What are LED
and How do they Work?
By now, most of us are familiar with LED in one form or another, they’re present in our televisions, our alarm clocks, our street lights, our cellphones, and so much more. But, how many of us know how or why an LED works?
L.E.D. stands for light emitting diode. The ‘light emitting’ part of the name is easy enough to understand, but what is a diode? The defining characteristic of a diode is that it only allows current to flow in one direction. Electrons enter the diode through a positively charged lead called an anode and leaves through a negatively charged lead cathode. The electrons' movement through the substrate release little packets of energy called photons as the electrons move from a higher energy state to a lower energy state in a process we will learn more about later.
When a current is supplied to the diode the electrons and holes cross into the depletion zone and some will collide in a process called recombination. When an electron in the conduction band meets a hole in the valence band, it settles into a lower energy state within the crystalline lattice, and its extra energy is released into a unit of light known as a photon. Because this is where most of the light is created, in an LED this zone is called the active region.
The wavelength of the photon emitted is proportional to the energy that would be required for an electron to leave its position within the lattice. This energy is different for each element and is known as its bandgap.
Utilizing this knowledge, different semiconductors and impurities can be utilized to produce different colored LEDs. Below you can see a table showing some of the different elements we can utilize to make LEDs.
How do they Work?
Now that we understand a little more about what constitutes a diode, let's take a simplified look at how an LED works. An LED is what is known as a P-N junction. In a P-N junction, there are two or more different semiconductors, one has a net positive(P) charge and another has a net negative(N) charge. Simply put, the "P" side has extra spaces, or “holes” within its crystalline lattice (valence band) for free electrons, and the "N" side has extra electrons zipping around with lots of energy. Electrons with this energy level in an atomic region known as the conduction band. In between them, there is a “depletion zone” which is neutral.