light-emitting diode

light-emitting diode
Name	Light-emitting diode
Caption	A typical green LED
Type	Semiconductor, Light source
Working principle	Electroluminescence
Inventor	Nick Holonyak
First production	1962
Pin count	2 (anode and cathode)

light-emitting diode. A light-emitting diode is a semiconductor device that emits light when an electric current passes through it. This process, known as electroluminescence, occurs when electrons recombine with electron holes within the device, releasing energy in the form of photons. The color of the light is determined by the energy band gap of the semiconductor material used. Since their invention, these devices have become ubiquitous in modern technology, from indicator lights to advanced lighting and display systems.

History

The foundational discovery of electroluminescence was made in 1907 by Henry Joseph Round while experimenting with silicon carbide and a cat's-whisker detector. Decades later, Oleg Losev published detailed research on the phenomenon in the Soviet Union. The first practical visible-spectrum device was invented in 1962 by Nick Holonyak while working at General Electric Company, earning him the title "father of the light-emitting diode." Early commercial production was led by companies like Monsanto Company and Hewlett-Packard, initially producing only low-intensity red light. Key advancements followed, including the development of high-brightness versions by Shuji Nakamura at Nichia Corporation and the creation of efficient blue devices, which enabled white light sources and revolutionized lighting technology.

Working principle

The core operation relies on a p–n junction formed by joining p-type and n-type semiconductor materials. When forward voltage is applied, electrons from the n-type semiconductor region are injected across the junction into the p-type semiconductor region, where they recombine with holes. This recombination event releases energy; in direct bandgap semiconductors like gallium arsenide, this energy is emitted as a photon. The wavelength, and thus the color, of the emitted light is directly proportional to the bandgap energy of the semiconductor material. The entire process is highly efficient compared to incandescent or fluorescent mechanisms, as little energy is wasted as heat.

Materials and colors

Different semiconductor compounds are used to produce specific colors across the spectrum. Early red and infrared devices were made from materials like gallium arsenide phosphide and aluminium gallium arsenide. The breakthrough in blue and green technology came with the development of indium gallium nitride and gallium nitride. White light is typically generated either by combining red, green, and blue devices or, more commonly, by using a blue or ultraviolet device to excite a phosphor coating, such as cerium-doped yttrium aluminium garnet. Other materials include aluminium gallium indium phosphide for high-efficiency amber and red, and zinc selenide for some early blue devices.

Types and applications

Devices are categorized by their output and application. Standard brightness types are used as indicator lights in countless consumer electronics. High-power versions form the basis of modern solid-state lighting, including residential lamps and commercial fixtures. Miniature devices are essential in seven-segment displays and the backlighting of liquid-crystal displays. Specialized types include organic versions for television screens from Samsung and LG Corporation, and laser versions used in Blu-ray players and fiber-optic communication. Applications span traffic signals, automotive lighting on Tesla vehicles, billboards on Times Square, and grow lights in agriculture.

Efficiency and operational parameters

These devices are renowned for their high luminous efficacy, often exceeding 100 lumens per watt, far surpassing tungsten filaments. Efficiency is quantified by the external quantum efficiency, which factors in internal quantum efficiency and light extraction. Key operational parameters include forward voltage, which varies with material, and viewing angle, determined by the epitaxial structure and lens design. Performance is highly dependent on junction temperature; excessive heat managed by heat sinks can cause efficiency droop and reduce lifespan. They are driven by constant current sources, often provided by integrated circuits like those from Texas Instruments, to ensure stable operation.

Advantages and disadvantages

Primary advantages include exceptional energy efficiency, long operational life exceeding 50,000 hours, robustness, small size, and fast switching speed, making them ideal for Morse code transmitters and data transmission. They also offer excellent color saturation and are durable, withstanding vibration better than neon or fluorescent tubes. Disadvantages can include higher initial cost compared to traditional sources, sensitivity to voltage spikes requiring protection from electrostatic discharge, and potential for blue-rich white light to contribute to light pollution. Performance can also degrade over time, especially at high temperatures, and the light output of individual devices can vary, necessitating binning by manufacturers like Cree.

Category:American inventions Category:Optoelectronics Category:Light sources