Introduction

• A diode is a two terminal analog device that allows an electrical current to flow in only one direction. Standard diodes are made from silicone and have two sides, the anode and the cathode. The anode is the positive terminal, the one you would connect to the (+) side of a battery. The cathode is the negative side, the one you would connect to the (-) side of a battery.
  
  

• The diode symbol as used in schematics of electronic circuits.
  
  Diodes are manufactured by adding atoms of other elements to an otherwise pure silicone crystal in a process called doping. Doping results in negative, or N type material, at the cathode side of the diode, and positive, or P type material, at the anode side. The point at which the anode meets the cathode is called the P-N junction. The atoms introduced during doping are called charge carriers. These carry the current from the anode, through the semiconducting material, across the P-N junction and through the cathode. The doped atoms in the P-type material are positive carriers, which are free to move about the silicone crystal structure. The opposite is true of the N-type material, which has negative carriers, also free to move through the silicone crystal structure.
  
  

Inactive State

• In a non-operational state at the P-N junction, electrons from the N-type material move into the P-type material to join with the P-type charge carriers. The region they left is now depleted of electrons. This portion of the N-type material is called the depletion region. Since electricity is the movement of electrons, the depletion region becomes an effective insulator as it is unable to conduct electricity.
  
  

Simplification

• The mechanics of the diode break down to a simple principle often learned in grade school. Opposite poles attract each other and like poles repel each other, the same law that affects magnets. The N-type material is the negative pole, filled with negative charges. The P-Type material is the positive pole and is filled with positive charges.
  
  

Conductive Mode

• With the cathode connected to the positive battery terminal, current flows.
  
  If the positive terminal of a current source, such as a battery, is applied to the P-type material, it repels the positive charges towards the P-N junction. The PN junction becomes saturated with carriers and current is able to flow across the junction. As the positive charges are pushed through the material, they become attracted to the negative terminal of the current source and electricity is flowing.
  
  

Non Conductive Mode

• With the anode connected to the positive battery terminal, no current flow is possible.
  
  If the current source, or battery, is reversed and the negative terminal is connected to the P-type material, the positive charges are attracted away from the P-N junction. Similarly, the N-type material will be connected to the positive terminal which attracts the negative charges. Current cannot flow in two directions at once and the P-N junction is depleted of charges, therefore no current flow is possible at all.
  
  

Operational Current Usage

• A silicone diode requires about 0.7 volts to sufficiently saturate the P-N junction with charge carriers in order for it to carry current. It will slightly conduct (it is a semi-conductor) as the voltage increases from 0 to 0.6 volts. Beginning at 0.6 volts increasing to 0.7 volts, conductivity is increased and once 0.7 volts is reached, the diode conducts freely.
  
  

Application

• Diodes have a number of useful applications, the most common being in an AC to DC power converter. Power converter uses include charging laptops and cell phones and the power supply in your desktop computer. Alternating current (AC) first flows in one direction, then the reverses, changing direction 60 times per second in the United States. A diode or combination of diodes in conjunction with other components are used to change AC to DC (direct current), allowing flow in only one direction.