The
bipolar transistor |
Characteristics
of a bipolar transistor |
|
|
Return
to the synopsis |
To
contact the author |
Low
of page |
Created it, 06/03/17
Update it, 06/03/20
N° Visitors
3. - THE TRANSISTOR EFFECT
3. 1. - JUNCTION N.P.N.
Let us associate junction N.P. previously described a material of which conductibility is of type N in such a way that the continuation is obtained : N.P.N. (represented on the figure 7-a).
The junctions without polarization are not of great interest. In the case of the figure 7-a, there are several manners of organizing this polarization. For any statement, only one interests us and we will cross short while arranging it according to the figure 7-b.
On the basis of the left, junction N1P is polarized in the direct direction by the B1 pile. On the right, the B2 pile polarizes junction PN2 in the opposite direction.
Which are the currents which circulate in the external circuits connected to B1 and B2 ?
In that of B1, we note the presence of the flow of electrons of the majority carriers of the current IF.
Free electrons of tributary the N1 zone in the zone P where they are collected by the electrode connected to the pole + of the B1 pile.
In the circuit of B2, only a negligible current circulates. It is due to the minority carriers of the junction PN2 which is polarized in reverse.
This current, remember, is due to the space charge (galvanic effect), created close to the junction, by the presence of anions side P (negative ions) and of cations side N2 (positive ions).
The electric field e resulting, supports the circulation of electrons of the minority carriers of the zone P towards the zone N2.
Until now, this assembly is not of large interest. It thus should be done something so that that changes.
If one avoids with the electrons of N1, to recombine in the zone P and to leave by this place, they will be able to come to reinforce the current with electrons from the minority carriers caused by polarization with B2.
To arrive at this result, which artifice can we employ ?
Not to leave time to the electrons to recombine in the zone P and to be subjected to the field e which will transfer them towards the zone N2, implies a thickness of the very weak zone P.
Let us thin the zone P as much as possible and reconsider the currents (figure 7-c).
In the circuit of B1, from now on but one very weak current does not circulate any more. On the other hand, we note an important current in the circuit of B2.
The current of the majority carriers of N1 is transferred, mainly, in the zone N2 where it is attracted by the positive pole of B2.
Only a weak part of this current circulates in the circuit of B1. Some electrons have time to recombine in the zone P and are attracted by the positive pole of B1.
It is interesting to note that the sum of the currents leaving the zones P and N2 is equal to the current injected into N1.
This description constitutes what one calls : the transistor effect.
It is necessary well to be conscious that this physical phenomenon revêt a cardinal importance for electronics from the possibilities of miniaturization which it offers.
The sum of electrons collected out of N2 and P is equal to that emitted by N1. We have just found a name with each one of the electrodes connected to the N1 zones and N2 :
that of N1 will bear the name of transmitter
that of N2 that of collector.
The base of all this advance rests on the weak outgoing current of P. the electrode connected to this zone will bear the basic name.
Indeed, without the basic current, there are no current of transmitter and, consequently, no collector current (except for reverse current of junction PN2).
In addition, to further decrease the recombinations in the zone P, therefore to improve the relationship between the collector current and that of the basic current, one will dope less this zone.
This one having less holes, the recombinations will be less and report/ratio IC / IB will be improved by it.
If the geometry of the zone N2 (the collector) is such that it allows a better collection of the electrons, we will still improve this report/ratio (figure 8).
The movements of the free electrons not being inevitably rectilinear, if the surface of the collector is more important than that of the transmitter, the collection of the electrons can be only favoured. The transistor takes then the aspect of figure 8.
The dimension of the collector supports the collection of the electrons (from where a better output).
This type of junction transistor takes also the name of : bipolar transistor.
On the figure 7-c, we see that the collector current is in the following way made up :
most of the emitter current, approximately 0,9 times this one
the reverse current of the junction PN2, which one calls ICBO (residual current collector-bases).
In addition, the base current is consisted :
a weak part of the emitter current
current ICBO.
The base current thus consists of two currents in opposite direction.
We note, that using a weak current IB bases, under a weak tension B1 (direct polarization of junction N1P), one controls an important collector current IC, under a B2 tension higher than that of B1.
In short :
Using a very low power, one controls a power much more important.
The transistor is an amplifier of power.
It should be known that there also exists in other ways of organizing the material association of the type N and P.
Inter alia, that which leads to transistor P.N.P. the explanation of the phenomenon is similar but, in this case, doping taking the appearance of an injection of gaps or holes, one reasons by the displacement of these holes.
Figure 9 represents the currents of holes in transistor P.N.P.
Actually, the current of electrons circulates in the opposite direction (do not forget only if gaps move, it is because the electrons come to fill them).
4. - THE BIPOLAR TRANSISTOR
We know that there are two types which result from the organization of the materials N and P. It acts :
transistors N.P.N.
transistors P.N.P.
These two families can be made out of germanium or silicon. This last is the most widespread material from now on for the manufacture of the transistors.
Figure 10 represents a frequent symbolization of these transistors.
It is noted that the transmitter is differentiated from the collector by an arrow.
This arrow does not represent the direction of circulation of the electrons, but the conventional direction of the electrical current, i.e. the direction of displacement of the holes. It is thus necessary to take care not to make confusion between these two manners of representing the circulation of the electric charges.
For a question of standardization, in the diagrams, we must use the conventional direction.
The applicability of the bipolar transistors extends from the D.C. current at the radio frequencies (U.H.F. : Ultra high frequency).
The power dissipated on their collector goes from a few hundred milliwatts to several hundred Watts according to types'.
According to the destination of the assemblies, in which they intervene, we find three assemblies fundamental, allowing to use their characteristics as well as possible and enumerated below :
common transmitter
common collector
base common
Figure 11 symbolizes these three assemblies.
In the common transmitter, one applies the input signal between base and transmitter, one collects the output signal between collector and transmitter (the electrode of transmitter is common to both signals). It is the assembly more spread, it makes it possible to obtain between the entry and the exit a profit of power.
In the common collecting assembly, the input signal is applied between base and collector, the output signal is collected between transmitter and collector. It allows obtaining a profit while running.
In the assembly bases common, the input signal is applied between transmitter and bases, the output signal is collected between collector and bases. It makes it possible to obtain a profit in tension between the entry the exit.
Figure 12 gathers principal qualities of these three assemblies.
The principal electric quantities which characterize the transistor are 4 :
Two for the input circuit :
basic current IB
the tension base-transmitter VBE
Two for the output circuit :
the collector current IC
the tension collector-transmitter VCE.
The two sizes referring to the input circuit correspond to the characteristic of a diode (junction base-transmitter). Consequently, one finds polarization in the direct direction (allowing obtaining a collector current) and polarization in the opposite direction, with the phenomenon of avalanche when this tension becomes too high (figure 13).
In general, this value is given by the manufacturer because it is destroying and does not have to be reached (VBE (BR)).
The electric quantities of exit, running collecting IC according to the tension collector-transmitter VCE, are raised for various currents of entry IB.
This characteristic comprises three zones :
the zone of operation to saturation
the linear zone of operation
the zone of avalanche.
Figure 14 represents one of these characteristics.
One gathers the static characteristics of the transistor in networks of curves on four quadrants, whose figure 15 gives an example.
Quadrant 1 of
this figure represents the characteristic of exit: IC
= function of VCE for various values of IB.
It makes it possible to determine the following elements:
the resistance of exit
profit while running
line of load.
Quadrant 2
represents the characteristic of direct transfer: IC
= function of IB. It allows the study of the
behavior of the transistor:
for weak signals
for strong signals.
Quadrant 3
represents the static characteristic of entry : VBE
= function of IB. It allows the study
:
differential resistance of entry
line of attack.
Quadrant 4
represents the opposite characteristic of transfer : VBE
= function of VCE for various currents
IB.
It is the coefficient of reaction of the exit on the entry.
A great number of parameters make it possible to define a transistor. Among those, we find the parameters h (hybrid parameters, drawn from matrix algebra).
These are those that one most frequently uses, also, we will enumerate them as an indication and, we remind to you that the latter were largely explained in fundamental electronics (see this lesson).
h11 ; it is drawn
from quadrant 3
It represents the differential resistance of entry :
h11 = DVBE / DIB = Re expressed in ohm
This report/ratio represents the value of the tangent to the characteristic traced for the VCE considered, compared to the variations of VBE and IB.
h12
; it
is drawn from quadrant 4 and represents the
coefficient of reaction of the exit on the entry :
h12 = DVBE / DVCE = µ it is the relationship between two tensions, therefore without unit.
h21 ; it is drawn
from quadrant 2 and represents the direct
transfer or profit while running :
h21 = DIC / DIB = b relationship between two currents, therefore without unit.
h22 ; it is drawn
from quadrant 1 and represents the reverse
of the resistance of exit or conductance :
h22 = DIC / DVCE = 1 / rs expressed in mho
In theory, the manufacturer gives the networks of quadrants 1 and 3.
The parameters h are often followed, in index, of an E which assigns these parameters to the assembly into transmitter-common.
Example : h21E = 100 per IC = 5 mA.
What means, that in transmitter-common assembly, this transistor has a profit while running from 100 for one IC of 5 mA, which corresponds to a basic current of 0,05 mA.
To avoid encumbering this lesson, we will continue the procedure of the bipolar transistors with saturated mode and blocked rate corresponding to digital electronics…
|
Click
here for the following lesson or in the synopsis envisaged to this end. |
High of page |
Preceding
page |
Following
page |
Daniel
