Diodes are like electronic check-valve for current flow.That is ,it allows current flow in one direction, but blocks it in the other direction. When a diode's cathode is connected to a negative terminal of a power supply and its anode to a positive terminal, current flows through the diode.But when the polarity is altered, it blocks the current flow.This is due to the semiconductors which makeup a diode.The most common semiconductor is Silicon.It has four valence electrons which make bonds with the neighbouring atoms through covalent bonding.Now the silicon behaves like an insulator.But when an external energy in the form of voltage is applied to it, some electrons in the bond break, and become free electrons.Due to the availability of free electrons, now the silicon behaves like a conductor.This is why silicon is called a semiconductor.
A process called doping is done to silicon, to improve conductivity. There are two types of doping called p type and n type doping.Phosphorous and boron impurities are used in p and n type doping respectively.
A diode has three regions called p region ,n region and depletion region.When p region is connected to positive terminal and n region to negative terminal,the charges crush the depletion region. At 0.7 volts, the depletion region breaks and current flows through diode.This is called forward biasing of a diode.When the polarity is altered, the electrons and holes are attracted towards the terminals and depletion region widens .This blocks the current flow.This is called reverse biasing of a diode.
This is how a diode works

свернуть