Bonds between atoms |
Table of MENDELEYEV |
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Created it, 05/10/15
Update it, 06/01/13
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SEMICONDUCTORS “1st PART”
In this lesson, we will start to occupy us of the semiconductors, materials which made it possible to carry out important devices (diodes, transistors, integrated circuits…) able to ensure of the multiple and complex functions.
To include/understand perfectly the operation of the transistors, it is necessary to know well the structure of materials used for their manufacture.
We will start with a rapid examination of the internal structure of the crystals.
Throughout this development, one will often have recourse to figures to represent in an analogical way the phenomena which occur in the semiconductors. The recourse to these figures is useful all the times that it is not possible to visualize the phenomenon directly to be studied. Even if this representation is always approximate and does not reproduce the phenomenon accurately, it can however, in a certain way and for a given time, to represent as well as possible the known phenomena.
1. - BONDS
BETWEEN ATOMSAll the chemistry experiments show that the atoms of different bodies, attract each other or are pushed back, when they are placed at small distance from/to each other.
It should be specified immediately that when it is about distance between atoms, one uses as measuring unit the angström (symbol Å) equivalent to the ten millionth of millimetre or the millimicron (symbol mµ) corresponding to millionth of millimetre.
The attraction which appears between the atoms is called attraction of VAN DER WAALS; it is in general relatively weak and starts to be exerted only when the atoms are with distances of a few angströms (figure 1).
With shorter distance, when the external orbits of the electrons are touched, it can be established a closer link, called chemical bond, or on the contrary it can occur a force repulsion.
The attraction of VAN DER WAALS is primarily a phenomenon of electrostatic nature, due to the electric force of attraction, being exerted between the core electrically positive and the electrons of the close atom, electrically negative.
The chemical bond and the repulsion are on the contrary due to the constitution of the last layer of the electrons, turning around the core, i.e. the external layer. In general, the internal layers of the electrons are stable and are not concerned with the chemical and electric phenomena.
The external layer on the contrary can have a certain instability, because it can lose or gain electrons or share them with the electrons of the external layer of a nearby atom.
To explain the formation of the chemical bond in certain cases and others the repulsion, it is necessary to refer to a physical, known principle under the name of principle of PAULI.
According to this principle, in each electronic orbit placed around the core of an atom, there can be only one electron having a given energy. However, if it is admitted that all the electrons are exactly equal between them and, that an electron on its orbit turns also on itself in a direction or the other, one can modify the preceding expression of the principle of PAULI and write: on the orbit of a core, it cannot turn more than two electrons and the latter turning on themselves, must turn in direction reverses one of the other (principle of PAULI generalized on which we will base ourselves in the explanations which follow).
We will admit this principle without showing it because it requires very major knowledge in outgoing nuclear physics of the framework of this lesson.
Let us imagine now that two hydrogen atoms approach one the other. One can represent the atoms of hydrogen as on figure 1 ; they consist of a core and only one electron turning on the layer K (figure 2 : uranium atom comprising here 7 layers on which revolve of the electrons; the layer K has 2 electrons in this example).
When the cores of the two atoms are at a distance of a few angströms (approximately 5 Å), the attraction of VAN DER WAALS starts to appear.
If the distance decreases, the electrostatic attraction increases and starting from 0,75 Å, the chemical bond intervenes. This bond ensures the stable union of the two atoms and thus determines the formation of the hydrogen molecule.
The chemical bond is not solely consisted a strong electrostatic attraction between the core and the electrons, but also by an energy exchange between the atoms, exchanges occurring via the electrons passing successively and indifferently from the orbit of one atom to the orbit of another atom.
At the time of the continual passage of the electrons from one atom to another, it happens obviously that on the orbit of a hydrogen atom he is two electrons instead of one.
This possibility is evoked in the generalized principle of PAULI since in each orbit one can find two electrons.
Let us consider another case now: let us imagine that two helium atoms approached one the other.
The helium atoms consist of a core and two electrons revolving on the layer K. This layer, i.e. the layer nearest to the core consists of only one orbit.
For helium, this layer is complete, because it includes/understands two electrons and cannot, according to the principle of PAULI of accommodating others of them.
When the cores of the two helium atoms are at a distance of a few angströms, the attraction of VAN DER WAALS can appear; however at a weaker distance, there is a strong repulsion instead of the chemical bond. This repulsion between the two helium atoms is due to the interaction of their electrons. Indeed, we know that the electrons have all the same load and that they are pushed back between them. In addition, it is impossible to obtain a stable distribution of the electrons around the two cores as in the case of hydrogen, because according to the principle of PAULI generalized, the orbit of each helium atom is regarded as complete.
We examined only simple atoms up to now, comprising only one layer of electrons, the layer K.
In the case of atoms with two or several layers (figure 2), it is not only necessary to take account of the principle of PAULI but also owing to the fact that the maximum stability of the electrons is obtained, when the external layer includes/understands eight electrons revolving out of four orbits.
The elements having eight electrons on the external layer are joined together in group IX of the table of MENDELEYEV given figure 3.
On the other hand, if an atom with less than eight electrons in an external layer, different from the layer K, it will tend to accept the chemical bond with other atoms. This capacity of the atom to bind chemically with other atoms is known as valence. There is a certain relation between the valence of an atom and the number of electrons which includes/understands its external layer.
This criterion to distinguish the valences is however not always valid; it y capable many exceptions.
For example, antimony, arsenic and phosphorus have only five electrons on their external layer and can consequently be trivalent above according to the criterion, but can also have a valence five. In this case, they are known as pentavalent.
One finds also atoms being able to have a valence six (hexavalent) and a valence seven (heptavalent).
It is not necessary to look further into the general concept of valence, concept related to very complex research leaving the framework of this lesson, but it is useful on the other hand to highlight well the chemical bond between the atoms of the same element.
We previously considered the bond which is established between two hydrogen atoms, when their respective cores are at a distance from 0,75Å. This particular type of chemical bond between atoms of the same species is known as covalent bond or homopolar bond.
Let us see how can be established a covalent bond between atoms, having more than two electrons and less than eight in their external layer.
Let us take for example some atoms of a tetravalent element, i.e. an element having four electrons on its peripheral layer.
To simplify the representation, one made be reproduced on the drawing of the figure 4-a that the core of the atoms and the four outer-shell electrons (electrons of the other layers not being concerned).
Of course, actually the four outer-shell electrons of each atom, occupy several orbits, but for more clearness it is enough to represent only one of them.
Between the outer-shell electrons, one also drew four holes, which represent the free places, in which could come four other electrons, in order to supplement the external layer.
The sketch of the figure 4-a can represent all the atoms of the tetravalent elements classified in group IV of the table of MENDELEYEV (figure 3), i.e. carbon, silicon, titanium, germanium, zirconium, tin, hafnium and lead.
Let us imagine now that four tetravalent atoms approach an atom of the same type. When these atoms are with a few angströms, the forces of VAN DER WAALS appear ; but at shorter distances it is established a covalent bond between the atom considered and each of the four atoms which approached.
The four covalent bonds are represented figure 4-b by the four rays in dotted lines. This phenomenon is easy to include/understand. Indeed, in the center we have an unstable atom with four electrons on the external layer which can either recover some, or to yield some.
Near these atoms, four other atoms of the same type approached until the orbits are in contact the ones with the others.
Quite naturally the atom of the center thus fills its four holes (see figure 4-a) with the unstable electrons of the four other atoms.
The atom of the center thus has its complete external layer with eight electrons. Actually, these eight electrons do not belong exclusively to the same atom; they are subdivided in four couples and each couple of electrons belongs obviously to the central atom and a peripheral atom.
The covalent, existing bond between the central atom and the four peripheral atoms, consists of a repeated exchange of energy due to the passage of electrons from one atom to another.
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