This is entry #4 of my 52 Project 2012: Foundational Electronic Components Crash Course series.
What is a diode?
A diode is a simple device which allows energy to flow through it in only one direction. It is also the simplest type of semiconductor in existence, and is the foundation of the all-important transistor.
How does a diode work?
A diode is made by placing two layers of specifically doped materials together. Doped materials are substances that have intentionally been injected with impurities. In the case of diodes and many other semiconductors, a conductive material is injected into a non-conductive material, resulting in “semiconductive” substance. This substance may contain a surplus of electrons (known as “N-type” material), or it may contain a shortage of electrons (known as “P-type” material). “N” and “P” stand for negative and positive, respectively; since electrons have a negative charge, a surplus of them creates an overall negative charge in the material, and a shortage creates an overall positive charge.
A substance that has a perfect balance of protons and electrons is not conductive, since all particles are drawn to their counterbalancing partners. Such a substance is typically called an insulator. But if there is an imbalance, then some electrons (negatively charged particles) will be able to flow from a negatively charged area to a positively charged area. This property, the flow of electrons, is what makes conductive materials like wires able to carry electrical current. Diodes are built by bonding an N-type semiconductor to a P-type semiconductor and attaching one terminal to each layer.
When a negative source (for example, the negative terminal of a battery) is connected to the N-type layer and a positive source is connected to the P-type layer, then the current flows freely. The electrons are attracted through the N-type layer to the P-type layer, and vice-versa for the positively charged areas.
However, when the connection is reversed—a negative source is connected to the P-type layer and a positive source to the N-type layer— then a non-conductive gap is created in the center of the diode, where the two layers are joined together. The negatively charged particles from the source create a build-up of positively charged particles in the P-type layer, and the positively charged particles from the source create a build-up of negatively charge particles in the N-type layer, leaving what is called the depletion zone in between where no current flow can happen.
One common use of diodes is to provide protection against reversed power connections. If you put a diode in line with the power supply, then a backwards battery will not damage the circuit (though it still won’t work right until you fix the connection again).
Diode ratings
Although we have seen quantities for resistance, capacitance, and inductance, there is no directly equivalent “diodance.” Due to the simple function of diodes—unidirectional flow of energy—there are two main factors to consider.
First is a rating called “forward voltage drop.” Due to the nature of semiconductive material and the functionality of a diode, there is some reduction of voltage that occurs. This is often quite small (0.5 volts or so), but especially in low-voltage circuits, it is critical to know. (This demonstrates the same effect as a resistor, remember: current flow stays the same, but voltage drops some.)
Second is a rating known as “peak inverse voltage.” Diodes have a limitation by nature, in that if you apply a voltage big enough in the wrong direction, the depletion zone “barrier” breaks down, and current flows anyway. It is extremely important to know this rating as well. In low-voltage circuits, the PIV rating is usually much higher than any voltage that is designed to be used.
For a description of this 52 Project series and links to the all existing entries, go to the 52 Project 2012: Foundational Electronic Components Crash Course page.