Their underlying technology enables a vast range of applications and benefits
The LED is a special type of diode that emits light when current flows through it in one direction but does not light up when current flows through it the opposite way.
BY JEREMY LOSAW
From the tips of our cell phones, the dashboards of our cars and all the way to the Las Vegas strip, LEDs illuminate our world and are found in a huge proportion of electrified products.
The humble light-emitting diode is a simple component of an electrical circuit that creates light when current is placed across its terminals. Despite LEDs’ simplicity, they have some unique technology inside their small lenses, and their development began more than a century ago.
The use of LED technology has skyrocketed in the past two to three decades, so it is vital for anyone building electrified prototypes to understand the technology and capability.
In this two-part series, I will start our exploration of LEDs by revealing their history and underlying technology, and discuss their merits and applications. In Part 2, I will discuss them in more practical terms and reveal how to use them in your next prototype.
What is an LED?
In a light-emitting diode, a diode is like an electronic check valve. It allows electric current to flow in one direction but not the other. This is an important task in an electronic circuit because it can prevent current from flooding into sensitive areas of a circuit.
There are myriad types and applications for diodes beyond the scope of this discussion. However, the LED is a special type of diode that emits light when current flows through it in one direction but does not light up when current flows through it the opposite way. Unlike other types of diodes, they are used not for circuit protection or signal rectification but rather primarily as indicators for what is happening in a circuit or simply for the light they create.
How they work
To understand how an LED works, you must understand what a semiconductor is.
A conductor is a material that conducts electricity very easily (such as metal), and an insulator is a material that resists the flow of electricity (such as plastic). So, a semiconductor is a material that conducts electricity relatively poorly unless provided with unique conditions.
Of course, this is all relative; any material can conduct electricity if enough power is available. The obvious example is that air is non-conductive until enough energy builds up in the atmosphere to induce a lightning strike that carries electricity from the upper atmosphere to the ground.
Semiconductors are made by taking a typically insulating material and adding or doping it with trace amounts of a conductive material. For example, red LEDs use aluminum gallium arsenide (AlGaAs). This allows semiconductors to flow electricity under special conditions.
LEDs use two different semiconductor materials that are placed adjacent to each other. One of the materials has extra positive charge; the other has extra negative charge.
When voltage is placed across the terminals of the LED, the excitation is enough to cause the negatively charged electrons to combine in the gap between the materials with positively charged “holes.” When the electrons and holes combine, energy is released in the form of light.
If current is applied to the LED in the wrong direction, the semiconductor is not excited properly, no current flows through the LED, and no light is formed. Different colors of LEDs can be created by changing the material of the semiconductor.
The foundational technology of the LED dates to 1907. British scientist Henry Joseph Round created the first examples of electroluminescence, creating faint yellow light by applying voltage across a piece of silicon carbide. However, it was not until the 1960s that LEDs became viable production components, and it took decades of research and development to create LEDs that would emit light in the visible spectrum.
The first commercially available LEDs were red; blue and yellow ones were developed in the 1970s. These first-generation LEDs were not very bright, extremely expensive, and only viable for use in high-end lab equipment.
Since then, LED technology has accelerated rapidly. We now have access to high-brightness, low-cost LEDs in an array of colors that include UV and infrared. We have RGB LEDs, which have all three primary colors of light (red, green and blue), as well as addressable LEDs with tiny integrated circuits inside to give us infinite color and brightness control. We are truly living in the golden age of LED technology.
LEDs have many advantages over other types of lighting devices, such as incandescent bulbs. The primary benefit is that they are the lowest-powered way to generate light.
A single-component LED draws about 20-30 milliamps of power, and LED light bulbs use about 10 times less power to generate the same amount of light as their incandescent equivalents. This makes them economical for home use and much better for battery-powered devices with a fixed power capacity.
Because LEDs use less power, they generate less heat. This makes them much more friendly and less dangerous in the home environment than incandescents.
Lower heat is also good for the companion circuitry, because electronics usually run more efficiently when not under duress from excess heat. LEDs rarely require additional cooling elements such as fans and their associated power consumption, unless many high-powered LEDs are put into a large grid or fixture.
LEDs are also long-lived components. A standard LED can have a useful lifespan of about 50,000 hours, which is more than 5.5 years of ‘round-the-clock use. For many applications, this means they are essentially immortal. It is very rare to replace a burned-out LED on a circuit board unless it is mistreated.
The final advantage of LEDs is their versatility.
Component LEDs come in many different sizes. The most common is the 5mm domed LED, but there is a whole spectrum of sizes. The smallest is the 0201 series—just .002” x .001” in plain view—but they can go up to 20mm in readily available packages.
This means they can be used in the tiniest of electronics, all the way up to giant TV screens. The largest LED screen is at Charlotte Motor Speedway in Concord, N.C. The Panasonic-built screen uses more than nine million discreet LEDs to display live video to fans during races.
LEDs come in many sizes and packages, making them convenient to package into many different applications.
LED lighting has endless applications. In consumer products, home lighting is a huge category.
However, LEDs are often a component of other common devices. TV remotes use IR LEDs to send control signals, and they are commonly used for status indication and for backlighting buttons on consumer electronic devices.
Outside of consumer products, LEDs are huge in automotive—from headlights to interior dashboard lighting—and can be found in civil devices such as street and traffic lights. They are in almost anything that has a circuit inside it.