The periodic’s table elements and specially metals have a large number of weak bond electrons, which are identified with their conduction band. Due to the ease with which they can circulate through metals, they are considered conductive. On the other hand, there are materials that have atoms with valence electrons strongly linked and that do not move easily, these materials are called insulator and therefore do not conduct large electric currents.


As we mentioned before, there are conductive and insulator materials, although we can also find semiconductor materials that have intermediate properties to conduct or not the electric current; group IV elements of the periodic table.  Semiconductors are chemical elements that allow the flow of electrons, but with a conduction capacity inferior to a metallic conductor and superior to an insulator material.

The most commonly used semiconductor is silicon. In practice semiconductor diodes are commonly used to learn silicon diodes, other semiconductor materials used are germanium and selenium.

It should be taken into account that silicon and germanium are semiconductors with current carrying characteristics. This depends on the temperature of the amount of light that hits on them.


In a semiconductor crystal, a valence electron can jump to the conduction band leaving an empty space in the valence band which is called a gap. Allowing an electron from another nearby atom to move into the gap, leaving another gap and so on creating a chain with filling and emptying of gaps, as a result you get a current that can be considered as the movement of gaps in one direction or electrons in the other.


By adding impurities to semiconductors, we cause them to have a better electrical conductivity. This process is known as doping, it can be in two ways:

  • Pentavalent impurities: These are elements that van be found in the periodic table whose atoms have five valence electrons in their outer orbital. Among these elements we can find phosphorus, antimony and arsenic.
  • Trivalent impurities: these are elements whose atoms have three valence electrons in their outer orbital. We can find among them boron, gallium and indium. 

When an element enters the silicon crystal lattice that has five electrons, the four valence electrons that are needed to reach equilibrium are completed and therefore there would be a fifth electron that allows the element to be much better conductor. A semiconductor doped with pentavalent impurities is said to be N-type.

If a trivalent impurity is introduced into the silicon crystal lattice, three covalent bonds are formed with three neighboring silicon atoms, leaving a fourth silicon atom with an unbound electron, thus causing a gap in the crystal lattice. A semiconductor doped with trivalent impurities is said to be P-type.


By introducing a P-type impurity on the one hand and N-type impurity on the other hand in a semiconductor material, a PN union is obtained. In the N region positive ions are created and negative ions are created in the P region. This distribution of charges in the union stablishes a potential barrier that repels the holes in the P region and the electrons in the N region, distancing them form each other.

In the case of PN junction, there are two forms of polarization:

  • Direct polarized PN junction
  • Inverse polarized PN junction

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