An electron hole exists where an atom lacks electrons negatively charged and therefore has a positive charge. Semiconductor materials like germanium or silicon can be "doped" to create and control the number of electron holes. Doping is the adding of other elements to the semiconductor material to change its properties. By doping a semiconductor you can make two separate types of semiconductors in the same crystal. The boundary between the two types is called a p-n junction. The junction only allows current to pass through it one way, this is why they are used as diodes.
LEDs are made using p-n junctions. As electrons pass through one crystal to the other they fill electron holes. They emit photons light. This is also how the semiconductor laser works. Above: A laser also creates light, but through a different construction. Read more about semiconductor devices used in electronics here. To understand p-n junctions and semiconductors better you will need to invest a good amount of time in a lecture, it is not a simple phenomena and far too lengthy to cover here.
See a 59 minute introduction lecture to solid state semiconductors here. Phosphors are used to help filter the light output of the LED. They create a more pure "harsh" color. Engineers had to figure out how to control the angle the light escapes the semiconductor, this "light cone" is very narrow.
They figured out how to make light refract or bounce off all surfaces of the semiconductor crystal to intensify the light output. This is why LED displays traditionally have been best viewed from one angle. The metal tabs on the sides of each help distribute the heat away from the LED.
Photo: Whelan Communications. This type of display is only usable for large area applications and decorative backgrounds in small spaces. The human eye can only effectively perceive the image at more than 6 meters distance.
The tricolor array is arranged in the close-up at the top right. There are also orange, green, blue, violet, purple, ultraviolet LEDs. For more details on elements used for each color go here.
Above: Two different types of LEDs, both in a strip mount configuration. Above: this experiment of a tunnel diode atop a GaAs semi-insulating substrate convinced Pittman and Biard that there must be light emission going on, and resulted in further experimentation. The early years of the s consisted of a 'race' in the field of semiconductors. Gallium arsenide and germanium were some of the first semiconductors uses before silicon became the preferred material in the industry.
These devices were being developed as diodes since they can pass current in one direction by not the other. Biard and Gary Pittman. Gary had been working in the related field of solar cells since In their efforts to try to make an X-band GaAs varactor diode they created tunnel diodes which had been developed first at Esaki. They placed the tunnel diode on a GaAs substrate and discovered that there must be light production going on during forward bias operation.
Using an infrared detector just brought in from Japan they tested it and discovered that the devices lit up brightly! The LEDs were first used with IBM computers to replace tungsten bulbs that controlled punch card readers infrared light was sent through the holes, or blocked by the card.
Today there is a myriad of applications for the LED. The First LED patent click to enlarge. Above: Walter T. Matzen top and Bob Biard bottom worked on parametric amplifiers, this helped lay groundwork for the LED. Later Gary Pittman and Mr. Biard worked on varactor diodes which led to the LED as we know it. Below is a closeup of a WS The bigger square IC on the right controls the colors individually. What is this magic? An LED with a built-in resistor? That's right. There are also LEDs that include a small, current limiting resistor.
If you look closely at the image below, there is a small, black square IC on the post to limit the current on these types of LEDs. So plug the LED with built-in resistor to your power source and light it up! We have tested these types of LEDs at 3. As electronics get smaller and smaller, manufacturers have figured out how to cram more components in a smaller space. Because they're so small, and have pads instead of legs, they're not as easy to work with, but if you're tight on space, they might be just what the doctor ordered.
You would probably not to manually solder all of those components by hand. These are brighter than the super brights! These are the fancy LEDs that you find in really nice flashlights. Arrays of them can even be built for spotlights and automobile headlights. Because there's so much power being pumped through the LED, these often require heatsinks. A heatsink is basically a chunk of heat conducting metal with lots of surface area whose job is to transfer as much waste heat into the surrounding air as possible.
There can be some heat dissipation built into the design of some breakout board such as the one shown below. High-Power LEDs can generate so much waste heat that they'll damage themselves without proper cooling. Don't let the term "waste heat" fool you, though, these devices are still incredibly efficient compared to conventional bulbs.
To control, you could use a constant current LED driver. There are even LEDs that emit light outside of the normal visible spectrum. You probably use infrared LEDs every day, for instance. They're used in things like TV remotes to send small pieces of information in the form of invisible light! On the opposite end of the spectrum you can also get ultraviolet LEDs.
Ultraviolet LEDs will make certain materials fluoresce, just like a blacklight! They're also used for disinfecting surfaces, because many bacteria are sensitive to UV radiation. They may also be used counterfeit detection bills, credit cards, documents, etc , sun burns, the list goes on. Please wear eye protection when using these LEDs. With fancy LEDs like these at your disposal, there's no excuse for leaving anything un-illuminated. However, if your thirst for LED knowledge hasn't been slaked, then read on, and we'll get into the nitty-gritty on LEDs, color, and luminous intensity!
So you've graduated from LEDs and you want more? Oh, don't worry, we've got more. Let's start with the science behind what makes LEDs tick We've already mentioned that LEDs are a special kind of diode, but let's delve a little deeper into exactly what that means:. It's a chip of semiconductor material that's doped with impurities which creates a boundary for charge carriers. When current flows into the semi-conductor, it jumps from one side of this boundary to the other, releasing energy in the process.
In most diodes that energy leaves as heat, but in LEDs that energy is dissipated as light! The wavelength of light, and therefore the color, depends on the type of semiconductor material used to make the diode. That's because the energy band structure of semiconductors differs between materials, so photons are emitted with differing frequencies.
Here's a table of common LED semiconductors by frequency:. Truncated table of semiconductor materials by color. The full table is available on the Wikipedia entry for "LED". While the wavelength of the light depends on the band gap of the semiconductor, the intensity depends on the amount of power being pushed through the diode.
We talked about luminous intensity a little bit in a previous section, but there's more to it than just putting a number on how bright something looks. The unit for measuring luminous intensity is called the candela, although when you're talking about the intensity of a single LED you're usually in the millicandela range.
The interesting thing about this unit is that it isn't really a measure of the amount of light energy, but an actual measure of "brightness". This is achieved by taking the power emitted in a particular direction and weighting that number by the luminosity function of the light. The human eye is more sensitive to some wavelengths of light than others, and the luminosity function is a standardized model that accounts for that sensitivity.
The luminous intesity of LEDs can range from the tens to the tens-of-thousands of millicandela. The power light on your TV is probably about mcd, whereas a good flashlight might be 20, mcd. Looking straight into anything brighter than a few thousand millicandela can be painful; don't try it. Oh, I also promised that we'd talk about the concept of Forward Voltage Drop.
Remember when we were looking at the datasheet and I mentioned that the Forward Voltage of all of your LEDs added together can't exceed your system voltage? This is because every component in your circuit has to share the voltage, and the amount of voltage that every part uses together will always equal the amount that's available. This is called Kirchhoff's Voltage Law. So if you have a 5V power supply and each of your LEDs have a forward voltage drop of 2.
Kirchhoff's Laws also come in handy when you want to approximate the voltage across a given part based on the Forward Voltage of other parts. Of course we would want to include a current limiting resistor, right? How would you find out the voltage across that resistor? It's easy:. So there is. This is a simplified example and it isn't always this easy, but hopefully this gives you an idea of why Forward Voltage Drop is important. However the technology and the materials used are key to understanding how a LED works.
Although the basic PN junction had been in use for many years, it was not until that the LED was developed and its action started to be understood. The semiconductor material used for the junction must be a compound semiconductor.
The commonly used semiconductor materials including silicon and germanium are simple elements and junction made from these materials do not emit light. Instead compound semiconductors including gallium arsenide, gallium phosphide and indium phosphide are compound semiconductors and junctions made from these materials do emit light.
These compound semiconductors are classified by the valence bands their constituents occupy. Energy Savings. How LEDs are Different. Key differences include: Light Source: LEDs are the size of a fleck of pepper, and can emit light in a range of colors. A mix of red, green, and blue LEDs is sometimes used to make white light. Direction: LEDs emit light in a specific direction, reducing the need for reflectors and diffusers that can trap light. This feature makes LEDs more efficient for many uses such as recessed downlights and task lighting.
With other types of lighting, the light must be reflected to the desired direction and more than half of the light may never leave the fixture. Heat: LEDs emit very little heat.
Lifetime: LED lighting products typically last much longer than other lighting types. LED Products.
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