Physics of a diode

Today we will learn about the letter P and the letter N.

As I mentioned earlier, diodes are made of one or more atomic elements to make a special semiconductor material.  This special semiconductor material is what makes the physical properties of a diode so unique and useful.  The semiconductor diodes are made of are usually made of different combinations of silicon, gallium, aluminum, and arsenide injected with several impurities.  This process of injecting impurities into these pure elements is called doping.

Once the doping process is complete a diode can fulfill its purpose.  The process creates two layers that these diagrams from HowStuffWorks.com can help you understand.  These two layers are properly named P-Type and N-Type.  Each layer is created by injecting a specific impurity into the layer, giving it the special ability to contain a concentration of protons (P-Type) and electrons (N-Type).  You now probably find it obvious that they named these layers by the charges (Positive and Negative) they contain.  Each layer represents a terminal of the diode, to be connected to any part of an electric circuit.

 

Above - The electron/proton activity in active state and the electron/proton alignment in its insulating state.
When a diode is absent of a voltage source, the protons and electrons are aligned in their proper layers (shown above) by default.  A region exists between the P-Type and N-Type layers called the depletion zone.  The size of the depletion zone in a diode changes as voltage or the configuration of a circuit is changed.

The depletion zone has some very interesting characteristics.  Because it is made of a material similar to the P-Type and N-Type material, it has "holes" that electrons fill in to prevent positive charges from going through, thus preventing current from flowing.  There is only one way the holes can be opened for positive charges to flow through and current to be produced.  This one way is thoroughly discussed below.

Forward-Biased: Positive end of the battery meets the positive end (the anode) of the diode, causing current flow.

  

 

  

Reverse-Biased: Negative end of the battery meets the negative end (the cathode) of the diode, no current flow occurs.

 
As you probably know, like charges repel and opposite charges attract.  Therefore whenever a battery is connected to a diode in a way such that the negative end is connected to the P-Type (or anode) end of the diode, charges will be driven to the edges of the diode and the depletion zone is at its largest size (above left).  This configuration gives the diode a behavior of what is called reverse-biased.  Because no protons are able to flow through the depletion zone, current does not flow in this state.  

On the other hand, if the battery's polarity is reversed and the positive end is connected to the diode's positive end, the electrons in the diode will repel from the negative end and free electrons will open the "holes" to allow protons to enter the N-type region and interact with the electrons.  This interaction gives a forward-biased behavior of the diode.  The interaction creates current and in the case of light emitting diodes, photons which create light.  In this "active" state, the depletion zone is at its minimum size.

More on the physics of diodes>>