Theory
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Conductions
Intrinsic and Extrinsic |
Junction
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Created it, 06/03/17
Update it, 06/03/19
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MATÉRIALISATION OF THE SWITCHING FUNCTIONS
INTRODUCTION :
Until now, we especially materialized the binary states using a switch, this one being opened or being closed.
The association of several of these switches makes it possible to fulfill the switching functions.
This switch is composed of a system of entry and a system of exit.
According to the presence or absence of an action on the entry, the exit can take two well differentiated states.
Each one of these states being indissociable one of the states of entry.
In addition, the technology of this switch, or element basic, relates to its realization, its physical matérialisation. From this matérialisation the origin rises from the energy which it is necessary to apply to the system of entry to modify the state of the exit. It also informs us about the type of energy usable at exit.
In the case of the switch, the energy used on the level of the entry is of mechanical origin, that usable at exit is electric.
This origin can be different, according to technology employed (pneumatic, hydraulic, electromechanical or electric).
According to the aim, one uses a technology rather than another, for his better adapted qualities.
One should not especially establish prestigious hierarchy in this choice; it is there initially question of technical criteria and also of cost price when several specimens should be carried out.
These two parameters are enough to understand that there is no final choice, but that this one can represent a compromise interesting for a given time, which is valid besides in other fields.
The realization of the switching functions with contacts is still used today in certain sectors like telephony or certain automatisms (ordering of elevators) but it acts especially of sure materials, which always fulfill their function, because the recent achievements are from now on electronic, the services rendered being, in addition, identical.
This technology of the contacts is not appropriate for certain uses where the speed of calculation is of primary importance and where the great number of functions would lead to systems of gigantic size.
Electronics brings a solution to the problem of the speed, but the lamp assemblies were very bulky and consumed much energy.
It was necessary to await the advent of the semiconductors so that numerical electronics takes its rise.
One carried out initially the diodes whose certain applications were directed towards logic, then came the transistors for which the applications in this field are numerous.
It any more but did not remain to integrate an increasingly large number of transistors on a few square millimetres to lead to the microprocessor, in partnership with an increasingly large control of the logical algebra by a greater number of technicians.
It appears essential to us to detail these stages a little, in order to better dominate the problem. We propose the following progression to you:
semiconductors
the diode and derivatives
the transistor effect
the bipolar transistor
the field-effect transistor
switching functions with diodes
switching functions with diodes and bipolar transistors
switching functions with bipolar transistors
switching functions with field-effect transistors
other logical families
integrated circuits and their evolution
circuits employed in industry
lexicon of the terms used in the catalogs manufacturers
various kinds of cases.
These various topics are treated in the four lessons of technology.
1. - THEORY
OF THE SEMICONDUCTORS
Generally, any device using the circulation of a flow of electrons takes the qualifier of electronics.
In these devices, a certain number of physical principles are implemented in order to support the birth of the flow of electrons. This one is then controlled using a signal of comparable nature.
Among the various existing bodies, with regard to electricity, we can classify them in two categories:
insulating, nonconducting materials of electricity
the conducting materials, which allow the passage of the current.
Between these two limits, the semiconductor materials, such germanium or silicon are intercalated.
The first transistors were carried out starting from germanium. Then, silicon was used.
Germanium is drawn from the sphalerites of which one also extracts zinc. There is of it little and it is difficult to produce.
Silicon exists in great quantity since it is drawn from quartz and silica. What explains, partly, the generalization of this material.
1. 1. - RECALL ON THE CONSTITUTION OF THE MATTER
The smallest part of a body still preserving the same properties that this one, names the molecule.
The molecule consists of atoms, in a variable number, function of the type of molecule.
The atom is composed of a core and electrons which revolve around this one on orbits, or layers, of which the number varies according to the atom considered as shown in the figure 1 for germanium and silicon.
The core consists of protons and neutrons. The electric charge of the neutrons is null while that of the protons is positive.
The electrons have a of the same load value than that of the protons but of opposed sign, i.e. negative.
The loads of the protons and those of the electrons balance, with the result that the atom is electrically neutral.
The electrons turn around the core, the image of our solar system and of the planets which make it up.
The successive layers on which these electrons circulate, constitute energy levels.
That means that the more the layer is brought closer to the core, the more one needs energy to tear off an electron to him.
It is the last layer, or sleep external known as of valence, which interests us because it is it which allows the connections with the atoms close thus authorizing the constitution to the molecule. In addition, it is on this layer that one will be able to cheat by adding or cutting off an electron.
The electric charge of an atom is neutral. One can break this balance by cutting off an electron to him from his peripheral layer. The remaining load becomes positive. It is said that the atom is ionized positively and it takes the name of cation.
In the contrary case, if one adds an electron to the layer of valence, the atom is ionized negatively. It takes the name of anion.
1. 2. - INTRINSIC CONDUCTION
The silicon atom has on its peripheral layer, or lay down valence, 4 electrons. Those allow the bond with 4 close atoms.
At the temperature of the absolute zero, all these connections are stable and the material can be regarded as insulator.
To tear off an electron of its orbit, it should be subjected to a force more important than that which binds it to its core.
This force can be the resultant of an electric field created by a tension, or a rise in temperature of which the effects appear by molecular vibrations. These vibrations result in the application of a system of forces on the electrons whose resultant can be enough to tear off those of their orbit.
At the temperature of 20° C, the crystal lattice of silicon is the seat of an important thermal agitation.
Electrons are then torn off their orbits and, in their routes, they recombine with positively ionized atoms (i.e. cations or atoms having lost an electron previously).
The quantity of free electrons is always equal to the quantity of “holes” ready to accept an electron, because the formation of a hole is the consequence of the departure of an electron.
An increase in temperature involves the formation of a deal even more great of pairs “electrons/holes”.
The mobility of those conditions the density of the current circulating in material (the electrical current is a displacement of electrons).
One notes, for the semiconductors, that when the temperature increases, conductibility made in the same way.
In germanium, this increase is more important, because the force necessary to tear off an electron of its orbit is weaker than for silicon.
1. 3. - EXTRINSIC CONDUCTION
If, into the crystal lattice of silicon, one introduces, in sufficiently small quantity not to modify this one, a body whose atoms have 5 electrons on their peripheral layer, these atoms of impurities use the grid of the crystal lattice, by imbricating 4 electrons of their layer of valence with 4 adjacent silicon atoms. In this organization, the fifth electron is in “excess”. At the temperature of 20° C, thermal agitation tears off this electron of its orbit. Thus, the atom of impurity becomes ionized positively (cation), since it loses a negative charge, the electron.
However, these opposite loads being contained in same volume, the resulting load is null.
Thermal agitation acts on the silicon atoms, intrinsic conduction always exists, but the presence of electrons in excess (due to the impurities) will tend to all the more quickly recombine the number of silicon atoms having lost an electron, in order to reconstitute the grid.
It will remain finally only little of missing connections in the network. These lacks are in numerical inferiority compared to the free electrons. They are carrying a positive load. To restore their balance, they must accept an electron.
In this type of material, they are the minority carriers or acceptors (they are the holes).
By opposition, the atoms of impurity having released an electron are called donors. These electrons, in greater number than the holes, are known as majority and since they are carrying an electric charge (negative), they will be called: majority carriers.
This operation, which consists in injecting impurities in silicon (or germanium), takes the name of : doping.
The result of doping that we have just described names :
The conductibility of the type N, in which
the majority carriers are the electrons, the minority carriers, holes.
A parallel reasoning can be held by injecting trivalent atoms of impurity (3 electrons on the peripheral layer of valence), in the crystal lattice of silicon. In this case, each atom of impurity joins 4 adjacent silicon atoms, but an electron, in this grid, is missing. This constitutes a place, or hole, likely to receive an electron.
Thermal agitation, leading to intrinsic conduction, provides the electron to this vacancy, in order to reconstitute the grid, thus creating a negatively ionized atom (anion).
It follows that the free electrons, in this case, are in numerical inferiority compared to the holes, they will take the name of minority carriers. The atoms having released them are the donors (which after this operation become ionized positively, they are cations).
The holes, with this type of doping, being in a majority, take the name of majority carriers, and the atoms to which they belong are the acceptors.
The atoms of impurity injected are electrically neutral. When they collect an additional electron, they take care negatively, but this load is compensated by the creation of another, of opposed sign, which had at the beginning of this electron of a nearby atom.
The unit electrically remains neutral. This doping makes it possible to obtain :
The conductibility of the type P, in which
the majority carriers are the holes, the minority carriers, electrons.
In short, the materials of conductibility N or P are electrically neutral.
In the material N, all occurs as if majority
electrons or carriers had been injected. Some lacks remain in the grid, had with
thermal agitation, they are the minority holes or carriers.
In the material P, all occurs as if majority
holes or carriers had been injected. Some free electrons remain, had with
thermal agitation, they are the minority carriers.
It is necessary to underline, for what will follow, that the holes do not move in the crystal lattice. Only, the electrons which are material elements, move.
The consequence of the departure of an electron is a gap or hole with the site which it occupied previously. There is thus circulation of electrons in a direction and the appearance of a displacement of holes in opposite direction.
In made descriptions, we spoke about bonds between atoms. There are several manners for the atoms of binding the ones with the others. In this case, they are covalent bonds (or connections of valence).
1. 4. - The junction
N. P.
We became acquainted with two types of conductibility :
the type N
the type P
If one associates, coast at coast, a material of the type N with a material of the type P, we carry out a junction.
This one indicates the mean zone in which conductibility passes from the type N to the type P (or the reverse).
Let us examine what occurs on the level from this junction in the following cases :
- without polarization
- with opposite polarization
- with direct polarization
1. 4. 1. - NON POLARIZED JUNCTION (figure 1 bis)
In figure 1 (a) like in figures 2 and 3 which follow, a certain number of signs were employed and which we will detail now :
Material N
- pentavalent atom of impurity, injected in silicon.
With agitation, this one becomes :
- a free electron.
- a cation entering the grid. On the drawings, for
more clearness, only the free electrons and the cations will be represented.
Material P
- trivalent atom of impurity, injected in silicon.
With thermal agitation, it collects the missing electron to enter the grid and
becomes:
- anion entering the grid
In two materials, we also find:
- silicon atom of the grid having lost an electron
by thermal agitation
- escaped free electron of the silicon atom.
a) - The diffusion current
The majority electrons carrying the zone N, diffuse in the zone P where they recombine with the majority carriers of this zone, by taking seat in the holes.
There is imbalance of the electric charges, indeed, in the zone N, the electrons having disappeared, the load of the donors or ions positive (cations) is not counterbalanced any more and this zone becomes positive.
The contribution of electrons in the gaps of the zone P modifies the electric balance of this zone with appearance of negative ions (anions).
The displacement of electrons of the zone N towards the zone P is named : diffusion current.
It is accompanied by a positive space charge on the side of the material N and of an equal load but of contrary sign on the side of the material P. Those create an electric field e.
b) - Current of conduction
The effect of this electric field e will force the minority electrons carrying the zone P, to circulate towards the gaps or minority carriers of the zone N, tending to counterbalance the loads lost by diffusion current.
A current of electrons is established zone P towards the zone N. This one is due, in this case, with the minority carriers of these zones.
The junction is the seat of two equal but opposite currents. It circulates no current in the external circuit.
The zone in which occurs the electric field e names :
c) - Barrier of potential
In this zone, the concentration out of carriers becomes identical to that of intrinsic conduction (at equal temperature).
1. 4. 2. - JUNCTION POLARIZED IN SENS REVERSES (figure 2)
Now let us apply the negative pole of a pile to the electrode of the material P and the positive pole to the material N.
The electric field created by the application of the tension of this pile is of the same direction than the electric field e of the barrier of potential.
These two fields are added and supported the circulation of electrons or minority carriers of the current of conduction. Moreover, the free electrons of the zone N and the majority carriers of the zone P (holes) under the effect of this field, will deviate from the junction.
It follows that the concentration in cations of the zone N and anions of the zone P will increase close to the junction.
The barrier of potential is widened and the circulation of electrons, majority carriers, is overdrawn compared to that of the minority carriers, the more so as the field e increases. A limitation of this current is established because the departure of the electrons of the zone P and their arrival in the zone N, create a space charge which limits this current by thwarting the increase in e.
It thus circulates a weak current in the external circuit. With constant temperature, for an increase in the tension of the pile thus an increase in E, the current remains practically constant bus it is due to the minority carriers resulting from thermal agitation.
Beyond a certain threshold, one notes that the current increases in a brutal way.
The increase in the field E confers on the electrons of the current of conduction a speed such as their kinetic energy reaches a value sufficient for, that in the event of shock, with an atom met on their trajectory, it tears off an electron of this one thus creating an increase in free electrons. These electrons are added to the first and the effect becomes cumulative.
This phenomenon takes the name of effect of avalanche. It is used for certain devices like the zener diodes.
In a junction which is not carried out for this effect, this one involves the irremediable destruction of the junction by breakdown.
The tension which creates the electric field E for which the phenomenon produces takes the name of :
- tension of reverse
breakdown or tension of avalanche.
In on this side this tension, if the temperature is increased, with constant tension, thermal agitation increases and the current of the minority carriers makes in the same way. Thus the reverse current increases. It is an important fact which it will be necessary to remember. We will speak again about it thereafter in the paragraphs devoted to the diode and the transistor.
The tension of breakdown is called : VRM
(R = reverse = opposite ; M = maximum)
- The current of conduction due to the minority
carriers, the junction being polarized in reverse, is called : IR (R =
reverse = opposite).
1. 4. 3. - JUNCTION POLARIZED IN DIRECT SENS (figure 3)
With a low value of the tension a weak electric field corresponds E, directed in opposite direction of the field e.
These two fields are opposed and the result is a reduction in e.
This one being at the origin of the current of conduction (minority carriers of the zone P), we note a reduction proportional in this current. Consequently, the diffusion current (majority carriers of the zone N) will become dominating and a weak current will circulate in the external circuit.
The departure of the electrons of the zone N tends to immediately create an imbalance of load in this material restores by the pile, which injects an equal quantity of it.
In the same way, the arrival of these electrons in the zone P tends to create an imbalance in this material (of sign opposed to the precedent), but the positive polarity of the pile applied on this side, aspires the negative charges in excess, restoring balance.
The progressive increase in the tension, does not lead to an appreciable increase in the current in the external circuit. However, when one reaches a certain threshold, whose value remains low despite everything, one notes an abrupt increase in the current.
The resulting electric field, confers on the majority electrons carrying the zone N (diffusion current), a sufficient energy so that they cross in great number, the barrier of potential, whose width is now very reduced.
The pile compensates for the departure of the electrons of the zone N and supports the arrival of those in the P zone.
The flow of the current is well established and for a weak increase in the tension, one notes a great increase in the current.
With this value of electric field E, which conditions the clear increase in current, corresponds a tension which one names :
VD or VF - (F = forward = direct).
The current which circulates in the external circuit and which corresponds to polarization in the direct direction (or passing) names :
ID or IF - (forward current)
The temperature has little influence on this current ; only the tension of threshold is affected, we will see how by observing the characteristics of the diode.
Now that you are familiarized with the concept of junction, it is necessary to give an order of magnitude to the physical values of which it was question. Moreover, the described principles, concerning various conductions, are schematized in order to allow of it an approach sufficient for our use.
It was especially question of silicon, because it is from now on the semiconductor material most widespread (the principle remains identical with germanium).
It should be known that the germanium atom has a core made up of 32 protons, electric charge + e and neutron (41) of neutral electric charge.
The atom, in general, being electrically neutral, that of germanium have 32 protons and also have 32 electrons of electric charge - e.
Those revolve on 4 layers, or orbits, around the core.
If one assigns to these layers a number, nearest to the core being the first, the maximum number of electrons being able to revolve on these layers is given by the formula :
N = 2 n2
With N = a number of electrons on layer n.
n = number of the layer considered.
Thus, germanium has : 2 electrons on the first layer, 8 over the second, 18 on the third and only 4 on the fourth (which could accept 32 of them).
The silicon atom is made up, as for him, of :
Around this core, 14 electrons of load revolve - e. The first two layers are complete, the third contains 4 of them (whereas it could accept 18 of them). See figure 1.
These two atoms have the same number of electrons on the peripheral layer, but silicon has a layer of less.
It is necessary to specify that the more one electron is on a layer far away from the core, the more the force necessary to tear off some is weak (at equal temperature) ; what results in a force measured into electronvolt (eV) about weaker 36 % for germanium than for silicon at the temperature of 27° C.
In the polarized junction in reverse, the current IR (due to the minority carriers of thermal agitation) will be more important in the case of germanium and, any increase in the temperature, will tend to increase this current in proportions larger than for silicon.
It is an additional reason for the choice of this material, because in the case of the diode (about which we will speak further), this current is to some extent a defect.
It was also question of doping, or injection of atoms "of impurity”.
In the conduction of the type P, these atoms are trivalent, i.e. they have 3 electrons on their peripheral layer. These atoms are those : boron (B), gallium (Ga), aluminum (Al) or indium (In).
In the conduction of the type N, they are pentavalent atoms (5 electrons on the peripheral layer) which are : phosphorus (P), arsenic (As) or antimony (Sb).
The proportion remains very weak and about some atoms of these bodies for million silicon atoms. This perhaps variable proportion as we will see it later in order to support certain phenomena.
In addition, we regarded this material (silicon) as being pure, which, in reality, is not the case. In the manufacture of the semiconductors, one will endeavor as well as possible to refine material, so that it does not contain, like impurities, that those which one injects to him.
We spoke about width of the barrier of potential, it should be known that it is approximately 1 micrometer called more usually micron (1 micron = 1 µm = 10-6 meter), which is already much on an atomic scale. The distance between two silicon atoms is about 10-10 meter (either 1 Å = 1 angström).
It is as advisable to say as if thermal agitation supports the departure of the electrons of their orbits, those do not remain indefinitely free. During their way, they meet a free left place, by the departure of a precedent, therefore undergo an attraction because the electric charge of this atom is broken (it is a cation thus positively charged) and there is capture.
It is what one calls : lifespan of the free electron before its recombination within an other atom (or rather cation).
It is wise to specify that a pentavalent atom of impurity, injected in silicon and using the grid, will tend to lose its fifth electron more easily, by thermal agitation, because this one does not enter the grid.
On the other hand, a silicon atom (ionized positively by thermal agitation) or a trivalent atom of impurity belonging to the grid, will attract in a more significant way its missing electron.
There would be to say much on all these phenomena, but those are the subject of very thorough studies on behalf of specialists and largely exceed the framework of this first lesson.
Figure 4 indicates the symbols of the various junctions which one can meet in the semiconductor devices.
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