What are diodes?
The function of a diode is to allow the flow of current in only one direction. Diodes are also called two-terminal devices because they have two pins that connect to the circuit. An example of a diode is a light emitting diode (LED), where a substance is used in construction that allows current to flow in the correct direction.
What are semiconductors?
A semiconductor is a material that has varying ability (between that of an insulator and a metal) to conduct electricity. Typically, a semiconductor is made from an insulator that has been doped in order to make it more conductive. Doping refers to the process of adding impurities into a material by adding additional atoms to change the balance of charge. An N-type semiconductor is one that has an excess of electrons, while a P-type semiconductor is one that has excess electron holes. In both cases, doping allows the material to conduct electricity to some degree.
How do diodes work?
Essentially, diodes are constructed by bonding a P-type semiconductor and a N-type semiconductor, side by side. Consequently, electricity can only be conducted in one direction. The junction is the area that splits the two types of semiconductors. Surrounding the junction, there is an area created called the depletion zone, where there are no excess electrons or electron holes. As a result, in this area, the material exhibits its original insulating state and charge cannot flow through the material.
If a power source, such as a battery, is connected to the diode (such that the positive terminal of the battery is connected to the N-type semiconductor and the negative terminal to the P-type negative voltage is applied), the result is that the positive holes are attracted to the negative electrode and the electrons are attracted to the positive electrode. As a result, the depletion zone increases and charge cannot flow through the material. Thus, the diode acts as an open circuit blocking the current flow.
However, if the battery is connected in the opposite manner (where positive voltage is applied and the negatively charged electrode is near the N-type region), then the electrons repel from this region and they are forced to travel to the other end. The positively charged electrode is doing the same with the electron holes. The width of the depletion zone decreases and the effective resistance of the diode is decreased. As a result, the diode acts similarly to a short circuit, allowing current to pass.