An LED dancing penguin in Locomotive Park, Lewiston Idaho. Created by Dr. Jesse Huso & Dr. Leah Bergman Department of Physics, University of Idaho Supported by: The National Science Foundation under Grant No. DMR

 Modern lighting is based on a device called a “Light Emitting Diode”.  We usually just call these LEDs (pronounced “el ee dees”).  LEDs are useful because they’re small, energy efficient and long lasting. Image courtesy of energystar.gov

 LEDs come in many colors.  These colors are determined by what the LED is made of, as we will see. The Visualizer Tree Locomotive Park, Lewiston Idaho

 While LEDs come in many shapes and sizes, they all work in similar ways.  Here are some parts.  When we apply electricity to the wires on the LED, it lights up! Colored lens (protects other parts and directs the light) Wires for electricity, called the anode and cathode Cathode Anode Semiconductor and reflector (more soon!)

 Let’s take off the lens…  And now let’s zoom in…  The tiny piece of semiconductor, called a die, is what emits the light we see.  There is also a reflector around the die to help light get out.  Let’s take an even closer look.

The semiconductor portion is made of two parts: a layer of semiconductor called “p-type” a layer of semiconductor called “n-type” There are also metal layers connected to the anode and cathode wires. These metal layers allow us to apply electricity to the semiconductor. p-type n-type Cathode Anode

 Let’s look at the semiconductor closely.  The n-type region has many extra electrons.  The p-type region is missing electrons: we call these “holes”.  The thin boundary is called the “depletion region”. p-type n-type Depletion region

 If we apply electricity across the die in the right direction…  The electrons and holes will move toward each other.  They recombine and the semiconductor emits light from the depletion region! p-type n-type Light

 Any semiconductor can be understood with a simple picture called an “energy band diagram”.  There are many energy states electrons can occupy.  The lower energy states are called the “Valence band”.  The higher energy states are the “Conduction band”.  The region with no energy states is called the “band gap”. Conduction band Valence band Band gap Energy states

Conduction band Valence band  Normally, the valence band is full of electrons and the conduction band is empty.  When a semiconductor is doped as n-type, extra electrons are added, and these electrons begin to fill up the conduction band. n-type

Conduction band Valence band  p-type doping is similar to n-type, but electrons are removed, leaving empty states in the valence band.  We call these empty states “holes”. p-type Missing electrons

p-type n-type Conduction band Valence band Band gap Depletion region p-type n-type Recall the n-type side has extra electrons in the conduction band… and the p-type side is missing electrons in the valence band. When we sandwich the n ‐ type and p- type layers, we get an energy band structure like this

p-typen-type When we apply electricity, we change the shape of the energy bands, making them look more like this. The electricity shrinks the depletion region and makes it easier for the electrons to find the holes. p-type n-type Depletion region

p-type n-type Conduction band Valence band Band gap The electrons combine with the holes and give off energy as the light we see. Light Depletion region

Medium band gap Light How much energy is released by the electrons is what determines the color of the LED. This amount of energy is in turn determined by the band gap. Light Small band gap Large band gap Light Each semiconductor material has a different band gap, and thus emits a different color. Indium gallium nitride Aluminum gallium Indium phosphide Aluminum gallium arsenide

And that’s how an LED works! We hope you learned something. MATERIALS PHYSICS IS AWESOME!

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