Fundamental assemblies of
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Transistors NPN, PNP and
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Created it, 05/10/15
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SEMICONDUCTORS 4 “8th PART”
The technique of the junction by diffusion consists primarily of an exposure to vapors and a moderate heating of the semiconductor intended for the manufacture of these transistors.
By choosing the temperature of the semiconductor suitably and by using materials with the vapor state, one obtains that a certain number of atoms diffuse in the crystal lattice of the semiconductor and form a zone of the type N or type P definitely distinct from the remainder of the semiconductor of origin.
If, initially the semiconductor is of type P, it is necessary to use with vapor the state of materials returning it of type N and vice versa.
With this method, it is possible to obtain very mean bases and to regulate their thickness with a remarkable precision by regulating the temperature and the duration of the process of diffusion. Moreover, in the bases thus formed, there is always a range of concentration of the impurities which causes the drift transistor effect thus.
By reducing the thickness of the base and by exploiting the drift transistor effect simultaneously, one obtains transistors for high and very high frequencies which one uses in the receivers with frequency modulation and in the television sets in the place of the transistors with alloy based on the drift transistor effect. The method of the diffusion however did not completely replace the process by alloy. Even currently, certain types of transistors for high frequencies are obtained by a process which combines the methods by alloy and diffusion.
On figure 10, one can see the internal constitution of a transistor manufactured with the mixed technique of diffusion and alloy.
These transistors are sometimes indicated by American initials MADT formed of initial of the words Microphone-Alloy Diffused-bases Transistor (transistor with micro-alloyage with base obtained by diffusion).
Briefly let us see the principal phases of the manufacture of these transistors.
At the beginning, one has sections obtained starting from a germanium P. monocrystal They are exposed to vapors of impurities so as to form a rather deep layer N. Then each section is divided into a certain number of pastilles and on each one of them one places two material amounts of impurity.
An amount consists of material which, while penetrating in the layer N obtained previously, returns it partially of type P ; the other amount must consist of material which leaves unchanged the fundamental characteristics of the semiconductor N.
By heating materials of impurity at their melting point, it is formed on the layer N two processes of alloy : one relating to the first type of material by which one obtains the formation of a layer P on the layer N of diffusion ; the other relating one to the second type of material by which one obtains a simple electrical contact for the connection external of this layer N.
The processes of alloy being finished, one cut out each pastille and one removes the higher edge. One obtains the illustrated form appears 10-a (see the diagram above).
The case of the transistor (figure 10-b) is filled with grease to silicone which, as we said previously, is used to protect the device from the chemical agents and the mechanical constraints.
In other types of transistor, the process by alloy was given up and the process of diffusion is repeated for the formation of the last electrode, i.e. that of the transmitter. With this method, one obtained the illustrated fundamental structures figure 11.
The section of a transistor with double diffusion of the mesa type is deferred figure 11-a. The name of this transistor comes from the Spanish term “mesa” which means flat or plate and which is usually used in America to indicate typical heights of the desert of California and other areas. These heights are made of walls more or less with peak finishing at the top by a plane surface rather wide. The profile of the transistor mesa points out them a little, from where its name.
The deferred structure figure 11-b points out it also the profile of the mesa but differs from the preceding one by the semiconductor used (silicon instead of germanium), by the succession of the layers (NPN instead of PNP) and especially by the division of the collector in two superimposed zones (zone with high resistivity and zone with low resistivity).
In general, the collector is consisted only one zone having raised an enough and uniform resistivity; thus when the collector current is close or equal to the maximum value, there are a voltage drop and a dissipation of electric output notable. But this disadvantage can be attenuated by reducing the resistance of the collector, i.e. by forming the collector with a principal layer with low resistivity by supplementing it with a layer with high resistivity in the zone of contact with the base (epitaxial layer).
The axial épit layer is obtained by depositing on a pastille of semiconductor with low resistivity another semiconductor of the same type but much purer. The deposit must be formed very slowly and under conditions of temperature such as they allow a normal growth of the crystal on the reticle of the pastille.
By the method of the double diffusion, the base and the transmitter are formed by leaving free a fine layer forming the collector jointly with the zone with low resistivity.
With the axial technique épit described, one manufactures transistors which are used in the circuits multivibrators of the computers and other circuits where the transistor must function with the maximum of collector current.
Briefly let us examine, now, a last method of manufacture of the transistors with the process of double diffusion, i.e. the method called usually technical double diffusion.
This technique is a process more recent adoptee for the preparation of the transistors and it envisages only the use of silicon.
As its name indicates it, the transistor double diffusion consists of a relatively plane surface obtained by diffusing the basic areas and of transmitter inside a silicon chip which is used as collector.
For the preparation of a transistor double diffusion to silicon, one superficially oxidizes a silicon chip which we suppose of type N (figure 12-a).
The oxide coating silicon constitutes the key of the process double diffusion. Indeed, this coating is used to protect material against the moisture and the dust, which could cause an instability of the junctions and consequently cause faulty operations of the transistors. This layer also prevents the diffusion of impurities in material during the manufactoring process.
On a face of the pastille, one removes by chemical attack part of silicon oxide so as to discover the semiconductor of origin (figure 12-b).
Through the window thus practiced in the oxide coating, one lets diffuse a certain quantity of boron in the silicon monocrystal of conductibility N.
Boron is a trivalent element and one obtains the base of P. conductibility thus Then surface is again oxidized, thus closing again the window (figure 12-c).
With the same process, one practices then a new window smaller than the preceding one (figure 12-d) and through this one, one lets diffuse phosphorus; one produces thus the transmitter of conductibility N. Lastly, surface is again oxidized.
At the end of these operations, one obtains three superimposed zones : one is consisted the semiconductor of origin (silicon N), another is formed by the first diffusion (silicon P) and another is created by the second diffusion (silicon N).
With the same method, one then practices on the transmitter and oxide coating two windows (smaller than the preceding ones) for the connections of the basic electrodes.
Thus the production cycle of the transistor double diffusion finishes of which the fundamental structure is illustrated figure 13.
For the preparation of a transistor to silicon, one can also use a silicon chip with a transition course épit axial as for the transistors mesa.
The manufactoring process is similar to that adopted for the normal transistors double diffusion.
One can see the structure of a transistor epitaxial double diffusion figure 14.
The normal transistors double diffusion as those with epitaxial layer have an excellent response to the frequencies raised with optimal characteristics of thermal stability, a high output and a great reliability of operation.
Their fundamental characteristics are similar to those of the transistors mesa ; they however have the possibility of controlling higher powers and they have leakage currents much weaker.
With the technique double diffusion, it is possible simultaneously to obtain a high number of transistors (two thousand and more according to the diameter of the silicon wafer starting).
The aspect that the transistors present is very varied, both for the shapes of the case dimensions.
The types of the most common transistors are illustrated figure 15 : high and ultra-high frequencies (figure 15-a), low frequencies and low powers (figure 15-b). On the other hand, some specimens of transistors of great powers used low frequency, with metal or plastic case, are represented figure 15-c.
4. - FUNDAMENTAL ASSEMBLIES
OF THE TRANSISTORSBy analyzing the principle of operation of the transistor assembled in common base, we saw that the collector current can be ordered by means of the current of transmitter.
So that the transistor functions thus, one must polarize his junction transmitter-bases on line and that collector-bases in reverse by using two distinct piles as indicated figure 16-a in the case of transistor PNP or as indicated figure 16-b in the case of transistor NPN.
By observing these circuits, one realizes that the electrode of the base is common to the circuits of collector and transmitter. Indeed, by regarding the transistor as a simple device having three terminals, one sees that the collector current IC (in the case of transistor NPN of the figure 16-b), provided by the B2 plaice, traverses the electrode of the collector, penetrates in the collector and must leave by the basic electrode to turn over to the negative pole of this pile. In the same way, by considering the circuit of transmitter, one sees how the current of transmitter provided by the B1 pile penetrates in the transistor by the electrode of the base and leaves by that the transmitter.
The basic electrode is thus traversed by the collector current IC and that of transmitter IE ; since these two currents circulate in contrary direction in this electrode, the basic effective current is given by their difference and is very weak (IC being not very lower than IE).
This way of cabling the transistor is called assembly bases common bus the basic electrode is common to the circuit of transmitter including/understanding the B1 pile as with the circuit of collector including/understanding the B2 pile.
Usually, with this type of assembly, the base is connected to the mass, one says whereas the transistor is assembled with the base with the mass.
The transistor can also be assembled in a different way as it in the case of is seen figure 17-a type PNP or like illustrated 17-b in the case of type NPN appears.
In this new assembly also, the junction transmitter-bases is on line polarized by means of the pile B1 and the junction collector-bases is polarized in reverse by means of the B2 pile which, this time, is connected either between the collector and the base, but between the collector and the transmitter.
The electrode of the transmitter is thus common to the basic circuits and of collector. It is traversed by the basic current and the collector current as indicated figure 17.
The total current in the transmitter is given by the sum of two currents IC and IB ; those circulating in the same direction, one thus has IE = IC + IB.
By analogy with the preceding circuit, the assembly of figure 17 is called transmitting assembly common or transmitter to the mass since in this case the transmitter is common to the input circuit (which are now that basic instead of that of transmitter) and to the output circuit (which is still that of collector).
The operation of the transistor assembled out of common transmitter is different from that of the transistor assembled in common base bus now the collector current is ordered by the basic current.
There is a third type of assembly of the transistor : the collecting assembly common or collector to the mass with which the input circuit is still that basic and the output circuit that of transmitter.
4. 1. - POLARIZATION OF THE COLLECTOR
In the common transmitting assembly, the tension of collector is applied between the collector and the transmitter with polarities such as the junction collector-bases is polarized in reverse.
Thus if a transistor NPN is considered, the positive pole of the pile is connected to the collector as it is seen figure 18.
In the circuit, it circulates a collector current which is called ICE0 to indicate that this current is that circulating between the collector and the transmitter when the basic current is null (indeed, the base is not connected and thus, on its terminal no current circulates).
This collector current is similar to the current ICB0 which we saw in the assembly at common base.
However, ICE0 is much larger than ICB0. To include/understand that, it is necessary to examine how tension VCE of the pile is distributed between the two junctions being in series.
Let us imagine that one replaces the transistor by two diodes with junction (figure 19-a). One, indicated by DCB, represents the junction collector-bases and the other, indicated by DBE, represents the junction base-transmitter.
These diodes, as it is seen figure 19-a, are connected in series and are laid out in contrary direction.
The junction collector-bases is traversed by current ICE0 in the direction NP, while the junction base-transmitter is traversed in direction PN.
More precisely, diode DCB is polarized in reverse while diode DBE is on line polarized.
Tension VCE is divided then into two, i.e. in a tension VCB at the boundaries of diode DCB and in a tension VBE at the boundaries of diode DBE.
To determine the order of magnitude of these two tensions, one can also replace the two diodes by their internal resistance which, in this circuit, have very different values. Indeed, diode DCB being polarized in reverse, has an opposite resistance Ri of very high value, while diode DBE being polarized on line present a direct resistance Rd of very low value.
In the diagram are equivalent illustrated appears 19-b, Ri and Rd thus behaves like a dividing bridge.
Two resistances being traversed by same current ICE0, the terminal voltage of each one of them is directly proportional to the value of these resistances, therefore the value of tension VCB is notably larger than that of tension VBE.
To take an example, one can suppose that Rd is 200 times smaller than Ri. Tension VBE at the boundaries of Rd will be then 200 times smaller than tension VCB at the boundaries of Ri.
(We defer the same diagram in order to better facilitate the reading above, namely figure 19).
One can see as well as the tension of the pile is not entirely applied to the junction collector-bases, but that a small part of this tension is localized with the electric terminals base-transmitter so as to polarize it on line. There is thus on behalf of the transmitter, emission of a certain quantity of electrons which diffuse in the base and which reach the collector mainly.
The collector current will not then only be consisted of current ICB0, but also of the current due to the emission of electrons on behalf of the transmitter.
To know the expression of ICE0 according to ICB0,
it is enough to use the formula already seen in the assembly bases common, i.e. IC
= 
IE + ICB0.
In this case, IC = IE = ICE0. It results from it that :
ICE0 + ICB0)
= ICB0)
x ICB0To have an idea of the report/ratio which exists
between ICE0 and ICB0,
let us suppose that
is equal to 0,98 and that ICB0
valley 5 µA. One obtains then ICE0
= 250 µA, i.e. ICE0 is 50
times larger than current ICB0
obtained with the same transistor assembled in common base.
Value 1 / (1 - )
depends obviously on the value
of the transistor considered and is all the more large as
is close to 1.
The values of ICB0 and ICE0 deferred previously are given as an indication and relating to transistors to germanium.
In the case of transistors with silicon on the other hand, current ICB0 is generally a smaller thousand of times and current ICE0 remains negligible although being consequently given formula.
4. 2. - POLARIZATION OF
THE BASE AND COEFFICIENT Of AMPLIFIER 
In the common transmitting assembly, the current of the input circuit is current the basic IB and the current of the output circuit is the current one of collector IC.
By increasing the biasing VBE of the base, one increases basic current IB and one increases proportionally the collector current IC. In other words, current IC is ordered by current IB.
One can thus define as coefficient of performance
of current the relationship between the collector current IC
(decreased by residual current ICE0) and
basic current IB. This coefficient called
beta (second letter of the Greek alphabet whose symbol is )
is given by the formula :
= IC - ICE0 / IBIn practice however, the value of ICE0 is very low compared to IC, therefore negligible.
The value of the coefficient
can be determined by the simple relationship between IC
and IB and one can thus write :
= IC / IBTo have an idea of the value of the coefficient
presented by the transistors, it should be remembered that basic current IB
is given by the difference between IE and IC
and that the current of transmitter IE are
not very higher than the collector current IC.
One deduces from it that basic current IB must have a very small value compared to IC.
The values of
must thus be very large and either lower than 1, as it is the case for the
coefficients .
Let us express
according to .
We know that :
IC =
IE + ICB0
=
IE + (1 - )
ICE0
=
(IB + IC) + (1 - )
ICE0
from where IC
(1 - )
=
IB + (1 - )
ICE0
(1 - )
(IC - ICE0) =
IB
that is to say IC
- ICE0 / IB =
/ (1 - )
from where
=
/ (
- 1)
This formula thus makes it possible to calculate
the coefficient of performance
when one knows the coefficient of performance .
For example for a transistor which has a
coefficient
= 0,98 one will have, with the common
transmitting assembly, a coefficient of performance
equal to :
= 0,98 / (1 - 0,98) = 0,98 / 0,02 = 49We thus see that ,
contrary to ,
is much larger than 1. That explains the great difference which exists between
the two types of assemblies (with being known, bases common and common
transmitter).
In the case of the common transmitting assembly,
it is enough to a weak basic current to cause a notable increase in the
collector current. Indeed, the collector current, which in the absence of
polarization of the base (IB = 0), with
value ICE0 (residual current), increases by
a value equalizes with
time the value of the current which one makes circulate in the base.
The collector current is given by the formula IC
=
IB + ICE0, that is to say by neglecting ICE0 : IC
=
IB.
The values of
which one records in practice for the transistors of low and average power, such
those used in the receivers of radio, spread out from 20 to 600 according to
types.
It is important to note that the coefficients of
performance
and
relate only to the static operation (in D.C. current) of the two circuits
considered (respectively at base common and common transmitter).
If one applies a signal to the entry of the two circuits, these two coefficients of performance are indicated by another symbol and take different values.
4. 3. - COMPARISON ENTERS THE TWO TYPES OF ASSEMBLIES
We saw that the transistor in the case of presents a greater profit while running the common transmitting assembly than in that of the assembly bases common and than one obtains a collector current raised with a relatively weak basic current.
To fix the ideas, one can notice that a transistor low frequency preamplifier assembled out of common transmitter has a current of basic entry about a few tens of microamperes, while for the same transistor assembled in common base, the current of entry (of transmitter) is about a few milliamperes.
Now let us consider the power gain, this one being defined by the report/ratio of the power provided in the circuit of the collector and that provided in the circuit of the base.
The power gain is larger for an assembly out of common transmitter than for an assembly in common base but on the other hand the residual current is more important in the first assembly.
5. - TRANSISTORS
NPN, PNP AND THEIR SYMBOLS GRAPHICIn the diagrams given until now, one always represented the transistor by means of three rectangles symbolizing the three zones of semiconductors of which it is made up.
In the electric diagrams of the transistor apparatuses, one is accustomed to on the other hand representing the transistor by a graphic symbol also indicating, in a conventional way, the type of the transistor, i.e. if it is NPN or PNP.
Among the many graphic symbols used in the past, those deferred figure 20 gradually spread.
In these symbols, the thick vertical line represents the zones of semiconductor from which the three electrodes leave.
On the left side, the small horizontal line represents the basic electrode ; on the right side, the two small tilted lines represent the electrodes of transmitter and collector.
To differentiate the transmitter from the collector, on the first, one draws an arrow whose direction has also as a role to indicate if it is of a transistor NPN or a transistor PNP.
The choice of the direction of the arrow was made so that the latter announces the direction in which the forward current of the junction base-transmitter circulates.
Therefore, by remembering that a junction leads in direction PN, it is easy to establish that the arrow is directed towards outside (i.e. base of the type P to the transmitter of the type N) in the case of transistor NPN, while in the case of transistor PNP, this arrow are directed towards the interior (i.e. transmitter of the type P towards the base of the type N).
To point out the polarities of the tensions to be applied to the transistors in the cases of the assemblies bases common and common transmitter for type NPN as for type PNP, the diagrams relating to the four possible cases were joined together figure 21.
a) - assembly bases commune-transistor NPN (figure 21-a) :
b) - assembly bases commune-transistor PNP (figure 21-b) :
c) - transmitting assembly common-transistor NPN (figure 21-c) :
d) - transmitting assembly common-transistor PNP (figure 21-d) :
According to these diagrams, one can note that the tension of collector is always positive in the case of transistors NPN and negative in the case of transistors PNP while the tension of the control electrode (transmitter and bases respectively in the two types of assembly) is of contrary polarity to that of the collector in the assembly bases common and is of the same polarity than that of the collector in the common transmitting assembly.
In the next lesson, we will examine the continuation of the transistors thus the continuity of the assemblies of this lesson but with other associated components. (Semiconductors 5).
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