting diode (LED) is a ubiquitous semiconductor device that emits light when an electric current passes through it. This process, known as electroluminescence, occurs through the recombination of electrons and holes within the diode's structure. LEDs are renowned for their ability to efficiently convert electrical energy into light energy, making them a cornerstone of modern technology. Their applications are vast and continue to expand, ranging from general lighting and flat-panel displays to automotive lighting and advanced medical devices.

When electrons and holes recombine, they can radiate light across the visible spectrum. This property allows for the creation of LEDs that produce different colors. For instance, Gallium Arsenide (GaAs) diodes typically emit red light, Gallium Phosphide (GaP) diodes emit green light, Silicon Carbide (SiC) diodes emit yellow light, and Gallium Nitride (GaN) diodes emit blue light. Chemically, LEDs are broadly categorized into two main types: traditional inorganic light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), which are based on organic compounds.

First developed in 1962, early LEDs could only produce a low-intensity red light. Over the decades, continuous technological advancements have led to the development of a wide array of monochromatic and white-light LEDs. Today, commercially available LEDs span the entire visible spectrum, as well as infrared and ultraviolet wavelengths. Initially used as simple indicator lights and in display panels, LEDs are now a dominant technology in high-resolution displays and general-purpose lighting, having largely replaced incandescent and fluorescent sources in many applications.

II. Parameters of Light-Emitting Diodes

1. Limit Parameters

• Allowable Power Consumption (Pm): This is the maximum power that an LED can handle, calculated as the product of the forward DC voltage and the current. Exceeding this value will cause the LED to overheat and fail.

• Maximum Forward DC Current (IFm): The maximum continuous forward DC current that can be passed through the LED. Exceeding this can permanently damage the diode.

• Maximum Reverse Voltage (VRm): The maximum reverse voltage that can be applied across the LED without causing breakdown and damage.

• Working Environment (Topm): The ambient temperature range within which the LED can operate safely and efficiently. Operating outside this range can lead to reduced performance and lifespan.

2. Electrical and Optical Parameters

Spectral Distribution and Peak Wavelength: An LED does not emit light at a single wavelength but rather over a range. The peak wavelength (λp) is the wavelength at which the LED emits the most intense light.

Figure 1: Spectral distribution and peak wavelength of an LED.

• Luminous Intensity (Iv): This measures the brightness of an LED in a specific direction, typically along the central axis. It is measured in candelas (cd) or, more commonly for LEDs, millicandelas (mcd).

• Spectral Half-Width (Δλ): This represents the spectral purity of the light. It is the width of the spectrum at half of the peak luminous intensity. A smaller value indicates a more monochromatic light.

• Viewing Angle (2θ1/2): This is the total angle at which the luminous intensity is at least half of the axial intensity. A wider viewing angle means the light is more spread out.

Figure 2: Angle distribution of luminous intensity for different LED types.

• Forward Current (If): The recommended continuous forward current for normal operation. For optimal lifespan, it is often recommended to operate below the maximum rated current.

• Forward Voltage (Vf): The voltage drop across the LED when it is operating at a specified forward current (e.g., 20mA). This value typically ranges from 1.4V to 3V and decreases as the temperature increases.

• V-I Characteristics: The relationship between the voltage applied across the LED and the current that flows through it. The LED begins to conduct and emit light significantly only after the voltage reaches a certain threshold.

Figure 3: V-I characteristics of a typical LED.

III. Working Principle of Light-Emitting Diodes

The core of a light-emitting diode is a semiconductor wafer composed of a P-type semiconductor and an N-type semiconductor. The region where these two materials meet is called a PN junction. In certain semiconductor materials, when minority carriers (e.g., electrons in the P-region) and majority carriers (e.g., holes in the P-region) recombine at this junction, they release excess energy in the form of photons (light). This process directly converts electrical energy into light energy. When a reverse voltage is applied, it becomes difficult to inject minority carriers across the junction, and thus no light is emitted.

The light emission process can be understood by considering three conditions:

1. No Voltage Applied

Without any external voltage, electrons from the N-type material diffuse across the PN junction to fill holes in the P-type material. This creates a thin insulating layer called the depletion region, where there are no free charge carriers (electrons or holes). As a result, no current can flow.

Figure 4: Depletion region in an LED with no applied voltage.

2. Forward Voltage Applied

When a forward voltage is applied (positive terminal to the P-type material, negative to the N-type), it opposes the depletion region's internal field. This allows electrons from the N-region and holes from the P-region to be injected across the junction. As they recombine, they release energy as photons, producing light. The energy released, and thus the wavelength (color) of the light, depends on the semiconductor material's bandgap. This is why different materials are used to create LEDs of different colors.

Figure 5: Recombination of electrons and holes under forward voltage, emitting light.

3. Reverse Voltage Applied

When a reverse voltage is applied (positive terminal to the N-type material, negative to the P-type), it reinforces the depletion region. Electrons in the N-type material are attracted toward the positive terminal, and holes in the P-type material are attracted toward the negative terminal, pulling them away from the junction. This widens the depletion region and prevents current from flowing, so no light is emitted.

Figure 6: Widened depletion region under reverse voltage, blocking current flow.

IV. Types of Light-Emitting Diodes

LEDs can be classified based on their structure, material, and application. Here are some of the most common types:

1. Based on Structure and Packaging

• Through-Hole LEDs: These are the classic, small, bulb-shaped LEDs with two long metal leads for mounting on printed circuit boards (PCBs). They are commonly used as indicator lights.

• Surface-Mount Device (SMD) LEDs: These are compact, low-profile LEDs designed to be mounted directly onto the surface of a PCB. Their small size and high brightness make them ideal for general lighting, displays, and backlighting.

• Chip-on-Board (COB) LEDs: COB technology involves mounting multiple LED chips directly onto a substrate to form a single module. This results in a dense array of LEDs that provides a uniform, high-intensity light source, often used in high-power applications like downlights and high-bay lights.

2. Based on Emitted Light

• Visible-Light LEDs: These emit light in the visible spectrum and are available in various colors, including red, green, blue, yellow, and amber. White light is typically produced by combining a blue LED with a yellow phosphor coating or by mixing red, green, and blue (RGB) LEDs.

• Infrared (IR) LEDs: These emit light in the infrared spectrum, which is invisible to the human eye. They are widely used in remote controls, night-vision systems, and optical communication.

• Ultraviolet (UV) LEDs: These emit light in the ultraviolet spectrum and are used for applications such as curing, sterilization, and forensic analysis.

3. Based on Chemical Composition

• Inorganic LEDs: The most common type, made from inorganic semiconductor materials like Gallium Nitride (GaN) and Gallium Arsenide (GaAs).

• Organic Light-Emitting Diodes (OLEDs): These use organic compounds to form the emissive layer. OLEDs can be made into thin, flexible sheets and are used to create vibrant, high-contrast displays for TVs, smartphones, and lighting panels.

Figure 7: Various types of LEDs, including through-hole and surface-mount devices.

V. Materials of a Light-Emitting Diode

The color of light emitted by an LED is determined by the semiconductor material used in its construction. Different materials have different bandgaps, which dictates the energy and wavelength of the emitted photons. The table below lists some common semiconductor materials and the corresponding colors they produce.

Figure 8: Internal structure of a typical LED, showing the semiconductor die and other components.

VI. Applications of Light-Emitting Diodes

The unique properties of LEDs—high efficiency, long lifespan, durability, and compact size—have enabled their use in a vast and growing range of applications. From simple indicators to complex lighting systems, LEDs have become an integral part of modern life.

1. Display Screens and Communication Signal Displays

LEDs are fundamental to modern display technology. Large-format video displays, billboards, and public information signs use arrays of RGB LEDs to create bright, dynamic images that are visible even in direct sunlight. In traffic signals and pedestrian crossings, LEDs have replaced traditional incandescent bulbs, offering greater reliability, longer life, and significant energy savings.

Figure 9: LED traffic lights are now a global standard for energy efficiency and reliability.

2. Automotive Industry

The automotive sector has widely adopted LEDs for both interior and exterior lighting. They are used in headlights, taillights, brake lights, and turn signals, where their fast response time enhances safety. Inside the vehicle, LEDs provide illumination for dashboards, infotainment displays, and ambient lighting, offering design flexibility and a premium feel.

Figure 10: Modern vehicles extensively use LEDs for both functional and aesthetic lighting.

3. LCD and OLED Display Backlighting

LEDs are the primary backlighting technology for liquid crystal displays (LCDs) used in televisions, computer monitors, and mobile devices. More recently, advanced technologies like Mini-LEDs have emerged, using thousands of tiny LEDs to create more precise local dimming zones, resulting in significantly improved contrast and image quality. This has positioned Mini-LED TVs as strong competitors to OLED displays, which do not require a backlight as each pixel emits its own light.

4. LED Lighting

General lighting is one of the largest markets for LEDs. LED bulbs and fixtures are now the standard for residential, commercial, and industrial lighting, offering dramatic energy savings and much longer lifespans compared to legacy technologies. The flexibility of LEDs has also enabled innovative lighting designs and the development of smart lighting systems that can be controlled remotely and customized for color and intensity.

Figure 11: LED bulbs are available in a wide variety of shapes and sizes to replace traditional incandescent bulbs.

5. Other Applications

LEDs are found in countless other products, including decorative lighting like Christmas lights, children's toys, and power indicators on electronic devices. Their low voltage requirements and durability make them safe and versatile for a wide range of consumer and industrial applications.

Figure 12: LED Christmas lights are popular for their vibrant colors, safety, and energy efficiency.

VII. The Trends of LED Lighting Technology (2025 Perspective)

The field of LED technology continues to evolve at a rapid pace. While many of the trends predicted in 2020 have materialized, new advancements are shaping the future of lighting and displays. Here is an updated look at the key trends as of 2025.

1. Ever-Increasing Efficiency and Lifespan

As of 2025, the luminous efficacy of commercial LED products has reached over 160 lm/W, with some high-end products exceeding 250 lm/W. In laboratory settings, researchers continue to push the boundaries, with theoretical efficiencies approaching 330 lm/W. Lifespans have also improved, with many commercial LEDs now rated for 50,000 to 100,000 hours of operation. This continuous improvement solidifies the LED's position as the most energy-efficient and long-lasting lighting technology available.

2. Closing the "Green Gap"

The "green gap"—the historically lower efficiency of green LEDs compared to their red and blue counterparts—has been a major focus of research. While the gap is not yet fully closed, significant progress has been made through new materials and device structures, such as V-defect engineering and novel carrier injection methods. These advancements are critical for improving the efficiency of color-mixed (RGB) white light systems and expanding the color gamut of LED displays.

3. Smart Lighting and IoT Integration

The integration of LEDs with information technology has given rise to a booming smart lighting market, projected to exceed $40 billion by 2030. As semiconductor devices, LEDs are easily controlled, making them perfect for IoT applications. Modern smart lighting systems offer features like remote control via mobile apps, automated scheduling, and dynamic adjustment of color temperature and brightness. Approximately 78% of residential LED strips now support RGB color-changing modes, and integration with smart home ecosystems has become standard.

4. The Rise of Mini-LED and the Evolution of Displays

While OLED technology continues to dominate the high-end display market, particularly in smartphones, Mini-LED backlighting has emerged as a powerful competitor for larger screens like TVs and monitors. By using thousands of tiny LEDs for backlighting, Mini-LED displays achieve superior contrast and brightness compared to traditional LCDs. Major manufacturers including Sony, Samsung, Hisense, and TCL have introduced RGB Mini-LED backlight systems in 2025-2026, creating a new tier of high-performance displays that bridge the gap between conventional LEDs and OLEDs.

5. Human-Centric Lighting

There is a growing emphasis on designing lighting systems that benefit human health and well-being. Tunable white LEDs, which can adjust their color temperature from warm (2700K) to cool white (6500K), are a key component of this trend. These systems can be programmed to mimic the natural changes in daylight, helping to regulate our circadian rhythms, improve mood, and boost productivity. This human-centric approach is becoming a standard feature in modern architectural and commercial lighting.

6. Advanced Color Rendering and Spectral Control

The traditional Color Rendering Index (CRI) is being supplemented by more advanced metrics as the spectral flexibility of LEDs increases. The ability to precisely control the spectral output of LEDs allows for the creation of light sources optimized for specific tasks, such as enhancing the color vibrancy of retail products, creating ideal growing conditions for plants in agricultural lighting, or providing optimal illumination for medical procedures.

7. Sustainable and Eco-Friendly Lighting

As global awareness of environmental issues grows, the lighting industry is placing greater emphasis on sustainability. LEDs already consume significantly less energy than traditional light sources, but manufacturers are now focusing on using recyclable materials, reducing hazardous substances, and designing products for easier end-of-life recycling. Natural lighting solutions that integrate daylight harvesting with LED systems are also gaining attention.

VIII. 2025 Update Summary

This article was originally published in 2020 and has been comprehensively updated in October 2025 to reflect the latest advancements in LED technology. Key updates include:

• Updated Performance Metrics: Luminous efficacy and lifespan figures have been revised to reflect current 2025 standards, with commercial LEDs now commonly exceeding 160 lm/W and offering lifespans of 50,000-100,000 hours.

• Smart Lighting and IoT: The section on trends has been significantly expanded to cover the rapid growth of smart lighting, its integration with the Internet of Things (IoT), and the rise of human-centric lighting systems that optimize for circadian rhythms and well-being.

• Display Technology Advancements: The article now includes comprehensive information on the emergence of Mini-LED backlighting as a major technology in the display market, creating a new competitive landscape alongside OLEDs for high-end TVs and monitors.

• Green LED Progress: Information on the "green gap" has been updated to note the significant research progress made in improving the efficiency of green LEDs through advanced materials and device structures.

• Content Enhancement: The entire article has been thoroughly reviewed to correct any errors, improve clarity, enhance readability, and ensure all technical information is current and accurate for 2025. All original images have been retained for continuity and reference.

The continued innovation in LED technology ensures that it will remain a critical component in energy-efficient lighting, advanced displays, and smart connected systems for the foreseeable future.