semiconductor!

                                                              What is a semiconductor?

       

                                                 A semiconductor:

                             A semiconductor such as silicon is a material that is not quite a conductor of electricity, nor quite an insulator. It can be either one or the other under various conditions.

                             The conductive or insulating character is rooted in the very structure of atoms: each element of the periodic table has a number of electrons are arranged around a core. It is this arrangement in the form of electron layers differ depending on the elements, which is responsible for the electrical conductivity.

            The electrons of an atom can have multiple roles within a structure of atoms:

                              heart electrons: they are close to the core and do not really interact with other atoms;

                              valence electrons: these are the outer layers of the atom and are used to create atomic bonds and form molecules;

                              conduction electrons thereof are responsible for the flow of electric current.

                    We can represent the whole in the form of layers. On the following diagram shows the valence electrons layers and conduction electrons:

          

                      It is seen that in a metal, some electrons are both in the valence band and the conduction band. This means that a metal can conduct current without further physicochemical treatment.

In an insulator, for cons, the two bands are separated by a space called "gap": this means that electrons can not be there. For insulation, the outer electrons are in the valence band and none is in the conduction band: these materials therefore can not conduct electricity. 

                  

                     Finally, in the case of semiconductors, in the middle, there is a gap well, but the latter is very thin. Just a little something for the valence electrons can move in the conduction band and make the semiconductor ... driver. It manages to do this by giving energy to the electrons in the exciting.

                                  So a semiconductor is an insulator, but can become a driver easily by exciting valence electrons: 

                                 The silicon has four valence electrons (such as carbon) into a crystal, it is therefore binds so tetrahedral four other silicon atoms: 

                     we do this by heating the material or by lighting, or subjecting it to a defined voltage.

For example, if a photovoltaic plate is illuminated, the plate becomes conductive and creates an electric current: it is the photoelectric effect.

                       So we have here a very interesting feature: it is an insulator which becomes conductive when the lights.

                       In a computer processor is a minimum voltage that can make the semiconductor insulation underneath and above driver.

Believe it or not, this is very simple principle is the basis of all computi

                                                 

                                                         Doping:

                     We have seen that there is a gap in the structure of semiconductors and insulators. A semiconductor has a small band gap that electrons can cross if given the necessary energy. Over this strip, the lower the energy required is small. This is interesting for the power consumption of our devices, but also from a point of view more technical.

                     In a silicon crystal (the most common semiconductors to date), it is an energy of 1.12 \ text {} eV 1.12 eV (or 1.79 \ times 10 ^ {- 19} \ text {J} 1.79 × 10

-19J) for placing a valence electron in the conduction band.

This is a very low energy, but it's still too much for the use we currently semiconductors.

                     Doping is a technique that aims to change the energy needed to make the driver a semiconductor. It is injected into the silicon crystals well chosen atoms to make it a little more conductive or less.

                         It is possible to replace some of the silicon atoms by other atoms, which will then change the conduction bands of the structure.

                         The silicon atom having four valence electrons can be injected an atom with five valence electrons: the atom will be in the crystal with 4 bonds but will have an extra electron. For that, we often use phosphorus (symbol PP).

Since this type of modification brings an electron more in the crystal, the conduction band is so negative overall: we speak of negative doping or doping N (the crystal as a whole remains neutral because the phosphorus contains one proton and more too; it's just the conduction band which is negative).

                        Another solution is to use a boron doping (symbol BB), which has only three valence electrons: boron will be there as well caught in a crystal with 4 connections, but connections missed an electron: it will therefore as "positive" (because it lacks such a negative charge), hence the name doping P:

                           With the N doping, the silicon becomes a little driver: in fact, the introduction of phosphorus has the effect of shifting the conduction band down: the silicon electrons are therefore more quickly conductors.

                           With the P doping, silicon also becomes a little more driver: boron certainly brings "an electron hole," but he may receive a neighboring electron which then leaves a hole behind him. The hole is then moved and this is a good way of moving loads ( "virtually" positive) and therefore increases the conductivity of the material also.

These two ways of doping a semiconductor antagonists are therefore: to provide an electron in addition to the semiconductor crystal and the other one withdraws.

                           By combining two materials doped differently, one can make electrical components such as diodes or transistors. But it will be the subject of a future article!

                           For now, remember that silicon is a semiconductor, because it allows only the current that excites if its valence electrons. This requires a voltage above a threshold minimum own material.

                           N doping is used (negative) or P (positive) to slightly change the threshold voltage but also to change other semiconductor properties (we will see in other articles).

                           Finally, just for culture, know that silicon is not the only existing semiconductor. Before him, the semiconductor properties of germanium (symbol GeGe) were already used in the very first transistors. Since then, other compounds were also discovered. The best known of them are gallium arsenide (GaAsGaAs) and indium nitride (InNInN), although there are many others.

                           Silicon is used heavily today mainly because it is very abundant on Earth: it is 25% of the earth's crust, and is simple to extract and use.