Junction transistors by
alloy |
||
|
Return
to the synopsis |
To
contact the author |
Low
of page |
Created it, 05/10/15
Update it, 06/01/14
N° Visitors
SEMICONDUCTORS 4 “7th Part”
In this theory, we will examine a component very much used in electronics : the transistor.
The principal manufactoring processes of the transistors will be exposed as well as the fundamental assemblies.
It is a device formed of three zones of semiconductor laid out the ones beside the others so that two consecutive zones are of different type.
With this system, one understands easily that one can obtain only two types of the device above : either one inserts a zone of semiconductor P between two zones of semiconductor N like illustrated appears 1-a, or on the contrary one intercalates a zone N between two zones P (figure 1-b).
The devices in question are then supplemented by three metal plates placed respectively on the ends of the side zones and the side of the central zone. They are indicated by the letters E, B, C.
To distinguish the two types, the device of the figure 1-a is called transistor NPN and that of the figure 1-b transistor PNP.
On the other hand in both cases, the central zone is called bases (from where the symbol B given to the plate and the corresponding connection) while the side zones are called respectively transmitting and collecting (from where the symbols E and C given to the plates and corresponding connections).
The characteristics of the two types of transistors are similar, i.e. they function in the same way. However, they require tensions of contrary polarity : transistor NPN functions with positive tensions while transistor PNP functions with negative tensions.
(We defer the same diagram in order to better facilitate the reading with knowing above figure 1).
By observing figure 1, one notices that the three zones form two distinct junctions; in transistor NPN, the transmitter and the base form a junction NP while the base and the collector form a junction PN.
In the case of the transistor PNP on the other hand, the transmitter and the base form a junction PN while the base and the collector constitute a junction NP.
When one applies any tension to the electrodes of the transistor, i.e. when its two junctions are not polarized, it appears the same phenomena there that one had in junction PN. (See semiconductor 3).
Each junction is the seat of a barrier of potential where the zone NR is made more positive than the P. zone the barrier of potential has a value such as it allows the passage of a flow of majority carriers equal to the flow of the minority carriers. Thus the total flow of the carriers crossing each junction is null.
For the normal operation of a transistor, that it is of type PNP or NPN, the junction transmitter-bases must be on line polarized while the junction collector-bases must be polarized in reverse. The polarization of the junctions can be obtained by means of a connected pile in an adequate way.
To polarize the junction directly transmitter-bases, the pile is connected as shown in the figure 2-a in the case of a transistor PNP, or in the manner indicated figure 2-b in the case of a transistor NPN.
In the circuits thus made up, in a way similar to what occurs in a diode, it circulates a forward current called current of transmitter and indicated by IE.
To polarize in reverse the junction collector-bases, the pile is connected as shown in the figure 3-a in the case of a transistor PNP, or in the manner indicated figure 3-b in the case of a transistor NPN.
In these new circuits, one also observes what occurs in a diode, i.e. one notes the circulation of a reverse current called current residual and indicated by ICBO.
This symbol (ICB0) indicates that it is about the current crossing the junction collector-bases when the junction transmitter-bases is not polarized (IE = 0).
Residual current ICB0, being due to the minority carriers, has a very low intensity. This one, which practically does not depend on the tension applied to the junction collector-bases, is on the other hand largely dependant on the temperature to which is the transistor and of its characteristics.
The EC what we have just said, it results that each junction of a transistor behaves like a diode. That is true only if the two junctions are separately polarized. So on the other hand, they simultaneously polarized like are indicated figure 4, the behavior of the transistor is different.
In this case, indeed, currents IE and ICB0 do not continue any more to circulate independently one of the other as indicated figure 2 and 3.
When the two junctions are polarized like illustrated figure 4, the current ICB0 which results from it increases to reach a value almost equal to that of current IE : this new current is called current of collector and is indicated by IC. It is thus seen that in a transistor, the current of transmitter can influence the collector current.
Thus, when the current of transmitter is null as in the case of figure 3, the collector current has a very low intensity (it is current the residual ICB0). When, on the other hand, the current of transmitter is not null any more as in the case of figure 4, the collector current increases by a value almost equal to that of the current of transmitter. To explain it, it is necessary to consider the interior of the transistor.
First of all let us examine transistor PNP of the figure 4-a by interesting us in the junction transmitter-bases.
Since this junction is on line polarized, two flows of the majority carriers which cross it are much more intense than two flows of the minority carriers. Those will thus be neglected. There will be thus a flow of holes circulating of the transmitter towards the base and a flow of electrons circulating of the base towards the transmitter. Electrons arrived on the recombining transmitter in their turn with the holes present while as many electrons is provided to the base by the pile connected between this base and the transmitter.
At the same time, the same pile attracts an equal number of electrons of the transmitter in which it is formed new holes replacing those which disappeared because of the recombinations with the electrons coming from the base. On the other hand, the flow of holes circulating of the transmitter towards the base behaves in a completely different way. Indeed, it should be known that the holes arrived in the base become minority carriers since this base is of type N.
In addition, the junction collector-bases being polarized in reverse, this polarization stops the flow of the majority carriers while it supports the passage through the junction of the flow of the minority carriers. Consequently, the holes coming from the transmitter and which arrived on the basis find forced to also cross the junction collector-bases. These minority holes, carriers, join the collector thus.
It is thus added, with the weak residual current of the minority carriers crossing the junction collector-bases polarized in reverse, the current much more intense due to the holes coming from the transmitter.
Previously, we said that the collector current increases by a value almost equal to that of the current of transmitter. Indeed, all the holes coming from the transmitter do not reach the collector because a small part of them recombining with the electrons present in the base. That thus contributes to form the basic current indicated by IB as represented figure 4-a.
(We defer the same diagram in order to better facilitate the reading above, namely figure 4).
To make the collector current most equal possible to the current of transmitter, it is necessary to seek to reduce to the minimum the number of holes recombining with the electrons present in the base.
To this end, the transistors are produced with a very mean base so as to reduce the course which the holes achieve there to reach the collector.
The possibility that these holes have to meet the electrons present in the base and to recombine with them thus becomes weaker.
For this reason, in the figures considered until now, the zone constituting the base is represented narrower than the two others forming the transmitter and the collector. Another means to reduce this possibility of meeting consists in carrying out this base with a little doped semiconductor. Thus, the number of free electrons present in the base is reduced.
The transmitter is called thus because it emits the holes which, after having crossed the base, are able at the collector. Its denomination comes owing to the fact that it “collects” the major part of the holes “emitted” by the transmitter.
What was known as is valid for transistor PNP. One can say the same thing for transistor NPN of the figure 4-b by holding account that in this one the transmitter consists of a semiconductor of the type N. It “thus emits” electrons which are, for a great part, “collected” by the collector after having crossed the base.
Since the electrons have a contrary load of sign to that of the holes, the two piles with which one polarizes transistor NPN are connected with the polarities opposite of those adopted for transistor PNP. Consequently, the currents of transmitter, basic and of collector are directed in contrary direction as indicated figure 4-b.
Now, we will specify the composition of the collector currents and basic while referring to us with a transistor PNP.
As we said, the collector current IC is formed of residual current ICB0 and mainly current of transmitter IE.
Current IC,
being a little lower than current IE, is
expressed by means of the product of current IE
by a factor slightly lower than 1 called
coefficient of performance while running and which is indicated by the Greek
letter 
(alpha). The value of the coefficient
depends on the characteristics of manufacture of the transistor ; it
generally lies between 0,920 and 0,998. Therefore, the part of the current of
transmitter which arrives at the collector of a transistor can be calculated by
product
x IE.
Example : if the coefficient
has a value of 0,98 and that the current of
transmitter IE is 5
mA, the product above becomes
x IE = 0,98 x 5 mA = 4,9 mA ; that means
that current of 4,9 mA arrives at the collector.
By adding to this current residual current ICB0,
one obtains the collector current IC ;
the expression of this last is thus IC =
x IE + ICB0.
As one can see it figure 5-a, current IC
at exit of the collector crosses the B2 pile
and reaches point A where it is divided into
two. A part, constituting current ICB0,
turns over to the base while the other part (
x IE) moves towards the B1
pile.
To point A,
current IE is added to the current indicated
figure 5-a by IE -
x IE. As one sees it on this figure, this current comes from the base
and it is due to part of the current IE
which does not reach the collector because of the recombinations of the holes
and the electrons in the base. This current, indicated by IE
-
x IE, is precisely equal to the difference between the current of
transmitter IE and part of this current
(i.e.
x IE) which reaches the collector.
On the figure 5-a, one can thus see that the
basic current, i.e. the current which circulates in the connection connecting
point A to the base of the transistor, is
made of two currents : the current ICB0
which enters the base and the current which in fate : IE
-
IE. Since generally this last A a value larger than current ICB0,
basic current IB is directed in the
direction indicated figure 4-a, i.e. outgoing of the base, therefore of the same
direction than current IE -
IE.
What was seen above for transistor PNP is also valid for transistor NPN with the proviso of reversing the direction of all the currents. As it is seen figure 5-b, that is due to the inversion of polarity of the two piles.
Of what was known as up to now, it is not yet possible to deduce which can be the practical utility of the transistor ; for the moment, we noted that current IC can be ordered by means of the current of transmitter IE. However, we saw that this last is larger than the first and thus, from this point of view, we do not find any advantage.
But one should not reason only while running ; it is also necessary to take account of the tensions brought into play. Indeed, it should be remembered that the junction transmitter-bases is on line polarized while the junction collector-bases is polarized in reverse.
It follows that, to make cross the junction transmitter-bases by the current of transmitter IE, it is enough to apply a reduced tension: consequently, the power brought into play in the circuit of transmitter is rather low. On the other hand, since the junction collector-bases must be polarized in reverse, one can apply a tension larger to him than that applied to the other junction.
The collector current IC being far from the lower than that of transmitter IE, the power brought into play in the circuit of collector are thus higher than that with the circuit of transmitter.
One can thus say that a transistor behaves like an amplifier bus by providing to its input circuit a low power, it is able to provide in his output circuit a power which can be hundred times higher. That is due to the fact that the transistor with the property to make circulate in the output circuit to great tension the same current (or almost) that that which circulates in the input circuit with low tension. Indeed, if the tension collector-bases were lower or equal to the tension transmitter-bases, the power of the output circuit would be lower than that provided to the circuit input and one could not thus have power gain.
The condition necessary so that a transistor amplifies is thus that the circuit of collector functions with a tension much higher than that to which functions the transmitting circuit. Therefore, if one applies to the entry of a transistor a signal of low power, one can take on his output circuit the same signal, this one having however a power much larger.
One can take as mechanical analogy the compressed-air assisted brake of a coach; the driver would not be able to exert a sufficient force to block the wheels, also is limited it to order using its foot a pneumatic valve regulating the surge of compressed air. This one is able on the other hand, to exert on the brakes the force necessary to the stopping of the vehicle.
The transistor behaves like a valve. Indeed, using a small power (provided to the circuit of transmitter), one can control a power in the circuit of collector much larger.
One can as note as the circuit of transmitter has a low resistance (it behaves indeed as an on line polarized diode) while the circuit of collector has a resistance much larger.
From this point of view, the transistor is a device able to transfer certain running from the input circuit to low resistance to the output circuit to great resistance. This property was indicated by the Americans in form condensed by the words transfer and resistor which then gave inception at the end transistor.
On the figures examined until now, the transmitter and the collector were always represented as of the equal zones laid out on both sides of the zone of the base. Moreover, one always regarded transmitter the zone of left and as collector the zone of right-hand side. Since these two zones equal and are laid out symmetrically compared to the base, one could think that one can use as transmitter the zone of right-hand side and collector that of left. In other words, one could believe that the transmitter and the collector of a transistor are interchangeable, i.e. the transmitter can provide the function of the collector and vice versa.
In fact, the transmitter and the collector are produced in a different way although they are of the semiconductors in the same way standard.
Thus, so that the coefficient of performance
is nearest possible to 1, one carries out notably different dopings for the
semiconductor which will have to function out of transmitter and for that which
will have to provide the function of collector.
The transmitter will be the semiconductor most strongly doped ; moreover, to facilitate the dissipation of heat developing inside the transistor, the junction collector-bases is carried out with a section larger than that of the junction transmitter-bases.
We will review some processes used for the manufacture of the transistors.
The first transistors were built according to the same principle as that used for the diodes with point; thereafter, since 1950, one had recourse to other methods which can be reduced to three: the process by alloy, the process by diffusion and the process double diffusion.
2. - TRANSISTORS
JUNCTIONS BY ALLOYThe technique of the junction by alloy primarily consists in putting a pastille of semiconductor in contact with a certain quantity of material of impurity and heating this material at a temperature higher than its melting point. It is thus formed an alloy consisted material of impurity and the surface part of the semiconductor.
After cooling of alloy and pastille, a certain quantity of atoms of impurities forms in the crystal lattice of the semiconductor a zone of the type N or type P definitely distinct from the remainder of the pastille. If the pastille is of type N, one uses materials of impurity allowing, after cooling, to obtain a zone P. Inversement, if the pastille is of type P, one uses materials of impurity ready to determine the formation of a zone N.
The junction between the zone which was formed and the remainder of the pastille constitutes a junction PN.
To obtain a transistor, it is advisable to form in the same pastille two junctions PN ; therefore, the same treatment is simultaneously carried out on the two opposite faces of the same pastille in order to create the two necessary junctions.
Figure 6 represents the cuts of two transistors with alloy ; they are transistors PNP to the germanium used formerly in the radio operator receivers with amplitude modulation and in the amplifiers low frequencies.
The transistor of the figure 6-a, was manufactured to function like amplifier of tension in the stages preamplifiers low frequency, in the oscillating stages and converters for medium waves, or in the amplifiers of intermediate frequency ; the same structure was adopted for manufacture of certain types of transistors used in the final stages of amplifiers of power lower than 1 Watt.
The transistor of the figure 6-b, on the other hand, was produced to function like final amplifier low frequency for powers equal or higher than 1 Watt.
The two structures were obtained with the same process by alloy in pastilles of germanium N monocrystals.
These pastilles constitute the basic electrode; the zones P, laid out on both sides on the opposite faces of the pastille, form the electrodes of transmitter and collector.
Each pastille is welded with a bored metal coil ; the electrode of transmitter is in the center of the hole so that the transmitter is not in contact with the band.
In the structure of the figure 6-b, the electrodes have dimensions higher than those of the figure 6-a ; that was done so that they can be traversed by more intense currents without there being an excessive heating. Between in addition to, always in the structure of the figure 6-b, the electrode of collector appears gone up on a rather broad metal support ; this provision was adopted to increase the power dissipated by the transistor during its operation.
On figure 7, one can see how the transistors (those of the type of the figure 6-a) in their envelope are laid out of glass.
The external surface of the envelope and the lower part of the base plate are in general covered with black painting the purpose of which is to prevent than the light can have an influence on the operation of the transistor.
The interior of the case is filled of grease to the silicone which is used to prevent any surface deterioration of the semiconductor and to damp out possible mechanical vibrations.
If it is about a transistor of power, one covers the envelope with a metal cylinder which, not only protects the transistor from the light, but is also used to improve thermal dissipation.
Figure 8 shows a transistor of power of the type presented figure 6-b.
The whole of the base plate and the cap form a metal case which has a good capacity of dissipation of the heat generated by the transistor. To improve thermal dissipation, one directly welds the electrode of collector, which is most prone to the heating, to the base plate.
Of this way, it can be used as radiator and replaces the terminal of the collector (C) which misses thus apparently. The transistor thus conditioned has a structure robust, compact and thus particularly resistant to the various thermal stresses and mechanical.
With the technique of the junctions by alloy, it was possible to manufacture transistors PNP functioning up to 15 MHz and of transistors NPN functioning up to 30 MHz.
In general, the response of a transistor to the high frequencies depends on the time of transit of the electric charges constituting the current in the base, i.e. the time which the electrons spend to cross the zone P of transistor NPN or the holes to cross the zone N of transistor PNP. By reducing this time of transit, one improves frequency response of the transistor.
The reduction of the time of transit in the base can be obtained in two manners : the thickness of the base and consequently the course of the loads are decreased, or one increases the speed even of these loads.
The minimal basic thickness that one obtains by the method of the junctions by alloy is about the micron (10-6 meter) ; but even with these extremely reduced dimensions, it is not possible to reach sufficiently short times of transit to have transistors with alloy functioning at frequencies higher than 30 MHz.
To still improve frequency response of the transistor to alloy, one has recourse to technological easy ways which make it possible to accelerate the loads in the basic electrode.
The acceleration of the loads is obtained by gradually graduating the concentration of the atoms of impurities along the thickness of the pastille intended for the manufacture of the transistor with alloy. Thus, in the base which preserves the structure of origin of the pastille, there is a range of the concentration of the atoms of impurities going from the transmitter to the collector: this range is called “gradient of impurities”.
On figure 9, one can schematically observe the range of impurities in the base of a transistor ; indeed, the atoms of impurities are more numerous side of the transmitter and their number falls while approaching the collector.
Let us recall that the base of a transistor NPN, such as that represented figure 4-b above, is consisted a semiconductor of the type P, i.e. by a semiconductor in which the atoms of impurities acquired an electron. These atoms are thus negative fixed loads.
The greatest density of the atoms of impurities on the side of the transmitter compared to the side of the collector causes a larger concentration of negative charges on the side of the transmitter than that of the collector. Thus, a certain potential difference VG exists between the ends of the base.
This potential difference, due to the range of impurities, is directed so that it exerts a force on the live loads, i.e. the electrons crossing the base. One thus obtains a larger acceleration of these loads and consequently, a reduction of their time of transit.
This phenomenon is usually called “drift transistor effect” (drift transistor is an English term meaning pushed, drift).
Various types of transistors with alloy for high frequencies were manufactured on the basis of drift transistor effect. They were used in the stages high frequencies and intermediate frequencies of the receivers radio operator AM - FM and the television sets and well of others…
|
Click
here for the following lesson or in the synopsis envisaged to this end. |
High
of page |
 Preceding
page |
Following
page |
